Using indirect for radiant

[mech644]

Hoping that the collective Boiler Room knowledge can pass judgement on what I'm intending to do.
At present the house is not heated by wood, but as this remodel moves along I intend on installing an indoor gasification boiler.
At present house is heated by an oil-fired Trane boiler pushing hot water out to baseboard. Boiler is fairly new, circa 2012, but it is still a old school big hunk of cast iron, etc.
Domestic hot water was heated on a separate zone in a 30gl Amtrol indirect tank, Grundfos 15-52 pump. 2 years ago I installed a heat pump hot water heater so the indirect has been unused since then.
Have recently redone a bathroom and walk in closet, approx 150 sq feet of floor, used Warmboard as the base.
To heat the Warmboard zone I've come up with the following thought:
Callefi 1725C1A manifold and mixing station to distribute heated water thru the pex that is in the Warmboard (https://www.supplyhouse.com/Caleffi...ld-Mixing-Station-w-UPS15-58FC-Pump-3-Outlets)
Use the indirect as thermal storage for the Caleffi to draw from: plumb it as closed loop between them with an expansion tank and pressure reducing valve to fill it up.
The boiler would respond to the 'stat in the indirect and reheat the tank as needed.
The indirect will get boxed in with 3 " of hard foam to augment its existing insulated jacket (already have the foam so may as well use it).
My hope is that doing the above would reduce short cycling on the boiler and also shield it from very low temp return water that a radiant zone would send back.

Please fire away with comments and constructive criticism. Thanks in advance to anyone who responds.

 

[maple1]

A wood boiler should have more than that for return temp protection. What are your plans for storage for an IWB? Likely an indirect tank could be used like you described but not sure it's the best solution, both sides would be pressurized and closed so an HX may not gain anything.

 

[mech644]

When I do IWB was going to fab an 800 gl tank. Have access at work to have things like that made.
I understand that what I'm proposing is not the ideal. If oil boiler was going to stay my understanding is using another mixing valve at the boiler to rewarm return water would be advised. But my thought is that would be drag on the system efficiency.
Also trying to build something now that doesn't require rework in the future. Looking ahead, the Caleffi mixing/pump/manifold combo could draw/return to 800 gal of storage same way as it would to the existing indirect.

 

[Bad LP]

My RFH system has a LP fired System 2000 and I'm using a electric water heater (not wired) as a buffer tank because the low mass steel boiler was short cycling so much it drove me crazy. You won't have as much short cycling IMO due to the fact you have a high mass CI boiler but using the indirect tank in the same manner will do the same thing. It's all in the piping.

I'm also using an indoor boiler and storage. The operation at change over from LP/wood/LP is 100% seamless as the primary loop doesn't care what the heat source is. When the tstat on the storage tank hits 115-120 the LP boiler disconnects itself and the wood boiler is now running the show until such time the tank temp drops out then the LP takes over again. The LP has nothing to do with the tanks.

 

[Fred61]

Essentially what I did when I removed my gasifier. The radiant supply was coming off the unpressurized storage through a mixing valve and returning to the bottom of the storage. I basically replaced the unpressurized storage tank with a 20 gallon superstore pool heater I picked up cheap and hooked the heat coil to my oil boiler and the output to the radiant runs. I installed the tank in place of a primary loop or should I say IN the primary loop for the same reason you mentioned, short cycling of the oil boiler. Works great! The aquastat on the tank allows me to set the temperature of the radiant without any mixers or anything else. It's basically an HX, an aquastat and a buffer tank all in one.

You will need all the hardware like expansion tank, air eliminator, etc.

 

[__dan]

If it were me I would avoid doing anything non standard just on principle. The next guy who has to troubleshoot it or service it cannot get hung up on trying to figure out something that may not have been ideally engineered to begin with. I would want to work with a standard reference design and then just copy that.

For me, key would be the boiler is not burning oil when it is off. So if it just turns off faster, that is not necessarily a downside. Firing longer to charge storage in mild weather adds up to a lot of extra gallons in mild weather.

For the oil boiler short cycling you can look at the nozzle firing rate and downsize the nozzle. Check that first. If the nozzle you have is over .75 gpm oil and you're short cycling under load, you can probably consult with the boiler manufacturer and go quite a bit lower to match actual demand better. It would not surprise me if you checked the existing nozzle fire rate and it is quite a bit higher than .75. Some guy changed it over the years and just put in what he had. Next guy changes it for the same part thinking it is the right one.

For the radiant, I believe you would have to have a boiler primary loop to maintain over 140 F boiler return water to avoid flue gas condenastion in the boiler, and then a (outdoor air temp reset) secondary loop mixed down for the radiant, usually a valve or injection pumped secondary.

When you go back to a wood boiler most likely you will want that indirect tank again as a load on the wood boiler (for the DHW).

Do the easy stuff first. Check the burner nozzle firing rate and downsize that quite a lot until it runs well under varying load conditions. Boiler piping must maintain over 140 F return water to the boiler.

Primary boiler hot water loop with secondary pumped mixed temp loop for the radiant would be kind of a necessary standard reference design. The indirect tank is an expensive unit, unless you need it then it is money well spent. Next guy comes along and says the indirect tank has gone bad and changes it for the same unit, when the money may have been better spent on the boiler header piping and secondary loop taps.

 

[Fred61]

My set-up isn't that complicated. Perhaps if your average attorney or an accountant were doing the troubleshooting it could be a problem but the guy with dirt under his fingernails would only need to walk by it to understand the concept.

 

[__dan]

Really not enough information to even guess at what the final permanent installation will be.

The 150 sf warmboard install sounds like a very low load low mass (distribution load) that is going to send warm water back, very low loop temp drop. But also very low heat into the space. With the low mass, the expectation would be it would want constant flow at a reset water temp and is going to cool off very quickly if it is running on an indoor stat.

With a fixed temp, mixed down radiant loop, you may want 130 F in the colder weather and 95 F in the many mild days. So too hot or too cool (probably too hot) for many days. A higher mass sytem could moderate temp swings or do the buffering. That's one of the reasons people keep an eye out for old cast iron radiators, and running CI rads with a lower reset hot water temp would be one of the ways to do it.

You can also try to OAT reset the water to the convection baseboards along with the radiant pex, but then you are back to primary (hot) boiler water loop and secondary mixed down OAT rest loop water.

Just not enough info. If I looked at it, being an older house, first thing I might suggest is stripping the siding and adding laytered rigid foam board and / or Roxul to the exterior sheathing, tied into new windows and probably a new roof. Design and Prioritize, then move in the direction of the satisfactory final result at a steady pace. Doing too many things as temporary arrangements, some get stuck over the long term without (the satisfactory result).

Just as a temp solution to get the 150 sf warmboard up and running, you could do the numbers for oil fired vs another electric DHW heat pump tank to source for the 150 ft pex. Probably, my guess is, it would not surprise me if a DHW heat pump for 150 sf of pex is very competitive with firing oil, especially with with the attached standby losses and system mismatch inefficiencies of the oil boiler. Fuel cost could be the same, electric to oil, but system efficiencies could be no contest.

There is probably an easy way to add an OAT reset control to a DHW heat pump, tank thermostat, to get the tank temp to reset to the (OAT reset calculated demand) and then a more constant flow, always on 150 sf warmfloor.

Design it first do the numbers, then redraw that draft 3 or 4 more times, and you will be closer to some buildable plan.