Radiant Heating Design Assistance for Hardwood Installation

cnaconsulting

Greetings! We're looking for some design assistance with an upcoming renovation we're working on. The home is small, less than 1100 sq ft and currently has forced hot water heating with cast iron radiators in each room. Besides the comfort that comes with under floor radiant heating, we were thinking too that since the rooms are on the small side, removing the cast iron radiators will free up some space. We will also be redoing the flooring, so we have some options when it comes to installation. So here are some of the design questions we have, thank you for your honest and open feedback:

We will be installing prefinished brazilian walnut 3 1/2" flooring throughout. The subfloor is 1x12 solid wood planks and floor joists are all 16" on center. A preliminary heat loss calculation gave us the result of needing 50 BTU's per square foot. The home is fairly tight for an older home (1940's).

1 - Being that this house is located in a cold climate area, are we crazy to consider radiant heat flooring in the first place? What we don't want its to end up having to put back the cast iron radiators because the radiant floor heat is insufficient.

2 - When it comes to the installation method, which would be best, under the subfloor (between floor joists) between subfloor and wood flooring? I've read about both methods, but would like to know which will result in more even/consistent heating, lower water temperatures, and less likely to be problematic or overheat the wood. Our flooring guy recommends that we add 1/2 plywood over the existing subfloor planking to sure up the floor. If we were to install the radiant heating under the subfloor, the combined layers of the floor itself would be at least 2 1/2" thick. Would this offer too much insulation or make installing under the subfloor not practical or effective?

3 - From what I've seen, the standard is to use 1/2 PEX with loops of 300' or less, keeping each loop close to the same length. The recomendation I see is to space each loop 8" apart. Is this correct? Will this allow for sufficient BTU's to heat the living space? Are there advantages to using smaller tubing with shorter spacing?

4 - Has anyone had experience using some of the different products available for installation, such as 4' aluminum heat transfer plates, joist trak, warmboard, etc? Any recommendations or pitfalls to avoid when choosing?

5 - We've also heard about radiant cooling, but have not been able to find too much information on it. Are there systems or some method that would allow us to use the same radiant underfloor tubing to both heat and cool the living space?

Thank you for any help you can offer!

STEVEusaPA

For starters you better take a closer look at that calculation 50/sq ft would be 55000 btu's need for just that room.
After were sure on the number, I would recommend Warmboard or equivalent (something with grooves for the tubing), right on top of the existing subfloor (adds 3/4"). Then the hardwood right on top.
The biggest installation mistake is installing the wood when it or the subfloor contain too much moisture. I'll have to look it up, but acclimate the flooring before installation, and make sure there isnt more than a 4% (I think) difference in moisture-confirmed with a moisture meter.
Insulate under the subfloor (joist bays).
The biggest operating problem is making sure the floor temp doesn't exceed 85°. Make sure that zone is piped properly with a mixing valve and that you have the ability to prevent the floor from getting too hot.
Solid Brazilian Walnut is not the best choice for radiant, an engineered product may be better.
Narrow planks are better than wider, but following the proper guidelines for installation and operation, you should be fine.
Don't forget you can also do a radiant ceiling.
As far as cooling, you're getting into a whole other animal. For starters the radiant loop would need to be isolated from the radiator loops.
It will also require a more stringent design.
You can seek out design ideas/help here:
http://www.radiantprofessionalsalliance.org/Pages/default.aspx

Excellent reading material:
http://www.caleffi.com/usa/en-us/technical-magazine

Gordy

Agree with @STEVEusaPA . First hurdle is the heat loss nailed down. No way is it 50 btus a sf. Tell us your approximate location.

cnaconsulting

Thank you both SteveUSAPA and Gordy. Ok, so sounds like We need to nail down the heat loss. The calculation I did was based on one room, which sounds like it is inaccurate. So I did it again, using the following parameters, the house is 2 story cape with full unfinished basement, 8' ceilings and is 32'x22', the sq ft of all exterior doors and windows is 170 sq ft. The house is located in the Providence, RI area. Based on that we would need 39 BTU's per sq ft. Does that sound correct?

Based on what I've read it seems like even with a product like warmboard the most BTU's we'd get would be 30 BTU's. I'm beginning to think radiant may not work. Thoughts?

Mark Eatherton

Not all woods can handle 85 degrees F and not all hardwood manufacturers will warrant their product when
placed over a RFH system. Radiant cooling requires hard impervious surfaces. As for heat load use the existing fuel consumption. Nothing better than real time feedback.

ME

KC_Jones

Quick question that relates to the heatloss. You are doing renovations, but not planning on doing any work on the building envelope? Insulation air sealing etc. Even 39 per square foot seems high if you are renovating. My house built in 1900 has original windows virtually zero insulation and barely hitting the heatloss you are calculating.

Adding insulation and air sealing gives HUGE payback in the short and long term. I suggest you consider this in your renovation plans. It also gets you into the realm of considering radiant floors.

Zman

I think you have an incorrect setting in your heat loss calc.
I am in zone 7 and have never seen a 30btu/ft house. You are in Zone 5.
I did a quick heat loss on the house you described with r-12 insulation throughout and single pane windows. It came out at 24btu/ft for your climate.
You would be a great candidate for in floor radiant heat.
I would be careful of radiant cooling unless you have a way to deal with humidity and condensation.

Gordy

39 btus a SF is still very high. For most homes of that period.

what indoor temp are you using in the program, and what outdoor design temp?

As @Mark Eatherton said earlier real time fuel consumption will paint a very vivid picture of your heat load. However there are still details to that method.

Gordy

Agreed. Even ignoring system efficiency will get you close. If you can weed out the other appliances on the same fuel.

Gordy

I'm thinking more long term calculations. Not by the hour, or day even.

Gordy

That's pretty darn close.....

cnaconsulting

Ok, so I downloaded the trial version of HeatCAD to more accurately determine the heat loss. Entering data as accurately as possible the software tells me that for 1181 sq ft the total heating load is 26,173 BTU's. So does this mean we would need 22 BTU per sq ft? (The load calculation method is Manual J8, indoor heating temp of 70f, 35-50% humidity, the local outdoor temp set by the software is 12f.)

To answer some of the other questions asked, the house has R-19 wall insulation and the and attic/eaves have blown insulation covering the joists. Also, since we just purchased the house, unfortunately we have no records of heating cost to base HL calculations.

Thanks all for your input!

cnaconsulting

Ok, so doable but challenging. So I need to calculate the cost to determine if this is going to be worth it. Design wise are we better off with a surface install using warmboard or a similar product and place the wood directly on top? Would it be better and more even to use extruded aluminum plates underneath between the floorjoists? Im thinking the latter would be better since the water temp could be higher and heat more wood mass, but then again it could be too much to try and heat as well.

Gordy

Over the top methods will always use a lower water temperature than underneath methods. If possible over the top is superior. This is especially beneficial with a modulating condensing boiler selection.

I'd be willing to bet your losses are even a bit lower in reality. Especially with that outdoor design temp of 12*.

cnaconsulting

Thanks. With over the top, do I need to put an aluminum plate or somethingover the pex to facilitate better heat distribution? Would something like the omega aluminum panels on top of the subfloor, but placed under the pex work? What about spacing? Space each loop 8" apart, 6"?

Gordy

Yes transfer plates help lower average water temps, and stripping. Look at sunboard, warmboard r, and many others with built in transfer plates. Minimum build up.

Room by room heat loss will dictate tube centers, and SWT.

Try to do 6 in areas where barefoot comfort is of higher frequency baths. Most other rooms 8" is sufficient. All though some panel manufactors are limited to 12" with their panels. Sunboard does 8", Roth panels do 6", or 12".

Gordy

What I like about Roth panels is they are insulated.