SHW & Existing Oil-fired Boiler w/storage tank

2 Posts
May 13, 2010 12:39 pm
SHW & Existing Oil-fired Boiler w/storage tank

I'm looking at SHW. I've got an existing 3 year old oil-fired boiler that provides radiant heat and domestic hot water. The domestic hot water is stored in a 40gal tank. (We use very little water each day - typically 50-100gal/day total - hot & cold for the 3 of us).

What I'm wondering about is the configuration of a SHW system. I'd like to have our cold water line run into the SHW tank, then have the outflowing hot water go into the existing 40 gal tank, then out to the supply lines to faucets, etc. I am not interested in electric backup on the SHW tank. In this reasonable? Thanks.
 
3 Posts
May 13, 2010 01:18 pm
Re: SHW & Existing Oil-fired Boiler w/storage tank

What you are describing is a very efficient and common setup to interface with an existing DHW or space heating system. The link below will take you to a possible system setup that could be plumbed to your 40g tank. The square footage of collector that would be required depends on your location/solar resource.

http://www.altestore.com/store/Solar-Water-Heaters/Climate-freezes-Closed-Loop-Systems/Closed-Loop-Systems-for-1-4-People/Closed-Loop-AC-Powered-w-Tank/Indirect-w-80-Gal-Tank-52-SqFt-Collectors/p8111/

The storage tank does have a 4500W back-up heating element in the top portion of the tank, but you can always leave it disconnected to avoid using it for supplementary heat.

Joey - AltE
 
2 Posts
May 13, 2010 01:39 pm
Re: SHW & Existing Oil-fired Boiler w/storage tank

Thanks. We're in Vermont with an unobstructed true southern exposure. Looking at a PV pumped system.
 
3 Posts
May 13, 2010 01:50 pm
Re: SHW & Existing Oil-fired Boiler w/storage tank

An 80g tank in Vermont needs about 1sqft of collector per gallon of tank. The link below is for an "IPV-80-80" system that uses a solar module and DC pump for controls and circulation. You might want to bump up to a 20W module in Northern areas instead of the 10W that comes with the package.

http://www.altestore.com/store/Solar-Water-Heaters/Climate-freezes-Closed-Loop-Systems/Closed-Loop-Systems-for-1-4-People/Closed-Loop-PV-Powered-w-Tank/AET-PV-w-80-Gal-Tank-80-SqFt-Collectors/p2325/

Joey - AltE
 
28 Posts
May 16, 2010 04:58 pm
Re: SHW & Existing Oil-fired Boiler w/storage tank

I am over in Northern NH (RT2) and I agree that a 20 watt panel is too small to run the PV pump. The 20 watt panel that came with AET kit would only run the pump about 3 hours per day and the flid got real hot when it wasnt running. I had an old 48 watts panel that has some degradation and it works fine. A 30 watt panel would probably work but they seem to be rare.   The other thing to look at is a DC pump controller. There are times in the morning and evening where even though the sun is out and the pump wants to run that the water being circulated is colder than the storage tank, and therefore it cools down the storage. I installed an Art Tech controller after the original installation and it is set up so that the pump only runs when the water in the collector is warmer than the SHW tank.

I have the same plumbing setup but do have a bypass around the oil fired storage tank for summer when I normally shut down the furnace. I also have a thermal switch on the output of my SHW tank that locks out the oil fired storage tank when the SHW is hot enough during spring and fall. Unfortunately that means that without a bypass, the nice hot water from the SHW tank has to go through the now cold Oil heated storage tank unless I have a bypass. The alternative is to install a recirculation pump between the tanks, but that is more complication. I have the parts but not the motivation to automate the bypass, but currently I just go turn two valves.

 
462 Posts
Jul 14, 2010 07:30 pm
Re: SHW & Existing Oil-fired Boiler w/storage tank

If you plan on using a PV powered pump, I would suggest adding a charge controller and battery. Hot water will be made any time the collector water is hotter than the storage water. This can even happen at night. Solar radiation is always there. So  having enough power to run the pump will not be a problem if there is battery storage. Use the Pv to charge the battery instead of the pump directly. A differential controller is always best and should be considered. Otherwise, as it seems you already know, there may be times when heat is available but enough direct sunlight to power a pump is not...
 
Aug 22, 2010 03:26 am
Re: SHW & Existing Oil-fired Boiler w/storage tank

Reprinted


I'm looking at SHW. I've got an existing 3 year old oil-fired boiler that provides radiant heat and domestic hot water. The domestic hot water is stored in a 40gal tank. (We use very little water each day - typically 50-100gal/day total - hot & cold for the 3 of us).

What I'm wondering about is the configuration of a SHW system. I'd like to have our cold water line run into the SHW tank, then have the outflowing hot water go into the existing 40 gal tank, then out to the supply lines to faucets, etc. I am not interested in electric backup on the SHW tank. In this reasonable? Thanks.
 
462 Posts
Aug 22, 2010 11:12 pm
Re: SHW & Existing Oil-fired Boiler w/storage tank

Amy, you got it. But also consider adding a  bypass so you can use solar hot water only and turn the furnace off if you can when you can.
 I usually set it up so you can use either solar only, backup only or solar preheating backup as you suggest. I also incorporate an electric bypass using an aquastat so that whenever the solar hot water is hot enough, it turns the power to the boiler off keeping it from running just to maintain temperature.
  Adding a small electric tank can also save you money. Then you can turn the boiler off until you need heating saving on fuel, and wear and tear, that way. then the electric tank will stay off if the incoming solar hot water is hot enough.
 

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