On demand domestic hot water using water to water heat exchanger?

[Fether Hardwoods]

 I would like to use a plate heater to make the hot water for my home, my question is how big of an exchanger would I need to get my domestic water from about 50 degrees to 130 degrees assuming a flow of 2 gallons a minute?
Thanks

[azmtnman]

A btu is the amount of heat it takes to raise 1 lb of water 1 degree Fahrenheit.
Water weight=8.35 lbs./gallon
You need to raise your temp 80 degrees
80x 16.7 (2 gallons) =1336 a minute or 80160 btu/hr  (the terms we typically use).
A gallon of liquid propane (LP) is roughly 96,000 btu per gallon.
Your heat exchanger will need to be sized for your water flow and the temperature of your "supply heat"--in this case, your plate heater (I'm not sure what exactly that is)--may be available flow rate/btu/hr.
 I am a redirected commercial HVAC mechanic with 20 years experience.

[Fether Hardwoods]

It's a water to water heat exchanger, using the stove to heat my domestic hot water. I can get a 125000 btu unit or 225000 btu unit, I am thinking of getting the 225 to be safe.
Thanks for your help

[azmtnman]

The mfg should have all the information you need. I know plenty of people who have heated their hot water with their wood boilers.
One thing about choosing a size--if you get way too big, the unit will not run "full throttle" enough to burn clean and it will soot up a lot.
 

[jfaulring]

I have one of these on my setup:  http://www.supplyhouse.com/FlatPlate-FG5X12-8-8-plate-3-4-Thread-20-GPM-Heat-Exchanger-5-x-12

Mine's sitting as a "side arm" on the electric hot-water tank that we use in the off season when the OWB is't being fired. It's sitting as the first item on the loop from the boiler being fed by by a Taco 009 pump. I purposely oversized it a bit to compensate from the inevitable mineral build-up on the potable side of the exchanger from our well water. I usually end up needing to de-scale the exchanger once a season to get things back to maximum flow again; I built in some isolation valved and tap points where I can re-circulate some white vinegar with an external pump through the exchanger without needing to pull it all apart. This setup keeps the domestic tank up at about 180 degrees (which prevents any bacteria growth in the domestic hot water tank) and there's a mixer valve at it's exit to bring the temp back down to 130ish to feed the house.

[red oaks lumber]

i have a 11 plate exchanger for my domestic water

[upsnake]

I have a ten plate hx.
It works good. It will take the well water ~50 degrees up to about 130.
I have my setup going from the plate to the water heater, and use the water heater as a storage buffering tank.

I can run a shower and washing machine or two showers etc and keep the temps up over 120.

This is where I got mine from.
http://www.ebay.com/itm/Stainless-Steel-Copper-Brazed-Heat-Exchanger-Mounting-Studs-Water-Boiler-Radiant-/351148034715?pt=LH_DefaultDomain_0&var=&hash=item906f6f376f


Jfaulring - do you have any pictures of your setup. I am going to need to clean mine at some point it has been in service for 4 years.
I put unions on the copper so I can take it out, but something to just run vinegar through would be nice.

[r.man]

Two gallons a minute sounds low, where did that number come from?  I have never heard of anyone doing on demand with a water to water exchanger.