Will This Boiler do the Trick?

[Russell Stevenson]

Hello Everyone,

Avid reader of your forums, first time poster. I have gleaned lots of information from the forums and sincerely appreciate the knowledge contained herein.

I bought a late 1800's 1100 s.f. brick house. I have removed the rotten wood floors and joists and I am going to pour a polished concrete floor in which I will run tubing for radiant heat. I will over-insulate the the attic and the slab, put in new Marvin windows and doors, but the house will have no insulation on the exterior walls. as I want to keep the plastered brick. I plan to add 2 or 3 Mitsubishi mini-splits for cooling and to augment the radiant heat. And I might add a wood burning stove at a future date. I realize I will have have substantial heat loss. The house is located between Nashville and Chattanooga.

I have been reading for weeks about radiant heat (overwhelming) and I would be forever grateful if someone would tell me if the Thermolec B-6TMB would be a good choice for my application? It is a 240v, 6kw, 24,000 BTU unit. I will be upgrading to 200 amp service and will likely have an electric tank water heater for domestic use. The house will be all electric.

Many Thanks,
Russell

 

[Dana]

To do this right you really need to do a room by room heating & cooling load calculation on the "after improvements" condition of the house.

You are in US DOE climate zone 4A. It's unlikely that you would be able to even hit the IRC 2015 code minimum R49 for the attic, let a alone "...over-insulate..." R49 takes about 13-15" of fluff on the attic floor, and you probably don't have a foot of clearance from the roof all the way out to the exterior edge of the brick walls.

Arithmetic: 6kw x 3412 BTU= 20,472 BTU, not 24,000 BTU

Assuming you have something like 1000 square feet of available radiant floor area (not covered by cabinets, or closets, etc) 20,472 BTU/hr would be 20 BTU/hr per square foot, which would still be a comfortable enough floor temperature at full power. If you only have 800' of available radiant floor area that would be 25 BTU/hr per square foot, which is still comfortable, but edging into the the 80F for a 68F room. Going much over that could be a hot-foot situation, but I think you're going to be fine with that boiler, even if it isn't enough to cover the full heat load on it's own. This short manual is useful for figuring out the limits of just how much boiler you want to put into how many square feet of active radiant floor. From a comfort point of view I personally prefer to hold the line at 30 BTU/hr per square foot of active floor, even if that isn't enough to fully heat the space. The next size bigger 9kw Thermolec B-9TMB would be arguably too much boiler if it's 800' of active radiant floor, but still fine if it's 1000' of radiant.

For more money than the boiler (but maybe not more than a boiler + a few mini-splits) a modulating 2-ton Chiltrix reversible chiller and some fan-coils for additional heat emitter (and cooling) is probably about the right size for your loads. It will run the radiant floors at mini-split type efficiency, and could use the wall-coils as second stage heating, as well as cooling.

 

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[Dana]

BTW: For the sub-slab insulation you can save a lot of money by going with reclaimed roofing EPS rather than new EPS or XPS. XPS is usually labeled R5/inch, but as it's HFC blowing agents diffuse out over time it eventually drops to the same ~R4.2/ inch that you get with EPS of similar density. EPS uses much more enviromentally friendly hydrocarbon blowing agents, most of which escapes the foam (and is recaptured) at the factory, and has no affect on it's performanc. Most roofing EPS is 1.25 lbs per cubic foot "Type-VIII" at about R4.15/inch or 1.5lbs "Type-II" at R4.2/inch.

This guy near Chattanooga is sitting on 250 sheets of reclaimed 4" EPS (about R16.5-R17) price at about half the box-store pricing of R5 EPS. For 1100' of slab it's worth buying 40 sheets/1280 square feet (to cover any damaged sheets and deal with scrap rates, etc)., which is still less than $500 (not counting what it costs to bring them to your job site). He's willing to go lower in price for more than 25 sheets at a time.

Even though there is a lot of reclaimed material out there for cheap, DO NOT USE POLYISOCYANURATE foam for under-slab insulation. Use only EPS or XPS. Polyiso is somewhat hygroscopic, and can wick & retain ground moisture over time, cutting into performance. It's fine to use in the interior side of basement walls, etc, but should never be in contact with the ground or isolated from the dry interior air by a concrete slab/wall.

Type-VIII EPS is typically rated 15 psi, Type-II EPS is typically 20 psi, compared to 25 psi for 1.5 lb density XPS. Newbies often get nervous about that, but for an under-slab application it doesn't really matter at all, since it's not supporting the weight of the building, only the slab, the partition walls and the room contents. (More discussion on sub-slab foam here. ) Even the 15psi stuff is "walkable" without leaving permanent footprints unless intentionally stomping hard on your heels. (Since its usually under a flexible membrane roof, the foam is almost always specified at a walkable compressive strength.) Once it's covered by a reinforced slab the rigidity of the slab distributes the weight well, an you can park your Caterpillar D8 on a 4" slab without damaging the foam or cracking the slab (assuming that's what you want for your living room chair instead of a LA-Z-boy. )

When pouring a slab over foam, put a 6 mill plastic sheeting on top of the foam as a vapor barrier, not under the foam. When the vapor barrier is under the foam it inevitably gets pockets of liquid water collecting on top, between the foam & vapor barrier, and takes forever to dry through the foam toward the interior, which risks damaging the finish flooring.

A more in-depth discussion of the slab /foam/ vapor barrier stackup lives here.

Tie the radiant tubing to the reinforcing steel (be it mesh or rebar) so that the tubing is at or above mid-depth in the slab, which makes the radiant slightly more responsive.

 

[Russell Stevenson]

Thanks very much for the replies, Dana - gives me lots to think about.

I sent the fella in Chattanooga an email about the insulation. If he still has it I will go pick it up and stash it in the garage until I'm ready for it.

That Chiltrix reversible chiller is really nice. I've been reading about HVAC for weeks and thought I'd seen it all, but the Chiltrix is new to me. I wonder if I would have difficulty finding someone to install it properly? I like the small electric boilers because it seems like I might be able to do a lot of it myself. I have a hard time finding trades people. Everyone is very busy.

I've never been in a house that had radiant heat, other than my own. In another life, I lived in a house in MI and I had under floor radiant heat installed and it never worked very well. I don't think it was installed properly.

I am going to meet with the inspector on Tuesday and discuss my plan.

Thanks again for the help. I shall return.

Russell