control a side arm heater

[Jim Post]

Is there a way to control or repipe my sidearm so that it only raises my dhw heater tank to 130 degrees or so...it seems like a waste of wood to let my sidearm heat my electric water heater tank to 180 degrees and then use a tempering valve to get it down to 120 for use. Wouldn't it be better to not heat the tank so hot to begin with?

 

[MarcM]

Is there a way to control or repipe my sidearm so that it only raises my dhw heater tank to 130 degrees or so...it seems like a waste of wood to let my sidearm heat my electric water heater tank to 180 degrees and then use a tempering valve to get it down to 120 for use. Wouldn't it be better to not heat the tank so hot to begin with?

Ways to reduce heat transfer in a coxial opposing flow heat exchanger:

Shorten the length
Increase the volumetric flowrate of the cold fluid
Decrease the volumetric flowrate of the hot fluid

The only loss you get from heating your DHW above the temperature at which it is used are the stand by losses... if you take the appropriate steps to thoroughly insulate and minimize stand by losses from your DHW tank, I wouldn't concern myself with it much...

 

[Eric Johnson]

I think you could connect a 3/4-inch zone valve to the inner (DHW) tube and power it with the water heater's thermostat. The idea being that when the thermostat calls for heat, it opens the zone valve and allows the convection to begin. I'm not sure if there would be enough time before it closed to heat your water adequately, but it's something to consider.

But basically, I'm with MarcM. The more heat you can stash in your hot water heater, the longer it's going to provide hot water, even after your boiler cools down and quits circulating water through the sidearm. Think of it as auxiliary hot water storage--it works both ways when your pump is on, so that when the water in the hot water heater exceeds that of the boiler (happens), it's actually giving heat back to the system, allowing it to maintain a more even temp.

 

[Tony H]

I thought the same thing only in reverse ... what if the weather is moderate so the boilers not running much and the wife goes to take a shower will the hot water hold out ? Well if its up to 170 + and being mixed back to 120 thats alot of available water and plus since I have no other storage it acts as a little storage tank being it has more water (40gal) that the whole rest of the system it helpls keep the temp up a bit so when the heat kicks on it has some hot water ready to supply the forced air heAt exchanger until the boiler gets pumping again, this is all projected operation from others experiences as my system is still under construction. Like Eric says a zone controller could work and if you were to connect it to an outdoor therm so it only restricts heating when the boiler is running often, or if you are running off storage an aquastat to only heat to the temp you desire to meet your need.