BaseBoard to Runtal Wall Radiator retrofit

[ThomasD.]

I have a 2 story open main floor with standard 3/4"hot water finned base board. The existing BTU output doesn't allow me to get the temperature above 67 degrees when it's colder than -20 outside. I realize that other factors are at work such as window exposure, wall insulation, boiler output etc., they will be dealt with. Have always hated the look of baseboard so this is where I'm starting. We are remodeling the basement so all 3/4" heating lines are accessible now. I've run the calcs, discovered that the existing baseboard only provides 20- 25 btu's per square foot. Will be replacing all finned baseboard with Runtal wall panels with increased output to accommodate 35-40 btu's per square foot. My question is since the existing baseboard is 3/4" cu. and the Runtal panels are 1/2", will there be any issues reducing the 3/4" stubs to 1/2" ? Planning on using Pex-al-pex with all plastic Uponor expansion fitting to make the transitions. I'll reduce the 3/4" below the floor and stub up with 1/2. Any other suggestions? Thanks

 

[ThomasD.]

Guess I should proof read more often. Planning on using HePex not Pex-al-pex. Pex-al-pex is almost unavailable up here and nobody carries the necessary fittings or tool.

 

[Sponsor]

 

[Dana]

"...the existing baseboard only provides 20- 25 btu's per square foot..."

Why are you looking at BTUs per square foot of conditioned space (an almost meaningless concept)? The total heat load of the space in BTU/hr and the output specs & amount of the heat emitters are what's more relevant.

That said, a load/area ratio of 25 BTU/hr per square foot of room @ -20F is on the extremely high side, as is 35BTU/hr @ -40F (roughly your 99% outside design temp in Fairbanks.) Even at IRC code-min foundation R should be in single-digits BTU/ft at -20F for most basements low low double-digits at -40F. Is this a walk-out basement with a big glass slider or something?

Or are you only referring to the above-grade first-floor? Even then 25 BTU/hr per square foot @ -40F would be on the high side unless you have a lot of window area. With your existing baseboard you're able to hold on to a ~87F temperature difference, and -40F to +70F indoors for design temps would be a 110F difference. The ratio of 110F/87F= 1.26, so increasing radiation output by ~25% should be sufficient, and it seems like you're bumping it up by more than 50%. (?).

But to answer your question, dropping back to half-inch for the Runtal baseboards isn't likely to be a problem- most 3/4" baseboard in basement zones is over-pumped. How many total feet of baseboard, and how many feet of 3/4" plumbing?

 

[ThomasD.]

Let me preface this by stating that I'm not a heating or plumbing contractor. I'm an Electrical contractor. I'm dealing with a 900 sq. ft. +/- , 17' high 2nd floor area w/ 70% window coverage on the south facing wall & 25% window coverage on the north facing wall. Having talked with 3 Heating contractors, they each said that with this much exposure and no radiant floor heating possible that the 35-40 BTU sq. ft. was minimal using perimeter baseboard/ radiator. They only used the BTU sq. ft. reference, stating that if it was a new construction project, that's how they would figure the required baseboard. The existing linear footage of standard baseboard calcs out to approximately 25 BTU sq. ft. And again, cannot raise indoor temp above 67 * @ -20 outside temp.

 

[Dana]

Ah, so this isn't a basement, it's all above-grade high-ceiling high glazing fraction stuff? In that case 35 BTU per square foot might be the ratio, but probably not 40.

HVAC contractors sure seem to love their "BTU per square foot" rules of thumb, which allows them to disengage brains, but reliably oversizes the systems. It doesn't take rocket science to run a Manual-J heat load calculation correctly using off the shelf software packages, but it does take some time. Since you have a heating history on the place you COULD use fuel-use against heating degree-days calculations to come up with heat load & total radiation sizing, but it would be most accurate if you used fuel used during the fall shoulder season when it was still keeping up with the heat load, not when it was cooler than -20F.

When you put it in terms of BTU per square feet it requires us to infer the actual amount of baseboard, which is the more relevant number. 900' x 25 BTU/hr= 22,500 BTU/hr, yes? Most fin-tube baseboard is good for about 600 BTU/hr at an average water temp of 180F, so that means you have something like 22,500/600= 38 feet of baseboard?

It sounds like the existing baseboards can deliver an 87F difference (67F minus -20F is 87F), as mentioned in the first post, and it really only needs to clear a 110F difference (70F minus -40F is 110F), which is where only needing a 26% upsizing of heat emitter capacity to cover your 99% heat load comes from.

If you increased the 38' of baseboard by 26% that would mean you only need a total of 48' of baseboard to cover the load at -40F with a 70F interior, which is only 10 feet more.

Assuming you're getting something like 22,500 BTU/hr out of the baseboard at a temperature difference of 87F, and you only need 26% more at temperature difference of 110F, that implies the 99% heat load is 1.26 x 22,500= 28,350 BTU/hr, or (28,350/900'=) 32 BTU/hr per square foot of conditioned space, not 35, and not 40.

Runtal UF-3 is good for about 770 BTU/ft, or about 28% more than the 600 BTU/ft fin-tube, so simply replacing it all with Runtal UF-3 with NO additional length would deliver design-day heat. Is that what you were planning to do?

Or were you planning on all UF-4?

UF-4 delivers 930 BTU/ft (x 38' = 35,340) which would be 35,340/900'= 39 BTU per square foot of conditioned space, and a bit overkill for a ~28 KBTU/hr heating load- at the very high end (or slightly over) ASHRAE's recommended maximum oversizing factor.

BTW: You CAN get 40 BTU per square foot out of radiant if it's all exposed floor, but whether you have sufficient expose floor area to use as radiator sufficient to cover the true heat load takes more analysis. The contractors may be correct that you can't heat the place with the floor without frying your feet, but the odds that you really need 900 square feet x 40 BTU= 36,000 BTU/hr to cover the heat load at -40F or even the ~28,000 BTU/hr implied load from your 67F, -20F temperature difference. is a dubious proposition, even with the high ceilings and extra glass. But if you have enough exposed floor to deliver even even 8000 BTU/hr (likely) you could drop back to UF-2 on the Runtal (or keep the ugly fin-tube) and still have some margin on the total heat load AND the much higher comfort of a radiant floor.