Solar hot water and wood boiler

[Carl Webber]

I currently have a Tarm OT-50 set up with a superstor. I dont yet have thermal storage. Because i dont yet have thermal storage i use wood or coal for the winter and i use oil during the warmer months. I was trying to think of ways that would improve my current system but also would integrate with my storage when i do get it. At some point i plan on getting either 3 300 gallon propane tanks or 4 250 gallon propane tanks and converting them to roughly 1000 gallons of thermal storage. I am not there yet. I was thinking that i might be able to use one or two solar water heating panels on my roof to maintain the temp of my boiler during the warmer months. Of course i would leave the oil turned on in case the solar cant keep up. This would serve 3 purposes. One i wont be burning oil to maintain boiler temp. Two the boiler will stay up to temp to be ready for hot water production and wont corrode. And 3 i get my hot water. During the summer it would greatly reduce my dependance on oil and during the colder months it should also reduce the wood or coal need to heat the house. Is it possible to use the 90 gallons of water in my boiler as thermal storage for solar? Has anyone ever heard of this? Is there any good reasons why people dont do this?

 

[peakbagger]

I have SHW in Northern NH. I never run out of hot water with a 80 gallon storage tank on my SHW system. I have an indirect hot water heater on my wood/oil system. My hot water system is valved so that the indirect hot water heater is fed preheated makeup water from the SHW system in the winter and then when he SHW system is warm enough in the spring I valve out the indirect heater.

On a closed system as long as the wood boiler circuits are left full of water there is no rust so no advantage to linking the systems.

 

[velvetfoot]

But, don't you have any backup for dhw in the summertime?

 

[peakbagger]

Never needed one, my SHW storage tank has a backup electric upper coil. I have had the wire and the breaker for 15 years but never installed it. Worse case is, I turn on the oil boiler and flip on my zone circuit and the indirect hot water heater is back on line. I then swap two valves and I am back in business. I keep the indirect ho water heater thermostat cranked up to its max setting (with a downstream antiscald /blending valve) and am feeding it with warm water from the SHW tank so the actual amount of hot water used from the indirect storage tank is pretty minimal. I also practice demand management, If the tank temp is low after a few days of clouds, I hold off on the dishwasher.

 

[Carl Webber]

Maybe i missed something but i thought it was a bad thing if the boiler got below 140 because condensation would cause corrosion inside the boiler? I also thought that the big shift in temp from cold to hot or hot to cold would cause my gaskets to start leaking eventually?

 

[Zef_66]

I am considering doing the exact same thing. I'm in the planning stages right now, so just watching this thread to see if anything develops.

Thanks!

 

[maple1]

Maybe i missed something but i thought it was a bad thing if the boiler got below 140 because condensation would cause corrosion inside the boiler? I also thought that the big shift in temp from cold to hot or hot to cold would cause my gaskets to start leaking eventually?

That's if you have a fire burning. If you let it go cold, there are no combustion byproducts being made to condense.

I would just let it go cold, and install a conventional electric water heater for DHW in the non-burning months. That's IF the boiler will go cold without any drips or leaks appearing. My old one used to drip around the DHW coil gaskets, when it got much below 140 or so. I replaced those gaskets I think 3 times in the 17 years I had it. An oil boiler is terribly inefficient for doing DHW with. We do our with conventional electric for about $20-25/mo at $0.18/kwh. Vs. about 3/4 gallon per day for the oil, more or less. My new (not so new now) boiler will be going cold here in a couple of weeks - I think - and not for the first time. So far so good. It doesn't have gaskets though. One thing to consider might be a heat pump water heater for maintain your boiler temp - especially if you would otherwise use a dehumidifier or a/c in the summer. (A HP water heater does some of that stuff too while it makes its hot water). It likely won't get it much hotter than 120-130 or so though. I considered solar hot water at one point, but the numbers didn't make sense for us - would have been a looong time recovering the upfront costs.