Hello, I'm a DIYer. I don;t know much about these systems. I am remodeling my kitchen. I was thinking about removing my old radiator and connecting a PEX system to the pipes, installing with the aluminum thermalboard over wood subfloor then covering with either ceramic tile or linoleum. The thermalboard recommends backer board + waterproof crack isolation membrane + thinset with the tile or plywood backer with linoleum. Do I need to use manifolds or pumps? I am thinking the length would be one loop approx. 200 ft. Can you recommend anything that would help my heat transfer as far as underlayment advice?

We design and install radiant floor panels for kitchen remodels every week here in Minneapolis. Unfortunately nearly half of our work involves correcting other contractor's mistakes. First and most common when removing an old cast iron radiator is not performing a proper Manual 'J' heat load before proceeding to install a new radiant floor panel to replace it.

As the first order of business in removing an old cast iron radiator during a kitchen remodel is to fill the new found floor space with cabinet or appliance, the already "free floor area" gets even smaller, making the heat output of the old radiator harder to replace.

First, the heat load, then the radiant panel output at available delivered water temperature. Ideally, the radiant panel should be sized to the design water temperature of the radiator it is replacing. Zoning is the next issue. If floor coverings are similar to those in the rest of the zone (area controlled by one thermostat), you are all set. If you change from, say 3/4"oak to tile the output will change and the outcome may not be what you hoped for.

We look to the heat source and current control system before choosing a radiant panel system. Thermal Board is our choice in many applications but Thermo-Fin C is the best fit if the sub-floor is open and design temperatures fit. Once in a while we even install a radiant ceiling since the highest potential output and the lowest design temperature is always in a radiant ceiling panel.

The subfloor is open, but I feel that all of the heat would radiate into the basement, and I am not near finishing the basement, if that is even in my long term plans.

Also, I am not filling the space fromt he radiator with an appliance, i am planning to install a sliding door to the porch with the extra wall space.

Great, even more heat load and likely no floor area to make up for it :-).

The sub-floor radiant panels are always insulated whether over basement or crawl space.

Posted By BadgerBoilerMN on 22 Sep 2012 08:33 PM
Great, even more heat load and likely no floor area to make up for it :-).

The sub-floor radiant panels are always insulated whether over basement or crawl space.

... or even over another fully conditioned room, to isolate the zones.

Or at least they should be. Some people seem to be into wishful thinking or "97% reflective" foils, or somehow believe heat is formed of anti-gravity and can only flow up or other bizarre notions.

Above fully conditioned space R11/R13 batts is usually enough for zone isolation.  Over unvented semi-conditioned basements cheapie low density R19s are "good 'nuff", but other situations will often call for more/better.

If you want both floors heated (sounds like you don't), then you can get a radiant floor and a radiant ceiling in one shot - no insulation is an option. I don't recall the details I used (probably in-floor, like the OP), but 2/3 of the heat going upward is the figure I came up with.

I'm such a non believer in zone isolation that I leave most interior doors open :-). And my entire house is controlled by a single thermostat (all one zone but air).

that's a really bad idea Jon. unless your heatloads work out perfect you are going to be unhappy in most cases on one side or the other. you can't control one emitter from two spaces with any precision.

Except in this case we are talking about a small area. So, assuming an open floor plan, there is some other primary heating system that could hold upstairs/downstairs temperatures right at the desired values.

I have seen people do this by accident Jon, but they are much luckier people than I...or maybe they are just smarter.

Something to consider here- we have hydronics under gypcrete in our home in the high sierras. We used to have fiberglass insulation under the subfloor, but I use the crawlspace under the house for extra storage and it was getting disgusting with the mice running through the fiberglass. So to be "green", I replaced the fiberglass with soy foam insulation, not expecting a huge improvement in my ability to heat. Surprise!! That evening they finished adding the soy foam, the house felt over 10F warmer with the same heat settings. The soy foam made a HUGE improvement in the ability to heat the home. It's was expensive, but I think with the heating savings, it may pay for itself sooner than later.

Mice will chew through foam too, but until they do you won't have the air-drafts that can walk right through low-density fiberglass. It's not nearly as common as the nests you'll find in fiberglass, but rodents will even gnaw out a little cave in fairly hard closed-cell foam. If you don't have some sheathing between the foam and the crawlspace it's worth putting some up to protect the foam, even though the foam is pretty air-tight.

BTW: The soy content of soy-based foam is pretty small- only a fraction of just one of the two components that get mixed during the spray, but it's decent stuff.

@ bluebeard, how were you certain that soy spray foam would not cause damage to your radiant tubing? Everything I've read says the pex may deteriorate or weaken, voiding any warranties that may be in effect.

Could you show us a few of those readings that say the PEX may deteriorate with soy spray foam? Crosslinked polyethylene (PEX) is a remarkably stable material. That's why it is used for plumbing. My Uponor Wirsbo manuals stated categorically that the tubing was not affected by even the most harsh (polyurethane) foam sealants. When you get into code mandated fire blocking, it is another thing, primarily because of the fire resistant characteristics of the blocking and not so much the PEX compatibility.

Another situation might be when applying (large) amounts of sprayfoam. The heat of curing can be significant on deep applications when the PEX is embedded in the foam and may exceed the recommended PEX temperature. One consideration might be to limit the amount of spray foam that is applied at any one time, like 2" lifts. However, that would be up to the spray installers to know and watch for their particular products.

Pex will break down when in contact with petroleum-based adhesives, generally speaking. but you should send an MSDS to your tubing manufacturer for any products you are going to put in contact with PEX and get an official statement from them on the matter.

On the other hand, they make gas tanks out of polyethylene.

gas is also a very poor adhesive

Posted By NRT.Rob on 24 Dec 2012 02:50 PM
gas is also a very poor adhesive

But gasoline makes a great solvent for making pretty-good polystyrene adhesive, which is how EPS flotsam has become the third-world fisherman's adhesive and hull sealant of choice.  (First-world folks sometimes use citrus oil as a less flammable less toxic solvent for using scrap polystyrene as an adhesive.)