In slab and baseboard fed at the same water temp?

[Max Power]

Problem: Plumber says that in order to run a baseboard radiator in our new bonus room at 180 degrees and keep the in slab running at 120, it will require about 30 hours of labor to replumb the boiler and add in mix valves etc.

we have a Navien 240e combi boiler that is heating a 24x30 very well insulated basement and is now going to be heating the slab of our very well insulated 26x32 new garage and 12x10 breezeway.

I could save about $2000 by running the baseboard at the same temp as the floor heat. I have 20’ of wall space that I can fill with radiator. Considering Runtal 4 tube unit.

Apparently I need about 10,000 btus in the bonus room to hit design temp. The Runtal unit at 130 degree water temp puts out about 8900 btu.

The guy at Runtal asked if my boiler had an outdoor reset control on the boiler. It is not installed. I guess he was thinking that the boiler could push hotter water on the colder days if it was installed... although I thought I’ve heard that you shouldn’t run anything above 130 degree water through a slab...

Any thoughts on what the best play is here? There’s also going to be a mini-split installed, so we are really just trying to cope with the worst of a Minnesota winter with this baseboard element.

 

[Dana]

There are cheaper flat-panel rads than Runtal baseboard, (Myson panel rads are usually "value priced", EcoStyle is cheaper than Buderus or Runtal). Typical inexpensive ~8-10" tall fin-tube baseboard delivers about 200 BTU/hr at an AWT of 120F. (20' x 200 BTU/hr per foot = 4000 BTU/hr, a large fraction of the estimated load.) How certain are you that the room's heat load is 10K? And at what design temperature?

How much does the planned mini-split put out at your design temperature? They're not all the same- a 3/4 ton Fujitsu -9RLSH is good for over 10K @ -15F , and never shuts down even when it's colder, still putting out heat (albeit without a specified capacity.) If the baseboard is just the "hail Mary" make-up heat for when the mini-split is running out of capacity 20' of the cheap stuff is probably fine, even at 120F.

 

[Sponsor]

 

[WorthFlorida]

Max, what will the heated garage be used for. For cars only or be using it as a rec room where you need it toastee? For cars and be able to work on them in some comfort can be anywhere above freezing temps. I worked many nights on my vehicles in a cold garage while I lived in upstate NY and Illinois. I always was looking for some heat to get the chill out.

 

[Max Power]

It’s definitely the Hail Mary scenario. I know the Runtals are not cheap, but I prefer the aesthetics of them.

How certain are you that the room's heat load is 10K? And at what design temperature?

Just going by what the plumber said. We design for -15 here.

How much does the planned mini-split put out at your design temperature? They're not all the same- a 3/4 ton Fujitsu -9RLSH is good for over 10K @ -15F , and never shuts down even when it's colder, still putting out heat (albeit without a specified capacity.)

They were planning to put in a Samsung 208/239V rated at 10,800 btu/hr... Not real sure of how it performs as the temp drops. I think it’s effectively useless at -5.

 

[Max Power]

Max, what will the heated garage be used for. For cars only or be using it as a rec room where you need it toastee?

Mostly just for cars, but I use it as a shop regularly as well. That in floor heat is going to be sweet! The studio upstairs where the baseboard heat is will be more of a working art studio.

 

[Dana]

It’s definitely the Hail Mary scenario. I know the Runtals are not cheap, but I prefer the aesthetics of them.



Just going by what the plumber said. We design for -15 here.

I wouldn't let a plumber run the heat load numbers- that would be like asking a dentist to test your blood pressure- some might be able to it right, but it's not part of the job description or training.

Even a paper & pencil or computer spreadsheet I=B=R type load calculation would have better cred, if you take the time to look up or calculate the U-factors on the construction.

They were planning to put in a Samsung 208/239V rated at 10,800 btu/hr... Not real sure of how it performs as the temp drops. I think it’s effectively useless at -5.

What model number Samsung? They're not all, the same. The 3/4 ton Samsung Max Heat AR09KSWSPWKNCV/ AR09KSWSPWKXCV is fully specified down to -13F/-25C.

I believe currently all Samsung mini-splits are manufactured by Midea (all of which use Toshiba compressors). But the compressor type make a HUGE difference. There are some Midea units with specified output fully characterized down at -22F/-30C, that also have the (necessary in your climate) pan heater and associated controls. I'm not sure if Samsung uses any of those Midea units under OEM arrangements, but Carrier uses some of Midea's cold-climate technology. The 3/4 ton Midea Premier-Hyper MCHSU-09PHH2/MEHSU-09PHH2 is good for 12,000 @ +5F, and something like 8000 BTU/hr @-22F (I'd need to dig up the capacity charts to be more precise or the number at -15F).

As noted previously, the 3/4 ton Fujistu cold climate heat pump is good for 11,000 BTU/hr @ -15F.

The upcharge from a vanilla simple compressor 3/4 ton mini-split to a cold climate version with enough output to fully cover the load at -15F is probably less than what you'd be spending on Runtal rads adequate to deliver the full heat @ -15F (even after getting more accurate heat load numbers.) In my area the Fujistu AOU/ARU9RLF3H runs about $3-3.5K (all in, fully installed w/warranty) in competitive bidding and a comparable Midea / Carrier would run a bit less than that.

 

[Dana]

BTW: This Midea is pretty much identical to this Bryant Evolution, and very similar to this Carrier, (a different indoor wall unit, same outdoor compressor), any of which would cover the entire likely load at -15F.

 

[Fitter30]

Fin tube doesn't really start heating till water temp is 120*. Floor radiant water temp run 5-10* over thermostat setting. Your floor covering all have temperature limitations and how the heat moves through them. 300' of 1/2" would be needed for 10k of btu's . Slantfin baseboard is 600 btu's a foot at 180*. Boiler reset will operating costs.

 

[Dana]

Fin tube doesn't really start heating till water temp is 120*.

Sure starts heating at lower temps than that- it starts heating whenever the water temp is above the floor temp. But it's output characteristics are very non-linear and variable below 120F (the convector cabinets are too low), which is why it's output is rarely specified at AWT below 110F. Just because there isn't a spec at 90F or 100F doesn't mean it's output is zero, or anywhere near zero at those temps.