Domestic Electric Water Heater as back up for Wood Boiler.

[Biglurr54]

I have an Outdoor Wood boiler with an oil furnace as a back up. So far I ran the furnace once last year to make sure it worked when I closed on the house. Since then I have been able to do all my heating with the OWB and a wood stove. Last month I built a side arm for my electric water heater. I now have great hot water and the breaker has been turned off to the heater since it was installed. I got to thinking about what would happen if i go away on vacation and the fire goes out. Typically I have people come load the boiler every other day. I was thinking that I might be able to let the water heater take over. The Boiler water goes from the Boiler through a taco pump up to the house and straight into the side arm on the water heater. It then goes to a plate exchanger which is hooked up to the oil furnace then back to the boiler. This circs 24/7. The side arm works off of a thermosiphon. My question is when the boiler water drops below the temp of the water heater water will the thermosiphon reverse. With the water heater breaker on and the thermostat on the heater turned up I could get 140 degree water from the water heater. Will this maintain the boiler to keep it from freezing when I'm away on Vaca? I'm just looking to keep pipe from freezing in the house and keep the boiler from freezing.

 

[taskswap]

I have an Outdoor Wood boiler with an oil furnace as a back up. So far I ran the furnace once last year to make sure it worked when I closed on the house. Since then I have been able to do all my heating with the OWB and a wood stove. Last month I built a side arm for my electric water heater. I now have great hot water and the breaker has been turned off to the heater since it was installed. I got to thinking about what would happen if i go away on vacation and the fire goes out. Typically I have people come load the boiler every other day. I was thinking that I might be able to let the water heater take over. The Boiler water goes from the Boiler through a taco pump up to the house and straight into the side arm on the water heater. It then goes to a plate exchanger which is hooked up to the oil furnace then back to the boiler. This circs 24/7. The side arm works off of a thermosiphon. My question is when the boiler water drops below the temp of the water heater water will the thermosiphon reverse. With the water heater breaker on and the thermostat on the heater turned up I could get 140 degree water from the water heater. Will this maintain the boiler to keep it from freezing when I'm away on Vaca? I'm just looking to keep pipe from freezing in the house and keep the boiler from freezing.

The thermosiphon - "it depends." That CAN work automatically but it depends a lot on your exact plumbing configuration and it's not a given. That said, it would be easy enough to add a circulator on the pipe that's triggered by either a cheap aquastat relay or just a relay on the water heater's coil itself.

This will work and I know two people who've done it. It is NOT efficient. DHW systems are designed to run at much lower temperatures than house heaters/boilers. Baseboards, for instance, are most efficient at around 180F. Most water heaters (like yours) are designed to run around 140F. But it's not just temp - they don't produce enough BTUs (most are on a 20A or similar circuit) to really heat the house. That is, they won't be able to keep that water 140F as it circulates through all the baseboards. But they'll definitely keep the pipes from freezing, and you'll probably come home to a 50F-60F house rather than a 20F house. For some people, that's all they want. Obviously you don't want it doing this much (emergencies only). It's going to be expensive as all get-out.

The nice thing about having a tank like this is if, some day, you want to add solar, you can plumb it to the same spot.

 

[Biglurr54]

well thats the plan to go solar in a few years. NY has great incentives to add solar. My father just put in a 7kw system and he got a little over 1/2 his money back through incentives. In 7 years at his current usage the system will have paid for itself and he will have free electric from then on. It really makes a ton of sense if you can foot the large initial bill. currently the system is thermosiphoning to heat the domestic hot water. If the heat is then applied to the other side I don't see why it wouldn't cycle in reverse. This summer i am putting in all radiant floor heat so the 180 temps won't apply any more. The only issue is if the heater can't heat the water fast enough to recover 9 (low BTU). I have extremely low electric rates right now so for a week away it would cost more to buy my buddy a case of beer then to have the water heater on the entire time.