Adding an old hot water heat tank to baseboard zone

[oceanjunkie]

I had a drainback solar hot water system installed and running for a while, but due to a recent renovation I took it down and don't plan to put it back up. I still have the 120gal drainback tank sitting in my basement. It is designed similar to an indirect fired hot water tank, except the internal coil is mounted high in the tank.....the domestic water would flow through the internal coil and get heated up by the 120gal of water in the tank which was heated by the solar panels. So what I'm considering doing is adding the 120gal tank of water to my existing single zone baseboard heating loop. That loop is setup for constant circulation through the heating season. If I add the 120gal to the loop, it should reduce the amount of times my burner has to fire, while increasing the amount of time it runs when it does fire.....which I think is a good thing for efficiency? And then the domestic water would run through the internal coil preheating it before it goes into the hot water heater (which is the same thing it was doing when setup for solar) which will add some capacity to our little 30gal indirect tank. We haven't had any capacity issues, but we just added a new full bath to the house so it could be an issue in the winter when the incoming water is real cold.
What do you think? Would this work? I wouldn't expect much of a decrease in oil usage, but since I already have the tank sitting there unused I just wonder if I should plumb it into the loop. Thanks in advance.

 

[Dana]

Adding a buffering thermal mass like a big tank of to extend the minimum burn times is indeed good for the boiler's net efficiency.

When adding that much thermal mass to the radiation it's important to protect the boiler & flue from having return water that's too cool for too long, creating copious acidic condensation in the flue, or on the boiler's heat exchanger plates. This can be accomplished with "system bypass" piping branch that mixes the output water with the return water, raising the temperature of the water that enters the boiler. This can be controlled with a themrostatic mixing valve if you like, or dial in they bypass flow by hand with a ball valve. For oil boilers vented into a tile lined masonry chimney the entering water temperature at the boiler should be no less than 140F. If it's a stainless steel flue liner with some sort of condensate disposal/management you can cheat that by 5F or so without risking the lifespan of the boiler, but don't run it as low as 130F EWT if you can help it, even if it's a "cold start" boiler.

If the house is set up for multiple zones you COULD set up the tank in the middle, of it all, with an aquastat on the tank being the only thing that fires the boiler. Most heating systems don't need anything like 180F water to heat the place, and you can set the aquastat to whatever temperature the radiation needs to be able to emit enough heat on design day, or a few degrees higher. That would lead to much more even room temperatures, and guarantees that the ~1000 lbs of water is involved with every space-heating burn (the indirect would still be it's own zone, and not be taking it's heating coil water from the buffer tank. There are buffer tanks with potable coils in them for exactly this sort of configuration, eg Ergomax:

In your case the surface area of the coils in the tank probably aren't high enough to deliver fully heated potable water the way an Ergomax would, so you'd still have two zones served directly by the boiler, the buffer tank, and the indirect.

 

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[oceanjunkie]

WOW, that was an awesome reply. Thank you! I don't know why I didn't receive a notification that you replied, but I am just seeing this almost 1 year after it was posted. I still have the tank in my basement and I'm still considering this project, even more so now that I saw your reply.

We have a Buderus G115 with Logomatic control and a stainless lined chimney. My understanding was that low return temps should not be an issue with this setup. If that's the case, then a bypass may not be needed? I think the Logomatic will cycle the circulator when return temps are too low. Except since I'm considering putting a relatively huge buffer on the system it could take long for the system to heat up, creating a lot of pump cycling?

Could I simply have the hot water from the boiler go from the circulator, enter the bottom of the big tank, and then have a line out the top of the big tank feeding the baseboard loop? And have the baseboard loop feed back into the return side of the boiler as it currently does.

FYI, the house is currently one zone.

 

[Dana]

A Buderus G115 probably still needs the entering water temp at the boiler to stay above some temperature to avoid destructive condensation inside the boiler. The stainless flue liner can tolerate exhaust condensation, but the boiler's heat exchangers usually don't tolerate it well. Not all Logamatic versions will auto protect. See section 2.8.1 on page 9. Seems they want the return water to stay above 130F on this boiler.

If it's all one zone and there's sufficient radiation to emit the full output of the boiler there probably isn't much efficiency to be reaped by adding the thermal mass, but if it's under-radiated there should be a measurable benefit by limiting the short-cycling.

Some amount of napkin-math would shed a light on this, starting with a fuel-use heat load calculation, and the total amount of baseboard on the loop. Dividing the calculated heat load by the feet of baseboard will tell you what the average water temp needs to be on design day to heat the house, and the water temperatures needed at any arbitrary outdoor temperature one choose can be inferred. In a constant circulation system most of the season the water temps don't need to be very high, and running it hotter than necessary wastes fuel in distribution losses.