electric hot water heater, sidearm on steriods - would this work?

[MrEd]

I had a thought, and in my mind it works, but before I waste my time and money I thought I'd run it by the woodboiler brain trust. I heat exclusively with my tarm, DHW currently comes off a 40G electric tank until I have time to add a superstor...now that most of my tarm is ready to go, I want to address the DHW side of my setup before winter.

I had thought about adding a 40/60 gallon superstor, and then possibly an on demand electric for when the wood isn't running (not sure if I am going to want to burn all summer).

Since a new superstor tank, and then a backup source (instant water heater) will end up costing me about $1500 when all is said and done, I was thinking about cheaper options...

A sidearm, as is often discussed here, would probably be of some help in cutting down the electric use/cost, how much I have no idea (anyone? will a sidearm actually heat a cold tank of water in reasonable amount of time? My guess is no), but what if I did the following...

Instead of adding a passive sidearm that depends on gravity flow to scavenge heat, what if I added a souped up 'sidearm/heat echanger' as follows: I would purchase a small flat-plate heat exchanger (about $100 on ebay), add it to the electric hotwater tank in the same way that a sidearm might be with DHW on one side and the boiler water on the other, use an available zone/zone valve on my distribution that would open whenever the tank dropped in temperature, and then use a small potable water pump to force circulation of the DHW thru the flat plat on one side (and the regular circs would push it through the other). Wouldn't this in effect give me a superstor tank, with an external heat exchanger, with the added benefit of having the electric kick on automatically when the woodboiler is cold?

The beauty of all of this is I already have an extra zone (with valve) ready to go because it was to be used for the superstor anyway, and two extra taco potable pumps as well...so my total cost will be about $150. Seems to easy, and too cheap...what am I missing? If this works, I may even pickup a bigger (used) DHW tank.

 

[kabbott]

The biggest drawback to a plate exchanger is the need for another pump suitable for the potable water, Which you already have.
Advantage to the superstore is no pump needed, less power to run.
It should work fine, The time needed to raise the DHW temp will be proportional to the size of the plate exchanger but should not
need to be very big.

Pretty sure this has been discussed a few times and I believe some have even had acceptable results with an iron pump.

My sidearm keeps up fine with family of four when it's cold and I burn everyday. My boiler output flows through the sidearm all the
time so the water does reach high temps sometimes. Mixing valve or temp controlled fixtures recommended.

 

[MrEd]

Are you saying your sidearm provides pretty much all the needed heat? I would have thought such a passive device would only help keep the water upto temp, or raise it slightly...if it really can transfer enough heat "most" of the time, that maybe the way to go.

 

[kabbott]

Yes. When burning at least a full load every day my sidearm keeps up fine. No electric used, I actually turn it off when winter arrives in force.
It usually See's four showers in a row in the evening, This is also when I am firing the boiler so not only do I have 40 gallons stored it heats as
the water is used.

One thing I should mention is that my sidearm is tee'd into the hot outlet on the water tank with the bottom connection in place of the bottom
element. When using water some flow is forced through sidearm, I believe this helps raise it's output some. Downside is on days with no fire
some flow through sidearm lowers overall outlet temp by mixing cold water from the bottom of the tank. In the summer I shut a ball valve on
the sidearm-tank pipe.

My sidearm is more than just a tube in a tube so output is a little better, How much is up for debate. See this thread.
https://www.hearth.com/talk/threads/26838/

 

[Mid Michigan]

My set up is uses a plate heat exchanger. The OWB circ. pump is always running so that side of exchanger is always hot. I put a small circ. pump on the domestic side to recirculate water from farthest point of DHW system back to the tank. It goes through the heat exchanger on its way back to tank. I even have the cold water make up going through heat exchanger and than into tank. I put a aquastat with a capillary tube down into my DHW tank. I took the anode out and used that opening for a well I made out of copper. Screwed in down into tank and inserted aquastat. I never run out of hot water. I could leave it running all day and it would be hot. It is nice though to know the aquastat is shutting down circ. pump when up to temp. Also it keeps my insulated pipes hot so I don't have to wait for hot water at the farthest bathroom. To be fool proof I should add another aquastat to the return line as the line will cool faster than the tank.

 

[dirttracker]

If you are going to use a plate HX, why not just skip the storage tank and put it in parallel with your electric tankless? I have my DHW set up this way only with an LP tankless. I believe I have am using a 10 plate HX and it is sufficient to make comfortable DHW with my storage tank down to ~130 F. The level of "comfortable" is a relative opinion, I have not measured the DHW temperature to have exact numbers. My setup has the plate HX plumbed in counterflow with a tempering valve on the DHW output. The HX/tempering halve in in parallel with an LP tankless water heater. I have ball valves on the inlet to each and switch over when necessary. In the summer I run the boiler ever couple of days to heat storage and use this for DHW. Occasionally I run the tankless if I don't have time to build a fire.

Originally I had the plate HX on the inlet to my 40 gal LP water heater. I ran this setup for 5 years with my OWB and every year I would shut off the LP to the water heater when I fired up the OWB. The plate HX was just plumbed in series on the inlet to the water heater, no circulating pumps or attempts to create natural convection through the plate. With this setup, I never had a problem with hot water availability. When I put in my Tarm, I took the old LP water heater to use as an expansion tank and went with the tankless setup to conserve space.

Just my 2c since you were thinking of buying a plate HX anyway.

Eric