Domestic Water Tank/Sidearm exchanger

[trumpeterb]

The past couple of days, it has been pretty cold in Western PA. I worked two 16 hour days in a row, and both days my Eko 40 was not reloaded until I got home, and it had burned out. The interesting thing is that my house was still warm, despite the fuel error message on the boiler. The pump on the boiler had shut off, but there is a secondary cirulation pump on the heating loop that still works when a thermostat calls for heat, and the boiler burning out has no effect on this pump. I was wondering why the house was still so warm even though the fire was out, and I got to thinking...the main loop goes through a sidearm heat exchanger on my domestic water tank. I wondered if the secondary circulator was pushing enough water through the main loop, boiler, and pex that the heated water in the domestic tank would actually heat the water in the heating loops, thus keeping the house warm. In this case, the sidearm exchanger meant to heat the domestic water would actually be working in reverse, and the domestic water would almost be acting as thermal storage of sorts. Does anyone think I am crazy, or is this possible?

 

[rwh442]

Very possible.

When I first started my system up last year I ran the boiler cold for a day to flush out the lines. Water temps were in the 60's. The next day i think they were in the 80's or 90's. The sidearm on my DHW heater was backheating the boiler. Shut off the sidearm valve on the DHW and that stopped.

I control a valve on the DHW to open/shut around 140 degree boiler supply to stop the backflow. If I don't my boiler can/does take back the heat from the DHW when it runs cooler after a long day.

 

[maplewoodshelby]

This has given me a thought and question. My DHW is running thru a coil on my wood boiler to 2 electric water heaters. I currently have the electric turned off and heating water with boiler. I'm heading out of town for one night and temps are in the teens. If I turn my electric water heater back on it will heat my boiler water once the fire goes out. I dont think anything would freeze with just one night of no heat but I would hate for that to happen so just a thought. My question is IS it safe because the electric water heater element will probably run the entire time I am gone. Is this detrimental to the water heater.

 

[Gooserider]

The past couple of days, it has been pretty cold in Western PA. I worked two 16 hour days in a row, and both days my Eko 40 was not reloaded until I got home, and it had burned out. The interesting thing is that my house was still warm, despite the fuel error message on the boiler. The pump on the boiler had shut off, but there is a secondary cirulation pump on the heating loop that still works when a thermostat calls for heat, and the boiler burning out has no effect on this pump. I was wondering why the house was still so warm even though the fire was out, and I got to thinking...the main loop goes through a sidearm heat exchanger on my domestic water tank. I wondered if the secondary circulator was pushing enough water through the main loop, boiler, and pex that the heated water in the domestic tank would actually heat the water in the heating loops, thus keeping the house warm. In this case, the sidearm exchanger meant to heat the domestic water would actually be working in reverse, and the domestic water would almost be acting as thermal storage of sorts. Does anyone think I am crazy, or is this possible?

No, you aren't crazy, as heat exchangers will transfer from hotter to colder, whichever way that is, so you WILL move heat from the DHW tank into a cooler heating system unless you take steps in the system design to prevent it (assuming that you do...)

In terms of the other question about reverse heating the boiler system with the electric water heaters - yes it will work. Assuming that the electric side is in good shape, the only harm SHOULD be to your electric bill - I don't believe there are any duty-cycle limits on an electric DHW tank.

However it could get quite expensive in a hurry, so I would strongly suggest attempting to design the controls so that you only circulate enough to prevent freezing, and not try to keep the system at full operating temperature...

Gooserider

 

[ewdudley]

...

In terms of the other question about reverse heating the boiler system with the electric water heaters - yes it will work. Assuming that the electric side is in good shape, the only harm SHOULD be to your electric bill - I don't believe there are any duty-cycle limits on an electric DHW tank.

However it could get quite expensive in a hurry, so I would strongly suggest attempting to design the controls so that you only circulate enough to prevent freezing, and not try to keep the system at full operating temperature...

Gooserider

Not SO expensive really, assuming 120 milli-dollars per kW hour, and 85% efficiency oil heat with oil at 140000 btu / gallon, we have:

You have:  120 milli dollar / kW hr
You want: dollar /  (0.85 * (140000 btu))
    $4.1850549

So with oil at three dollars a gallon you're looking at a 40% premium, no big deal for occasional backup use if it avoids the CAPITAL expense of another boiler.

--ewd

 

[Gooserider]

I'm working it from the other direction - CB said he had two heaters, and he expected them to be on pretty steady... Not sure just what a heater draws, as I've always avoided places w/ electric heaters since I learned about electric bills, but conservatively figuring 6KW each (Since they are usually on 30A 220V circuits, and knowing that 1500W is the standard for a 15A 110V circuit) that's 12KWh / hour X 24 = 288 KWh / day. At 0.15 / KWh, that's $43.20 / day worth of electric bill - not what I'd call an inexpensive backup when compared to the normal cost of heating w/ wood.... Even if my guess is off by a good bit, I think it still suggests an expensive backup, worth seeing if one can cut it down.

Agreed it's probably less than the expense of a backup boiler, but I don't know what CB might already have as an existing backup, that might be a lot less expensive to run. The key point I was trying to make is that it would probably be worth it to try and keep the heaters from doing more than providing freeze protection...

Gooserider

 

[ewdudley]

I'm working it from the other direction - CB said he had two heaters, and he expected them to be on pretty steady... Not sure just what a heater draws, as I've always avoided places w/ electric heaters since I learned about electric bills, but conservatively figuring 6KW each (Since they are usually on 30A 220V circuits, and knowing that 1500W is the standard for a 15A 110V circuit) that's 12KWh / hour X 24 = 288 KWh / day. At 0.15 / KWh, that's $43.20 / day worth of electric bill - not what I'd call an inexpensive backup when compared to the normal cost of heating w/ wood.... Even if my guess is off by a good bit, I think it still suggests an expensive backup, worth seeing if one can cut it down.

Agreed it's probably less than the expense of a backup boiler, but I don't know what CB might already have as an existing backup, that might be a lot less expensive to run. The key point I was trying to make is that it would probably be worth it to try and keep the heaters from doing more than providing freeze protection...
Gooserider

Is electricity expensive? Yes. For me it's about 115% the cost of oil, which is itself expensive.

Do I advise consuming mass quantities of expensive stuff? No.

But even at $0.15 / kWh, electricity is comparable to oil at $5.25 / gallon, which is hardly a catastrophic expense if all you're doing is trying to keep the pipes from freezing. Me, I'm looking at electricity at $0.10 / kWh, which works out to $3.50 / gallon oil, even less cause for alarm. A person can most definitely just lower the thermostat a much as he dares, go enjoy a nice vacation in Costa Rica, and not worry about it.

The mind boggles in speculating how you came up with $43.20 / day.

I do recall however that back when I earned my BSA Electricity Merit Badge my tutor stressed over and over that correct analysis of any problem required dimensionally consistent equations, which you lack, so that's a good place to start.

What you might try is to figure out how many btu per hour it will take to keep the pipes from freezing, convert that to kW, then multiply that times dollars per kWh to get dollars per hour, then by hours per day to get dollars per day.

--ewd

 

[101x81]

what about install a small grunfos circ. with a check valve on it and wired to the aquastat on the boiler so the pump run only when the boiler is hot and the check valve will stop the dwh migrating to the boiler when he is cold.
btw your electricity is very expensive, here in quebec it is 0,07 the kw/hr

 

[maplewoodshelby]

good discussion. I am still in the install process of my entire heating system so I'm kind of chained to the house this winter. I was just looking at using the electric water heater for one night to keep my system above freezing. Not to produce any meaningful heat. I have burnham gas boiler for backup but chimney issues have kept it off until spring when I can remedy the situation. We are at .9 cent per kwh here in WV but ti would still get expensive

 

[catnfool]

new guy here- I have been lurking quite a while!! I have 1 electric 55 gal 93% efficient dwh that has been heating my basement slab for 2 months while I finish the upstairs. My elect. bill was $160 more, but that was at -14*f for about a week, and we did not have a fire in the wood stove/2 electric bb heaters on for 10 days of that month. I plan on using my electric water heater as a backup for when we have to leave for a while.

 

[101x81]

what about install a small grunfos circ. with a check valve on it and wired to the aquastat on the boiler so the pump run only when the boiler is hot and the check valve will stop the dwh migrating to the boiler when he is cold.
btw your electricity is very expensive, here in quebec it is 0,07 the kw/hr

for security you may need to install a ranco sensor on the dwh to stop the pump when the water in the dwh is too hot..