Water heater size, how big with Proheat, Webasto ect

[Don4107]

I have a 45K BTU Proheat.  The water tank heat exchanger will be the first thing on the output of the Proheat.  How big does the water heater need to be?  Will the Proheat keep up for extended or multiple showers if I use a small tank like 6 gallons ?  What works for you?

Thanks,

Don 4107

[Jerry Liebler]

Don,
   Most of the water heaters that have heat exchangers for systems such as your Proheat are marine units designed to capture some engine heat.  The units from Seward and Atwood  are of this type and have VERY poor heat exchange from the coolant.  These units have very little recovery rate and only transfer about 5,000 BTU/h from the coolant.  This is fine if you run all day and then stop and use a small amount of hot water.  The electric element in these tanks is also not capable of much heat transfer(also about 5,000 BTU/h).  The only solution if you need significant amounts of hot water is to add a supplemental heat exchanger between the coolant and the hot water supply either going into or out of the tank.  I have a 10 gallon Seward water heater and a large enough double wall flat plate heat exchanger in front of it.  With a little planning I can fill my bathtub with hot water (it takes 40 gallons) and not run out.  To do this I must trick the Webasto system by dialing up a thermostat so it runs and actually starts the blowers before I start filling the tub.
Regards
Jerry 4107 1120

[donnreeves]

Don, I have the 45,ooobtu Pro-Heat hooked to an 11 gallon Atwood Marine water heater and it is very ineficient. It takes about 2 hours to heat the water from 60 degrees to 130. I then installed a flat plate heat exchanger on the inlet side of the water heater to preheat the water. This didn't work very well because water had to be flowing for it to preheat. At Bruce's rally, A fellow named Walt ( sorry don't know his last name) sugjested I move the heat exchanger to the outlet side of the water heater. The results are remarkable. From cold, within 2 minutes of firing up the Pro-Heat the water is hot and gets hotter the longer it runs. After twenty minutes it peaked at 139 degrees. The Atwood is slow on electric also, but we heat it up the night before departure and it stays hot for 24 hours. Enough to take two showers the morning after a travel day. If I were to do it again, I would use a small water electric water heater from Lowes of HD and the heat exchanger. Donn

[Don4107]

Thanks guys, I had no idea the heat exchangers were that poor.  We have a Raritan (Seward) in the boat and understand how slow the electric elements are.   It seams to heat pretty quickly when the engines are running.  The one in the old bus was not set up with a boiler, only engine and electric.  Both, like Donn said, will hold heat until morning after running.

I wonder what would happen if you reversed the installation and used the boiler/engine to circulate the tank full of water and draw the hot water trough the exchanger coil?  Would eliminate short cycling and provide some thermal mass storage.  Maybe a heat exchanger to boost the output if the internal exchanger doesn't get it.  Might not need to with a low flow shower head and 185 degree water.  The shower water does not need to be full hot.  Circulate the hot water from the tank through the external exchanger, if needed, until the temp of the tank drops enough to cycle on the boiler. 

Just another redneck idea.

Don 4107