Heating the slab

Is the slab insulated underneath? If not all the heat will go straight into the ground.

Maybe these would make the difference? http://www.radiantec.com/pricing/heat_transfer_plates.php

More details likely needed. Like supply and return temps, loop lengths & pipe sizes, flow rates, floor temp....

Let's start with temps. If you have a 1500 sq. ft. first floor, it likely delivers 30 btus/sf maximum.
My experience is that most staple up systems need relatively hot water (140F +) to get proper heat transfer.
180F is more likely needed, depending on floor covering.
Can you also measure the floor surface temperature?

Is it cold in the basement?

I would definitely heat the slab.. I have a 2" over pour on my main floor and it helps to heat the upstairs quite a lot.

The main floor surface temp must be about 10F above the room temp.
You can either up the main floor feed temps or install some extra radiation in the form of radiant panels in the area that is underheating.
Since you are working from storage at times, the feed temps are apt to drop below 160F. Obviously, it is not always practical to keep it hotter.
Heat the basement, which might offer a bump for the upstairs, but if you are not needing to heat the basement, the least expensive option might be
a radiant panel or two to keep things comfortable.

No expert on this topic by any means, but if your area is subject to wide and often fast outdoor temperature swings, and the main problem is getting more constant interior temp in spite of widely varying outside air temps, then I would agree with those who advise adding supplemental heat sources rather than trying to rely on the in-floor to handle all interior comfort needs.

When I did the research for the in-floor in my shop, I read and/or heard from a variety of sources that in-floor radiant, especially and maybe limited only to concrete, cannot be relied upon to provide all the heat needed at the times that heat is desired. Concrete responds too slowly, therefore under-heat and over-heat. I suppose reset parameters can help smooth this out, but still that's the advice I learned. Therefore, in my shop I also plumbed fittings for a unit heater to provide extra heat when needed and I follow a regimen of a constant floor setting (61F) to assure no overheat conditions. As it has turned out, I did not install the unit heater and in the shop air temp variation between a high of about 63F to a low in the low-50's are OK even when outside temps fall deep into the -20's to -40's.

By the way, as I write this (6.35am) the outside air temp is +26 and by tonight it will be -18F with a high tomorrow of -10F. And even colder tomorrow night. That's what I have to deal with.

Woodfarmer1 said:

Where is heaterman?

Click to expand...

Heaterman has been working his butt off.....lot's of 14-16 hour days the last few weeks. As the farmers say, "gotta make hay while the sun shines".

Brian it's pretty clear the underfloor heating in that area is just not generating enough btu/sq ft to heat the cubic feet in that space.
In an application like that it's helpful to imagine that area as double the actual floor space. To a certain extent you are heating the equivalent of double the actual floor space the room actually measures because of the tall ceiling. The heat loss will not actually be double but it will definitely be higher than the same sq ft with a standard ceiling height. In your case with 25'+ ceilings I'd guess a load of around 25,000 btu so you're looking at around 33-35 btu/sq ft.
A floor with staple up like your can't come close to generating that kind of heat. Nor would you want it to because of concerns about actual surface temp becoming uncomfortable and also the effect it would have on the hardwood flooring.

You just need to add more emitter in that area by whatever method makes sense in terms of cost and comfort. Another run of pex in each joist bay would gain you about 5 btu/sq ft so that's probably not going to get you where you need to be.

I'm thinking panel rads or a fan convector of some kind operating on it's own thermostat. This would give you a 2-stage system that would be able ramp down quickly when you get a lot of sun in there during the afternoon.

Check out the cabinet style convectors made by Smiths Environmental and see if somethig like that would work with your decor. They will put out decent heat even at water temps of 110-120*F.