Water System

[Mwilliamshs]

Looking for suggestions on the routing of the diagram moreso than the individual components.

Thinking my way through a water system for the van. This will all be installed on the passenger side, rear, with the tank just ahead of the wheel well for better weight distribution and the shower, fill points, tempering valve, etc just inside the right rear door built into a cabinet. This is so they're secure (locked up) and no extra holes have to be cut into the van body. The heater core is also already on the passenger side so no water should need to cross the body or frame and since the passenger door opens first in the back a fill-up or rinse off could be done without opening the whole rear end.
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What I want:
Hot shower
Easy access to water while cooking outside (side doors, under my awning)
Hot and cold sink in kitchen
Good filtration (under questionable conditions drinking will be bottled or 20L purified jugs)
Reliable, leak-free system (he's got hiiiiiiigh hopes)
Water and electrical efficiency (the main reason I'm not going Reverse Osmosis on filter, in which case I would use the tank for drinking water too)
Ability to fill tank from city water connection or by gravity fill or pumping from a bucket, etc.
Ability to use pump-pressurized water in shower, sink, etc either from tank or a bucket, stream, etc or from a city water connection (hose bib). This allows unlimited hot showers from a stream or lake, using a 20L jug (think office water cooler) of filtered water (super commonly sold in central american markets) instead of onboard tank, and filling tank by gravity (pour it in or stick a hose in without pressure [water bandit]) or filling tank from a bucket, etc by sucking from the bucket, through strainer, pump, filter, then filling the tank with the Spray-Away through the gravity filler. Also makes heating the entire tank of water possible should a large volume of hot water be needed or if the engine overheated this gives a 3 gpm liquid-to-liquid cooling system with a 20 gallon heat sink.
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I've decided to go with a liquid-to-liquid heat exchanger to obtain hot water from the engine. This is compact, effective, and energy efficient because I'll either be driving and get heat as a byproduct of a warm engine or l can heat water at idle and not use propane, etc. I've decided to use PEX lines. It's cost-effective, widely available and common in RV/Marine applications, and more durable than most alternatives especially considering unknown water qualities and temperature swings on travels.
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Going with the home-run style of plumbing and it uses manifolds (one hot, one cold) and solid legs from manifold to fixture (faucet, etc) without connections or fittings in the middle unless absolutely necessary for clearance (like an ell to make a tight-turn). Manifolds are like one big tee fitting with multiple outlet connections (see below).
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With an assembled camper interior the fewer connections and fittings hidden behind panels the better in my opinion, especially considering the temperature swings, bouncing around, etc that the camper will see. Home-run plumbing's advantage is that from the manifold to the faucet (for example) there's nothing but solid pipe so no likely leak-points. If PEX (or any pipe really) is gonna leak it'll likely do it at a fitting so best to eliminate as many of them as possible and consolidate as many connections as you can at the manifolds or at the fixtures. This puts any likely problem areas where they'll be easily accessed for troubleshooting or repair. The manifolds will have built in ball valves for each run so if there were a leak it could be isolated easily.
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Now about what's actually in the diagram:
The Tank will likely be used from a local RV scrapper. Goal is 25 gallons or so total capacity. For reference, Barker 11916. My van is 80 miles away on the farm so using Sportsmobile's DIY Studio it looks like the rear fenderwells are 9-10" deep and I'd like to stay flush with their inner edges for at least 8' from the inside of the rear doors so let's say I can have a tank that's 10" wide (fender will have insulation affecting its dimension and cabinet will not). DIY Studio also shows about 28" of length from the fenderwell forward to the center of the rear-most barn door, let's say I have a tank there 24" long. If that tank is 10" wide and 24" long and 20" tall its volume will be 4,800 cubic inches, which is 20.78 gallons. At 8.3 lbs/gal that's 172.47 lbs of water. The 20 gallon tank kit linked has a shipping weight of 14 lbs with threaded adapters, hoses, etc so call it 200 lbs total. The only way I can think of moving the tank any further forward is to put it on the driver's side and closer to the driver's seat which concentrates about 400 lbs (200 tank plus 200 driver) in a small area and I'd rather not do that. Mounting the pair of 6v golf cart batteries (~60 lbs each) and a Manchester #66-4941 propane tank (83 lbs full) directly across from the water tank should allow the two to just about perfectly cancel one another out side-to-side. (200lbs water on right and 200 lbs batteries and propane on left) Add the pump, filter, accumulator, etc and they'll hopefully balance the solar charger, inverter, etc on the other side. Method to my madness.
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The shutoff valves will be something like Watts WPBVD-08, #0653003, from their LFWPBVD line. They're full-port, lead-free, brass units with PEX ends, probably with waste drains (winterization).
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The Check Valves will likely be Sharkbite U2008-0000. They're very simple and frankly if there's a likely leak point it's probably the place where flow stops and pressure increases when anything goes wrong aka check valves and I trust sharkbites more than other PEX stuff.
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The Strainer will be a stainless mesh, probably Shurflo #255-313, as it's included with the pump (see below). Just a simple stainless screen to protect the pump from anything in the tank. They're servicable by disassembly and backflushing or replacing the screen.
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The Pump will likely be a Shurflo #4048. Flows 4.0 GPM max (~1.475 @ 45 psi) and draws 10.4 amps stalled @ 65 PSI (but draws~8.75@ 45psi & ~1.475 GPM)
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A 5 minute navy shower @ 45 PSI with the Oxygenics shower head below would use 1.475 GPM @ 45 PSI (pump limited) or 1.5 (head limited) 5 x 1.5 = 7.5 and since I like to be conservative let's say that's 8.25 (10% cushion) gallons. 8.75 amps x 1 hour = 8.75AH and 8.75/60 minutes= 0.14583 amps/minute and .14583 x 5 minutes is .73AH and that plus a 10% cushion is .8AH per shower. A 2 gallon accumulator @ 50PSI (for instance, probably needs to be 55ish, requires testing so accumulator is above pump "off" pressure) would allow the pump to run less than 3 minutes per 5 minute shower so electrical consumption would go down but is already pretty darn negligible.
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The Accumulator might be Shurflo #181-201 as it's very popular with RVs but I think an all-metal heavier duty and larger unit has some definite benefits. The larger the accumulator tank the less often the pump runs which is good for pump durability and power efficiency not to mention noise and vibration. I also think buying a bigger accumulator would be a good way to better utilize space by allowing a smaller holding tank thanks to the Accumulator's capacity. 20 gal main tank + 2 gallon acc. = 22 gallons, etc.
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The Coolant Filter is to keep the heat exchanger working as well as possible, as long as possible. Gonna use something like a Wix #24019 housing with a #24070 on it. This a simple engine coolant filter, rated at 27 microns, with no additives or chemicals involved. Just trying to prevent sediment from clogging up the exchanger.
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The water filter is a 10" whole-house water filter. There are a few different filter cartridges for the housings: carbon, pleated polypropylene, and spun cotton. These are inexpensive and typically sold in 2 packs and last 3 months or 15,000 gallons. I think packing 3 (1 in use) should definitely last a year, especially if I use an in-line RV filter to filter any imperfect water prior to putting it in the tank.
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The shower head will likely be an Oxygenics BodySpa #26781. Flows 1.5 GPM @ 45 psi and has great reviews among the water-conscious RV crowd and the tiny-house folks.
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The shower head will likely be connected to a D&W Spray-Away which is a coiled, self-retracting hose in it's own housing. The hose is fitted with a self-sealing quick-disconnect fitting.
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The shower "faucet" won't be a faucet per-se but rather an adjustable tempering valve like a Watts LFL1170-QC-M2. It connects to both hot and cold lines for inputs and has a large center-mounted knob that adjusts temperature by mixing the two automatically. This should prevent scalding (possible, especially with the engine heated heat-exchanger) and conserve water by not forcing you to run it and adjust it till it's just right then step into the stream but rather repeat a preset temperature again and again with no fear of burning. QC means it's push-fit onto PEX (sharkbite style). The LFL is the low temp range, which has a max output of 120* vs the LF model which will go up to 160*. Above those temperatures the tempering valves flow no water at all so burns should be nearly impossible and if it's too hot (very unlikely) ya just shut the motor off (wireless remote start/kill). The burning thing is a small concern for me (I'm an adult with no kids and never managed to burn myself yet) but the heat-exchanger can get very hot when you pause the shower (water sitting in heat exchanger as opposed to flowing) and the tempering valve should prevent that from being an excuse to step out of the stream and waste water.
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Here's a pic of a similar setup, the big tan knob is the tempering valve and its tempered output is the connector with the red ring around and there's also a direct inlet for a bucket. The blue ring surrounds a cold (non-tempered) output that the system's designer lated admitted is unnecessary because when unheated water is desired either the engine is allowed to cool first or the tempering valve is turned as cold as possible (~60*).

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The kitchen faucet/sink will be a nice long-neck Delta Bar Faucet I already have which is pretty basic but is a design that's been around for decades and uses readily available cartridges (I'll pack spares). The second faucet will be in the barn-doors on the side and will likely be the D&W Hot/Cold Spray-Port. It's a hot/cold faucet with the D&W quick-disconnect fitting like the Spray-Away so it can use the Spray-Away's coiled hose or a flexible spout they also offer to make it more like a regular faucet. No sink here, just the faucet. Can always use a bowl, bucket, etc if you want a "sink".

So what say you? Where did I screw up?

 

[Mwilliamshs]

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Made a change. Moved the City inlet to allow the tank to have fewer fittings and to eliminate 1 tee fitting (3 connections) and 1 ball valve (2 connections). Also eliminated the tank drain. If I can't pump it out, it probably wouldn't drain either and I'm not drilling a hole in the van just so the tank can have a drain that I'd be crawling under the van to use anyway.

 

[Bbasso]

Looks like a well thought out plan...
But!

My question is: Do you plan on staying in one spot for long periods of time?
if so the engine heat exchanger idea might be the weakest point in your system.
Because of all the fuel to heat up water after you have been parked for a while. And not having HOT water on demand.

Gray water tank?

 

[Mwilliamshs]

... My question is: Do you plan on staying in one spot for long periods of time?

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Yes and no. Sometimes a night, sometimes a week.
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...if so the engine heat exchanger idea might be the weakest point in your system.
Because of all the fuel to heat up water after you have been parked for a while...

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Heater circuit bypasses the thermostat so it delivers hot coolant before the engine reaches operating temperature. At idle in 30* weather that was 5 minutes or so last time I was home (was actually paying attention cuz I'd heard the 300-6's were cold blooded, doesn't seem to be). Not saying it'll fully heat the interior in that time frame but the heater is functional i.e. blowing hot air. I really don't know how much gas that is...but I can estimate/BS with the best of em...
5 minute warm up plus 5 minutes per shower of (100*-ish) water at 1.5 gpm per minute (shower head limited flow). That's 10 minutes total of 6 injectors capable of flowing a maximum of 14 lbs of fuel per hour (typically rated at ~80% duty cycle) but instead they're running at minimal duty cycle (idle). Calculated here each injector is squirting 4.544 cc per minute of idle. That 4.544 x 6 injectors = 27.264 cc/min x 10 minutes = 272.64 cc for warm up and 1 shower, which divided by 3,785.41178 cc (1 gallon) = 7.2% of a gallon of gas per warm up and shower. Gas was $1.999 per gallon here today, 7.2% of which is 14.4 cents. Ten minutes of idling costs 14.4 cents and an hour of idling costs 86.3 cents.
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... And not having HOT water on demand...

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Speaking of on demand, the most common on demand water heater I've seen for small campers, vans, etc is the Propane Eccotemp L5. It's rated input is 11kw, or 37533.562 BTU/hr. It's rated output is 37,500 BTU, so it's pretty darn efficient. At 1.5 GPM (my intended shower use rate, conveniently enough) it offers a 30-35 degree rise (not the 60+ of the engine heat exchanger but not bad). A 5 minute shower with the Eccotemp (assuming it's instantly hot and 100% effective) would consume 37533.562/60 = 625.56 BTU/min x 5 minutes = 3,127.8 BTU of propane. Propane is 91,500 BTU/gal, so 5 minutes of Eccotemp L5 use = 3% of a gallon and it's $2.35 a gallon here so about 8 cents for that shower. Roughly 50% cheaper than gasoline. Stock gas tank holds 22 gallons though, and propane will total only about 8 gallons, and that'll provide cooking and heating power in addition to water heating, so I'm okay with burning easily refilled gasoline for showers, especially since I'll arrive at each campsite with a fully heated system and the plumbing scheme above allows for preheating over 20 gallons of water.
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Not to mention the other benefits of idling the engine for 10 minutes: battery charging, cabin a/c and heat, running the max-a/c (on hot to get heat in winter) and interior dehumidification, etc.
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Gray water tank?

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Nope. The only drain is on the kitchen sink and if I need to contain it I can use a container inside the cabinet then dump and clean the container outside but my intention is to just let it run out through the floor as I don't plan to use campgrounds much, if at all.

 

[Mwilliamshs]

Don't be sorry No disrespect perceived.
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Simplicity is in the eye of the beholder I guess. My planned system has very few moving parts (just the pump chamber diaphragm), no electronic controls, no thermocouples, no blowers, no burners, no body perforations, only one fuse, only 2 wires, only 1 switch, and I think it's simple. I also think it's compact (no 5 gallon tank I only use on bath nights taking up space every day).
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Besides that one basic change (your 5 gallon tank for my 3"x10"x6" heat exchanger) I'd be surprised if our systems were much different at all. Surely you have a tank, strainer, 12v pump, filter, faucets, a hose connection, fill spout...but likely more than 2 tee fittings...
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Baja and Central America are the intended destination.
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Gonna quote myself here: Looking for suggestions on the routing of the diagram moreso than the individual components.
And follow that by saying I welcome input.
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Thanks guys!

 

[westyss]

Looks like the design is good, but there is room for a more simplified system, but for starters you said you are open to comments so that opens the door for some and that means no BS number crunching to make it look better, we get enough of creative accounting in our governments.

I really dont have too much, one thing to look at would be a hot water storage unit, stores and keeps hot water at engine coolant temps and typically they are fitted with mixing taps that like you already know mix cold with hot to not scald. Otherwise if you dont go this route I really dont see the need to have the hot water line go to two taps as there will not be any hot water unless the engine is running, and I am sure you wont start, warm up and run the engine to wash some dishes in reality.

Something like this:

It also has a 110V element if you are plugged in. This way if all you want to do is wash your hands you have hot water for it, or anything else. I have a bigger one and that will store enough hot water to have two good length hot showers and still have semi hot to warm water for dish washing several days later. I think this one is 2.5 gals and pretty compact. So start the van up every two to three days to get it hot for the next few days. Your wife will like you for that!

The other item I would delete for simplicity sake would be pressurized water from city, it does add a bit of complexity and you could simply top up the water tank and keep it simple and use the pump to pressurize. This could keep the overall pressure in the lines lower than city water pressure and as a result also maybe use less water and reduce the risk of leaks a bit.

One more comment and that is the capacity, 25 gals seems like allot of water but you may find it low, water, like electric power are always it seems under estimated, having another tank might be a good idea to have the option of more water if needed but it would also provide a place to pump or fill with some less than stellar water that could be used for showers or dish washing without contaminating the "clean water tank". Now the system gets complex but I have found from my own experience that I should have done something similar with my set up and may still do that. Redundency will be increased also(good).

I found that trying to fit a bought off the shelf tank to be very cumbersome and not very practical in space saving, having a tank that will fit into a little used and hard to get to space is ideal if you can do it, and several tanks may provide that too. Or build a tank to fit into a spot with limited wasted space.

I wouldnt worry too much about the size of pressure tank too much either, these pumps now are very capable of running slower or in a pulse kind of thing and really dont consume too much power. A bigger tank is just more weight that is not usable to you unless you are out of water and you drain the expansion tank as a last resort, otherwise it does not increase usable water.
Get a run dry pump!
And I am not too sure but I think my pump acts like a check valve so maybe one can be eliminated( if no city connection used) the other check valve just before the exchanger has me a bit stumped as to why you need it, Ill look at the design a bit more and see if I can figure out why you have it there, probably is obvious but ..
You have obviously got a good handle on this and put some thought into it.

Ok thats it, fire away!

 

[westyss]

One last thing I will be doing and that is to carry a spare pump, I have not seen anywhere to buy one in Mexico and Im sure less so the further south you go, they are cheap insurance.

 

[Mwilliamshs]

Good points.
Re: capacity, I'll carry a 20L jug (over 5 gallons) of fresh, purified water separate from the storage tank. They're sold everywhere down south. There are cheap little pumps, both AA and manually operated, for them available so you're not picking up a heavy jug to pour a cup of agua.
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Re: hot water, If I find hot water in short supply I can heat the entire 20 gallon tank easily and while it's not insulated the slow heat loss of water will keep it above tepid for a long time. The tank will inside the insulated van and inside a cabinet so while it's not insulated per se it's not prone to freezing.
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Re: starting the engine for heat, The van has wireless remote start so firing it up is effortless and as I've said, has side benefits. Also, this is a gasoline rig, not diesel, with a very quiet (moreso than I expected) engine so idling is not unpleasant. I made very successful concessions on the exhaust to make it quieter and added a catalytic converter so it wouldn't stink up the campsite.
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Re: pump/city water, The pump I linked to can run dry and prime to like 6' (pretty sure that's the number). One reason for the city water connection is as a pump backup, it also allows the pump to rest when parked where it isn't needed, and I visit state parks etc and can camp like the "real" RVs this way.
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Re: check valve, The heat exchanger requires a short bit of metal pipe on each side of it to protect the PEX from the heat (a precaution, not a problem) and the check valve allows this connection (as opposed to a plain coupling, etc) and also prevents hot water, which expands, from pushing back into the cold pipe when the engine runs with the hot taps closed. If the cold loses its cold with what shall we temper? (Matt 5:13) This valve would also protect the fresh water from antifreeze contamination if the heat exchanger had an internal failure.
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Re: accumulator, The accumulator does add to system capacity and instead of thinking of its contents as that last bit of water to be used why not think of those 2-4 gallons as the first out the pipe, leaving that last bit (which otherwise would not exist) till last? That is how it really works afterall: what's in the accumulator comes out the faucet first. The pulse capabilities of pumps is what the accumulator helps avoid. Start/stop/start is the least efficient way to operate the water pump, second only to running it slower. Slower on DC = less voltage and lower volts means higher amps. The fewer pump starts the better for both efficiency and longevity, not to mention the noise.
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Re: spare pump, I do plan to buy a used pump (local RV scrapper) and replace its diaphragm and carry it as a spare. The pump will be connected with flexible rubber hoses with threaded connections (pump silencer kit) so changing a pump would be simple and since I'd carry a spare diaphragm and pressure switch anyway, substituting the entire pump adds little net bulk to that. I've used these DC water pumps a lot (grew up RV camping often, herbicide sprayers on the farm, etc) and only ever had a pressure switch fail (always on) but a spare is simple peace of mind. I'll probably even mount the spare near the main so it's out of the way and can be utilized simply and rapidly if needed.
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Re: city water (ctd), The city water connection could be used with a MacGyver'd air-over-water tank as a pump backup too. Did this with a sandblaster for over a week in Siberia once. Fill sandblaster 75% with water, heat over coals, screw on the lid, pressurize with bike pump, shower. It impressed the locals.
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Re: 2nd check valve, The check valve after the pump is to prevent its diaphragm from holding back city water pressure, which I have an adjustable regulator for (regulator doubles as check valve, shown on diagram in city line). This should extend the life of the diaphragm and allows blowing out the water lines for winterizing. This valve could be eliminated and the ball valve at the tank could just be closed when city water was connected but this adds a little redundancy (to overcome forgetfulness) and protects the pump. It also keeps the accumulator from pushing back against the pump constantly and should ease starting loads on the pump.
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Re:less than stellar water utilization, That's what the bucket use line is for. Shower with local water, then run fresh to the tap, done.
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Thanks for contributing!

 

[westyss]

yeah like I said you have a good handle on this for sure.

My pump will not run full blast all the time but will slow as pressure mounts and I want to say it pulses but that might not be the best word for it, maybe it just starts to slow and will pump at a rate equal to the outflow, I pondered going to a larger accumulator but could not seem to justify it as necessary with these new pumps that seem to not even need the accumulator but I hear you, less on/off is good.

Maybe see you down in Baja sometime, spent two months there last year, week at a time parked on a beach not moving or starting the truck at all.....thats the best.

 

[Mwilliamshs]

Maybe you have a variable speed pump? Several on the market for a few years now. Microprocessor controlled, heatsink, fancy. Not terribly efficient but pretty slick in operation. They don't pulse on-off-on like the old pressure-sensitive pumps. They're flow-sensitive and don't even need accumulators.

...Maybe see you down in Baja sometime, spent two months there last year, week at a time parked on a beach not moving or starting the truck at all.....thats the best.

that all sounds good. I know on another forum I frequent there are stickers available that let fellow members recognize their kind on the road. Expo sell them too?

 

[Mwilliamshs]

Considerable inspiration came from here. Been looking for this thread to re-read for some time and just found it. http://www.expeditionportal.com/forum/threads/47516-LATEST-PROJECT-Truck-Drawers-Sleeping-Platform

 

[Weaselblade]

Good info! Your system looks a lot like what I'm aiming to do, though I don't intend to bother with city water hookup or 2nd sink. I'm adding a diesel Espar to minimize running my diesel engine while parked, though.

A few thoughts/questions:

Am I correct that you're installing the coolant filter inline? Any concern about flow restriction, esp. as filters approach replacement time?

How is this system pre-heating 20+ gallons of water? Or did you mean that stored heat on arrival at a campsite is sufficient to heat all the water in the tanks?

I'm still on the fence as far as a variable speed pump vs accumulator goes; what convinced you in favor of the accumulator?

The part that might not be immediately apparent about the accumulator, if I'm understanding correctly: the water in the accumulator is the first out of the tap; then, when you get to the end of the tank, the accumulator will continue flowing until it's out of pressure. When you run out of pressure, you still have water in the accumulator that's not accessible until there is more water available to the pump. However, the amount trapped in the accumulator is a fraction of capacity, because the accumulator was pressurized to begin with.

 

[Mwilliamshs]

...Am I correct that you're installing the coolant filter inline? Any concern about flow restriction, esp. as filters approach replacement time?

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Yes, the coolant filter is inline with the OE heater core and the hot-water heat-exchanger. No concern about flow restriction at all as these filters (those linked) are of the bypass variety and designed to filter only a portion of passing coolant and if it ever became entirely filled it will bypass entirely, like an engine oil filter. The coolant filter is more of a precaution than anything. My coolant now is perfectly clear (just did a full drain/refill and replaced all the hoses and had the radiator flushed by a shop while it was getting repaired) and after 25 years I'd expect very little original seditment to remain so the filter will just catch whatever's left and keep the exchanger work at peak efficiency longer.
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...How is this system pre-heating 20+ gallons of water? Or did you mean that stored heat on arrival at a campsite is sufficient to heat all the water in the tanks?

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Last question first, only one tank. IF I wanted to preheat all the on-board water I'd connect the Spray-Away and put its hose in the gravity filler then turn on the water pump while the engine ran. This would pump the tank's contents through the exchanger and back into the tank. I don't expect I'll ever do that but it's entirely possible.
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...I'm still on the fence as far as a variable speed pump vs accumulator goes; what convinced you in favor of the accumulator?

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Variable speed pumps pull lots of amperage (10+) and use it inefficiently by drawing it steadily any time they run. Anything that gets hot (except a heater) or has a heat-sink is wasting electicity. Variable speed pumps have big heat sinks and routinely put off considerable amounts of heat because they're pulling 10 amps and only giving the motor as much of that as it takes to keep up with demand, burning the rest of that energy as heat. Their microprocessors do not use PWM because it would wear out the pump under high starting loads (these pumps have practically no inertia) so they burn off the excess amperage like a motorcycle voltage regulator that shunts to the frame. Flow-sensitive pumps are smooth and quiet and practically seamless so I do think they're the way of the future RV but for a boondocker or dry camper they strike a poor balance between luxury (let's face it, running water is a luxury) and efficiency.
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...The part that might not be immediately apparent about the accumulator, if I'm understanding correctly: the water in the accumulator is the first out of the tap; then, when you get to the end of the tank, the accumulator will continue flowing until it's out of pressure. When you run out of pressure, you still have water in the accumulator that's not accessible until there is more water available to the pump. However, the amount trapped in the accumulator is a fraction of capacity, because the accumulator was pressurized to begin with.

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That's it exactly. Start with a dry tank, new pump, empty accumulator and fill the tank with exactly 20 gallons. Gravity will supply water through the strainer to the pump's inlet so long as the pump is below the highest point of water and/or piping. Turn the pump on and you now have water in the tank, strainer, pump and piping up to some point between the pump and the tap depending on how much air pressure is in the accumulator/lines. The pump's built-in pressure switch does this. Air is compressible but water is not. Open a tap and water will flow into the accumulator, fill it to the pressure of its captive air, then through filter, and tap with piping in-between. That first drop out the tap came through the accumulator. If you then close the tap and fill the tank the rest of the way it will take a water volume equal to the capacity of the lines, filter, accumulator, etc and the bigger the accumulator, the greater that capacity, so the greater the capacity of the system as a whole. If you have a 20 gallon tank and an accumulator with a 2 gallon acceptance volume you have a 22 gallon water capacity, period. Now when you run out of water in the tank and there's nothing left for the pump to pump, the accumulator with still hold its acceptance volume at pressure until a tap is opened. This water can't flow backward through the pump, and it can't flow forward until a tap is opened. When a tap is opened however, the water from the accumulator will run out until the atmospheric pressure (air pushing into the tap) is equal to that within the accumulator. 1 atmosphere is about 14.7 PSI at sea level and less anywhere above that so a 45 PSI preset accumulator will empty itself about 60% or more at elevation.
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The above assumes the system in question is assembled according to the diagrams I've posted here. There are certainly ways to make things work differently just by plumbing them differently. Insert cat skinning metaphor here.

 

[Weaselblade]

So, did this get built?

I'm finally at the ordering stage on mine, wondering how it turned out if you went ahead with it.

Especially that tempering valve; to paraphrase the Watts spec sheet, it 'cannot be used for tempering water temperature at fixtures, blahblah death scalding badness, blahblahblah, use something meeting ASSE 1016/ASSE 1070.' This one is only ASSE 1017, which 'does not address final temperature control at fixtures and appliances. It is concerned with valves used at the source of hot water for distribution to the supply system, and therefore allows wider variation of the outlet temperature at higher flow rates.'

If you got one, how is it working for you?

 

[Mwilliamshs]

I've assembled most of the parts but haven't used the whole system. None of it is installed in the van. A wall/window/flooring revamp must come before the water system and I'm waiting till Spring to strip the van.

I got the tempering valve specs from another build thread and several users have vouched for it in "portable-shower-with-engine-heat" applications. The 1070 spec includes pressure balancing I believe like residential shower valves.