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# GARN with solar

Member Posts: 50
I am going to hook up 6-4x8 solar panels to a GARN 1500 (outdoor wood heating system storing 1500 US gallons). Looking for opinions whether to go with a drainback system and no heat exchanger issues or closed loop system with anti-freeze and a heat exchanger. I am located at the 45th parallel in central Ontario.

• Member Posts: 20,725

you would want 1- 1.25 gallons of storage per square foot of collector. So around 180- 200 gallons.

With that much water, even at outdoor temperature I doubt you will see much temperature rise.

You can calculate that if you have the test data from your collectors and knowing how much actual volume you have in the Garn, piping and collectors. I'd guess those collectors might average 30,000 BTU/ day each at DHW temperatures, SRCC category C.

So the load is

8.33 X gallons (temperature hot-temperature cold)

8.33 x 1000 gallons (120- 70)= 416,500 BTU, not adding any losses.

Also you want to store thermal energy in a very well insulated tank if possible.

A boiler with a flue connected, as your storage, will be sucking away energy 24/7. What temperature you gain while the sun is up, may vanish before you get to it.

You may spend more \$\$ running the pump than you get as useable harvest from the array?

hr
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 50
re: In our area

Hot Rod

I am not trying to raise 1500 gallons of water - I was just hoping to maintain it at temp. The panels would only be required to offset standby losses and DHW for 2 people. Surely 6 - 4 x 8 panels could provide enough hot water for our DHW use - and to offset losses could it not? The GARN is very well insulated.
• Member Posts: 20,725
maintain it at what temperature?

unless the air temperature around the Garn is at or above the temperature you intend to maintain the tank, it will lose energy.

Hot goes to cold always. The rate of this exchange is based on the temperature difference between the tank and surrounding ambient.

I doubt the water would be much use below 105F or so for showering, etc. Unless you have a booster heater I doubt 1500 gallons of 80- 85F water will be a real crowd pleaser for the family. It's about quality, not just quanity.

So let's say you intend to maintain 1500 gallons at 110F. next you would need to know the average ambient temperature and the amount of insulation between that 110F tank and the air temperature. Additionally I think you will lose some, maybe a lot up the flue, depending on the draft. Even an induced draft appliance will lose some energy when the fan is off.

A simple basic 80- 120 gallon solar or indirect tank would be a much more efficient method and provide at least 60% or more of your DHW on an annual basis and perhaps 100% all summer when the Garn is not fired.

Go to RETscreen.net and download the free solar calculator. Then you enter your location and it pulls up all the data we are talking about, including performance data on your collectors if they are rated. Plug in the collectors, plug in the tank size, plug in your location, plug in you DHW needs, and out comes a SF% ( solar fraction).

I sized some systems up around the Toronto area and could easily show 55- 60% SF with 3 collectors and 80 gallons of storage. RET Screen allows you to change the number or size of the array, tank, temperatures, etc to play "what if" options.

I think if a "solar Garn" was "do-able" the folks that design, build, install, or own them would be all over the www talking about it

hr
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 50
Maintaining hot water temperature in a GARN

Agreed on the 105F - don't need any other reasons to downgrade my crowd pleasing rating - it is low enough as it is!

Tank is insulated to a minimum of R60 which should mean a slow heat loss between it and the surrounding ambient air given also that the temperature differential will be somewhat less than under winter conditions. Retscreen is a great idea but the panels are older and not listed so a bit of a guess on their performance. Certainly they would be on the lower end given their age.

As to GARN - they do talk about it but it certainly is not a part of their advertising - likely for the very reasons you mention. They do have a deal with a solar company to promote their integration but it is not well-known at all.

Guess I had better plan on plumbing in a smaller tank by all accounts and use the GARN as a heat dump if need be.
• Member Posts: 20,725
is a drainback possible

then you would not need to pipe a dump zone, and you could lose the glycol, expansion tank, etc.

I think you will be much happier with the smaller tank.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 50
Drainback may be possible but...

Thanks HR. I am definitely leaning towards the smaller tank - largely on your advice. Drainback, on the other hand, could be more of an issue because the panels are all below the GARN (or where the smaller hot water tank would be). I would then have insulate the drain back reservoir very well since it would be outside. I would also have to have 1-way valves to stop the smaller tank or GARN from trying to drain into the reservoir. At least that is my understanding of the situation.
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