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Author Topic: Need help with backup system  (Read 166 times)

Jon_E

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Need help with backup system
« on: March 26, 2018, 09:21:24 AM »

Hay all, I need some help with system design.  For the past 13 years I have been heating my house and producing domestic hot water with my outdoor wood boiler.  My old boiler had a propane burner in it, so when I didn't want to burn wood (or ran out), I would switch to propane.  The current G200 boiler does not have a propane system, so I have no backup.  At all.  I have no system in my house, such as a small propane boiler or electric water heater, that will provide me with hot water during the summer or even a backup in case of a major system failure or an extended vacation. 

So, this spring I have saved up some money and I want to install a heat-pump hybrid electric water heater.  I want it to serve two functions: First, to serve as primary domestic hot water from mid-April through early September.  The OWB would be shut off completely.  Second, to act as a backup if I have a system failure in the OWB, or I want to be away from my house for a few days in the winter and just need to maintain a minimum heat level.   I want to be able to manually switch modes (if necessary) between seasons, and manually switch to backup in case of a failure or vacation.  I have a generator to provide backup power so using electric backup is not a concern. 

Currently, I have a 40-gallon indirect water tank, which is fed from the primary OWB loop in the basement, has its' own small circulator pump loop, and the hot water flows through a mixing valve to domestic use.  The rest of the OWB loop is for heating.  Compounding matters, the indirect tank has developed pinhole leaks and is out of warranty.  I want to replace the indirect tank with the electric water heater.  I do not have room for both, so I need the electric heater to function as an indirect tank during the winter months when the outdoor boiler is running, and function as primary heat for domestic hot water during the summer.  I only need the electric heater to provide minimal heating if a backup is needed during the winter, and I would like to accomplish that with some sort of valved bypass. 

So, does anyone have any photos of systems that utilize both an electric water heater and an outdoor wood boiler, or have some sketches, or barring that, describe to me what I need to do?  I am on a very limited budget, and have some reasonable plumbing skills, so as long as I have an actual plan, I can put it together. 
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RSI

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Re: Need help with backup system
« Reply #1 on: March 26, 2018, 09:32:45 AM »

Where is the cold air going to go? If inside the house then it won't work at all for space heat in the winter. If outside, it won't do much when the temp is really low.

You can do either a sidearm or plate heat exchanger with the electric water heater to heat DHW with the OWB.

If you have a need for cooling in the house, you may want to setup the heatpump so it heats the water in the OWB instead of the DHW directly or set it up so it transfers heat back to the OWB from the domestic if you want to keep it cooling. You would probably need to use the electric water heater to get it up to a hot enough temp doing that though.

Another thing to consider is what water temps you can feed to the heatpump. As the temperature of the water going into a heat pump increases, so does the cost per BTU. Same applies to air temp except in reverse.

If you could get the water to stratify and pull cold from the bottom into the heatpump, it will cost a lot less to heat than if you are running warmer water into the heatpump.
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E Yoder

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Re: Need help with backup system
« Reply #2 on: March 26, 2018, 11:15:35 AM »

In this picture the red line coming out of the flat plate heading down feeds into the cold outlet on the water heater. Preheats all domestic water. Electric won't come on unless the OWB temp drops. This is a job from long ago but the basic method hasn't changed for me .
The heat pump water heaters I've seen cool the house to heat the domestic water. So like RSI mentioned I'm not sure how that could be used to heat the house unless you ran it on the electric backup.
« Last Edit: March 26, 2018, 03:33:32 PM by E Yoder »
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E Yoder

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Re: Need help with backup system
« Reply #3 on: March 26, 2018, 11:20:53 AM »

How do you heat the house off the OWB? Copper baseboard? Radiant? Maybe you mentioned that somewhere. To heat the house with the electric water heater you'd need to pump it back through the flat plate to heat the boiler water with a stainless or bronze pump... Not sure how practical that would be. A water heater doesn't produce a lot of btu's, but it might work.
« Last Edit: March 26, 2018, 03:29:51 PM by E Yoder »
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Jon_E

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Re: Need help with backup system
« Reply #4 on: March 27, 2018, 06:41:46 PM »

All my heating is radiant tubing, so the heat pump would be set to electric mode only if used to provide heat in the winter.  This would be pretty rare and only occur for a couple of days at most in the winter months.

The heat pump style water heater will be a bonus in the summer, because it will cool the space it's in (my basement) while providing domestic hot water.  Obviously no need for heating demand in the summertime.

I have been inquiring other places online and found that using another plate exchanger to heat the cold water inlet is the simple way to do it.  I might have to install a manual bypass, check valve and circulator pump into the system for those few days in the winter that I would need heat.  And it wouldn't need to be a lot of heat either, just enough to keep pipes from freezing. 
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E Yoder

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Re: Need help with backup system
« Reply #5 on: March 27, 2018, 07:53:49 PM »

 :thumbup:

Makes sense. It should work with the right piping, one element is about 15,000 btu's so not a major output for sure.
Never done it myself tho, most folks have an existing boiler or forced air system.
Routing some or all of the incoming OWB water through a second electric water heater would work too. If you have space.
« Last Edit: March 27, 2018, 08:11:37 PM by E Yoder »
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