Design: RV Recirculated Buffer Tank with Heat Exchanger

Drewfrd

Edit: apologies, is this on the jobs form?...

Hello,
I am prototyping a hot water system for my RV called home. When I get this working well I plan to open source the design/products/tools/code. I am here to ask if I have the correct setup ie, expansion tank, pump placement, check valves, psi etc. Please check out my diagram and pop any flares you see, it would mean a lot to me.

The idea is to turn the cold-in and hot-out into a heat exchanger loop. For the new hot-out I inserted a tee before the T&P valve which I hope will act as a makeshift hot-out line. This is an electric hot water tank, however consider it unplugged.

After reading Pumping Away, I placed the feed from the fresh water tank next to the expansion tank.

Questions I'm asking myself:
Will the fresh water feed line stay cold? Maybe I need a check valve.
55 PSI seems high, is this an okay pump? Should I regulate the psi with a valve?
Do I need a check valve on each side of the mixing valve?
How serviceable is this? Do I need a boiler drain valve?
What happens if I run out of fresh water? Will the air in the expansion tank expand and cave in the tank?
Where should I place temp/pressure guage(s)?

Thank you,
Drew

Zman

What exactly are your design goals?
Is the RV cooling system providing the heat? Is this intended to run off-grid or plugged in?
At a glance, this looks overly complex. The T&P should dump to the atmosphere. A small reverse indirect would simplify this considerably..

Ctoilman

Omg....it's a science experiment gone awry!!  

Drewfrd

@Zman Thanks for your response. This is intended to be off-grid. The glycol heat exchanger will be heated by either the van running or by an Espar S3 Hydronic B5E CS 12V (Gasoline) Furnace that will be mounted under the van which is supplied by a glycol expansion tank. The design goal is to store hot water when the furnace/engine is running in the 4 gallon tank. I can dump the T&P into a plastic bin on the floor instead of my vented water tank, would that be better because it would alert me to a pressure problem?

Drewfrd

@Ctoilman how do you know its awry?

Zman

I spend quite a bit of time tinkering with RV's.
If you time your showers after you have driven the rig, you could use all of the heat stored in the engine and coolant to heat your shower water. 4 gallons is not very much storage when you consider that you will harvest less than 3 gallons if you are not recovering at the same time. I think I would lose the storage tank, mixing valve, expansion tank, air sep, and ECM pump and go with just the heat exchanger using the vehicle to pump the coolant on the supply side. It would be an instantaneous hot water system using the reject heat from the rig.

Air elimination is not a big deal in RV domestic systems, it just burbs out. The typical 12-volt RV water pump at the tank is designed to handle the small amount of expansion you will get, it has a little bladder inside to buffer the pressure. The mixing valve should probably stay if you have young kids using the water, otherwise, I would lose it, it will make winterizing a PITA. A T&P valve is generally a good idea in case something goes wrong. You will know if it is leaking because your pump will cycle occasionally and drive you nuts. My guess is that if your system gets anywhere near 150 PSI or 210 degrees, something will blow before the T&P

Drewfrd

@Zman You have pinned my inner conflict about this very well. I've seen systems like the one you are describing. Its interesting to me that a simple idea of, "Hey, I'd like to store this hot water temporarily" explodes into complicatedness including the price tag. I've got to consider how much I like tinkering with RV's . Thanks for your words of wisdom.