Radiant Floor Heat in Slab-on-Grade

[Wes]

I have already searched the site for info and found lots of advice.

I have a client that would like to build a new home with a radiant floor heat system. He wants to have concrete floors throughout the first floor of the house with no flooring other than the finished concrete and a sealer, yep, no hardwood, carpet or tile. The site has a high water table, known termite activity on adjoining lots and sandy soil. I am in the bidding process of this now and would like to know some 'average' system costs for the heating system, i.e. tubing, boiler, manifold and all other accesories to make the system run. The house is about 1000sqft on the first floor with about 600 on top. I was thinking about pouring footings, using ICF's for 2' stem wall, backfilling with compacted rock 6 inches below top of forms, placing 6mil vapor barier, 2" ridgid insulation, rebar and pex tubing tied to the rebar.

The client does not require special 'zones' and would like the slab to heat at the same time. I read that there are limits to the length of each 'zone' and would want them to come on simultaneously so as to heat the entire floor evenly.

I assume that I would need to put radiant flooring in the 2nd floor as well.

I typically build with a crawlspace, would I be better off building my subfloor and pouring gysum on top with the tubing or go ahead with the slab? There are no radiant floor subs around and this would be my first. My plumber is putting in his first radiant system on another house but the contractor is running the tubing and concrete. It is also a slab on grade.

What I am really looking for is a cost for a radiant system in a slab above and beyond slab only. I would still build the foundation the same whether or not it had the radiant heat. Thanks for your help guys. This job is 30 miles from home so I am trying to find out if it is profitable to even take it on. Any help you guys can give is greatly appreciated!

[Mark G]

Re: Radiant Floor Heat in Slab-on-Grade

This website can give you a lot of info about the ins and outs of radiant heat plus price out a system for you: http://www.radiantdesigninstitute.com/

I suspect that an insulated slab would be the most effective especially with minimal floor covering. You could augment with solar and the mass would hold heat well into the evening.

[frenchie]

Re: Radiant Floor Heat in Slab-on-Grade

Hey, thanks, Mark. That's a great website.

[Don_in_Idaho]

Re: Radiant Floor Heat in Slab-on-Grade

If at all possible, radiant works best in a full slab. In the rocky mountain states, expecially areas where there is no AC needed almost all the new construction has in slab radiant.

In a well insulated house with 2" of foam under the slab, the standard that all contractors I've known use is 1/2" pex 12" oc and keep the loops to roughly 330'. This length maximized a 1000' roll and is easy to work with.

12" oc also equates to one linear foot of pex for each square foot of space. Easy part #1.

2" foam is roughly $2/sqft. Easy part #2.

Even if the client doesn't want different zones, they are required to ensure even heating. It's just not possible for a room with solar gain to be the same as on the back of the house. It's so easy to add thermastats to each major room that it's one of the most commonly overlooked aspect of a radiant system. Some people simply like to sleep at a lower or higher temp than others.

We stick to a simple layout with each zone equiped with it's own pump and each loop in that zone has a ball valve to adjust and balance the flow. Individual pumps for each zone means no control valves. No complicated control valve/pump setup means no expensive controller. Our heater's output temp is controlable internally so no mixing valves are used. Easy parts #3-#6.

A Takagi TK-Jr. tankless heater is $600 + vent pipe ($33/ft) and will electronically modulate the output temp up to 140,000 btu's, but also as low as 19k btu's.

This is the unit we use for everything and a big house could easily benefit from one on each floor. Easy part #7.

Convincing those who aren't used to hydronics in highly insulated houses that a system doesn't have to be any more difficult than this is the hard part.

One experienced person can probably prep the foam and lay 500' of pex in a good day if a pex stapler is used.

Not all pex is the same. Stick to regular pex for all in slab use. O2 barriers are of limited benefit, despite what the salesman says. 1000' of zurn pex at Lowes is likely to be 1/2 price of the local plumbing shop.

Best of luck

[Mark G]

Re: Radiant Floor Heat in Slab-on-Grade

Great to hear the practical side. Thanks.

Is anyone using electric boilers for radiant in conjunction with solar electricity and net metering?