I’m about to move into a new house that I’ve had constructed with radiant heat in a slab, but there are a couple of problems. The house is in Santa Fe, New Mexico, where in the winter the temperatures are very often below freezing but rarely below zero.
The heat has been on for a couple of months, and our bills are very high compared to what we were expecting. I did some looking around and every single web site with construction techniques says you have to insulate below the slab. I don’t think this was done. Does anyone here have experience what the real standard practice is on this?
The other issue is that when we run cold water from the tap it runs warm or hot for several seconds before running cold. It seems to be picking up a lot of heat during its run through the slab. Anybody here know if that is typical?
Thanks for any information.
-Steve Karmesin
Replies
RFH systems can take some effort to balance. Is the builder working with you to explain things?
If a cold water line is running through a heated slab, it should be insulated. With rfh slabs, I've joked before about needing a cold-water recirculating line.
Most people recommend insulating the slab, though I've heard from some who don't think it's important. The importance may depend on the design of your house...type of foundation, type of construction, solar exposure, etc.
In general, a load calculation should have been performed, and the heating system should be sufficient for meeting that load. Is the building heating sufficiently?
On what basis did you base your heating bill expectations? If the building is heating properly, and the cost is too high, then one possibility is that your expectations were...ambitious. Can't really tell without more info. Can you share some of your cost data...size of house, fuel source, utility bills, etc?
> RFH systems can take some effort to balance. Is the builder working > with you to explain things?I've talked with him about the hot water from the cold tap, and he's generally been OK about it. The leading theory right now is that the hot water recirculation pipes are laid right next to the cold pipes, and that since neither are insulated you get the obvious effect. I've let the plumber know how unhappy I would be if that is the case. That seems like a very basic error.> If a cold water line is running through a heated slab, it should be > insulated.It isn't.> In general, a load calculation should have been performed, and the
> heating system should be sufficient for meeting that load. Is the '
> building heating sufficiently?It is, it just seems to be using more fuel than I thought.> On what basis did you base your heating bill expectations? The house is an otherwise well insulated 2000 square feet. Our builder told us to expect about $100/mo, and we're seeing about $300/mo for natural gas. Temperatures have been in the 20-40F range mostly.-Steve
Steve, We are RFH users since 89 in 2700 sf house. Back then, highest nat gas bill here in NW Ohio for the floor heat only was 45.00, add 15 for dom. hw, dryer and cooking. This winter, 140 total in the coldest mo. Gas has gone up and surprisingly, way more than southern michigan. You need to figure it on comsumption, not so much $'s and cents. Additionally, what temp are you running, and what is the rest of the envelop like re: insulation, area of glass, potential for passive solar gain?
I insulated with 1.5" foam under entire slab whereas now some thought is to just come in about 4 ft from ext. walls. Supposedly the ground won't rob you of heat. I don't know, I would still fully insulate. Additionally, my heat runs in a slab, so I insulated between the slab and foundation, again with foam so heat wouldn't be wicked out of the slab. The cold water running in a heated slab is just plain dumb attack. Could they have run the water lines in the fill below the concrete? If so, maybe the circ hw is the culprit and not the slab. If it's down there and picking up heat from the slab, then you just answered the question of the ground stealing some of the heat out of an uninsulated slab.
Provide some more details on what runs on gas in that house? thickness of slab, floor covering , etcRemodeling Contractor just outside the Glass City.
Quittin' Time
>The house is an otherwise well insulated 2000 square feet. Our builder told us to expect about $100/mo, and we're seeing about $300/mo for natural gas. Temperatures have been in the 20-40F range mostly.Sounds expensive. Can you check the bill to see the energy equivalent? It'd be good to see the btu's used by the house. Can't imagine that an uninsulated slab could account for such a major gap by itself. You sure your windows are closed? No holes in the wall? I'm heating about 6000 sf in about the same temps (or colder) w/ rfh and a slab that's insulated in the perimeter 3' and running maybe 150-180/m for ALL utilities.
You should have the foam under the radiant, it keep the ground from pulling the heat from the pex. Here's a link that shows how it should look for all type of floors. http://www.tjsradiantheat.com/radiantheating.html
Also, when you say your cold tap water is hot for a few seconds, I seriously doubt that has anything to do with the radiant if it was done right. The water running through the pex radiant is only 75 degrees or so and your domestic hot for a faucet is 120 degrees max and 110 degrees max for showers (That's Mass code, your will probably be pretty close). Your not going to feel the 75 degree pex as anything more than lukewarm so the heat is coming from something else.
dday, are you running your water @ 75 degrees through that slab?Remodeling Contractor just outside the Glass City.
Quittin' Time
karmesin
I'm writing from the Canadian Maritimes. I've got a small chapel (650 sqft) with slab & inFRH. We used a 6mil vapor barrier, 2" of real stryofoam 4" of concrete and an electric bioler now running at 5kw total input. The walls are ICF and the roof has 9" of fiberglass batts. When the heater is off and the candles are burning for Mass the temp actually raises. Yes, the slab should have been insulated. What is the temp of the water entering the slab, what is the temp of the water returning? What is the control system for the rariant heat? I have a Techmar 262. How many zones? Hot water cross contamination: what is the actual temp that you begin to run the tap? There may be a bigger problem than just the heat from the slab. One more thing - radiant heat is not stopped by regular insulation but it is REFLECTED by a heat mirror such as shinny aluminum foil. I also used a product that is foil, two layers of bubble wrap and a layer of foil in the ceiling and coated the inside of the siding with foil to reflect the heat back into the building. Does it work? How about 68deg on one side and ice one the other in 0 deg f weather. Most of this is too late as the building is already up but maybe it will give some others an idea or two.
Most insulate with a layer of 2" rigid foam under the slab. I have gone a step further by getting a flexible insulation that is two 1/4" thick layers of closed-cell foam separated by a layer of foil. It's a radiant reflector, vapor barrier and insulation all in one. You should have the insulation under the slab. Basically your slab is heating the ground under it.
Take the difference between the heated slab and the average soil temperature and multiply it by the values in the chart based on soil conditions to obtain the heat loss without insulation.
Note the influence of moisture.
This is one of the critical areas we teach our students when it comes to radiant design...in <!----><!----><!---->Canada<!----><!----> the B214 Code requires slab insulation (a good thing).<!----><!---->
We also teach our students to be very careful of what types of insulation to specify under slabs. The standard of acceptance in the industry is extruded polystyrene insulation. Be forwarned that the foil bubble stuff has been in and out of the industry over my 25 year career and though it has applications - under the slab in this writers opinion is not one (at this point). Have been waiting for over a decade to see a long term integrity test by a reputable testing agency...something that can prove these products will be as good in year 5, 10, 15 and 20 as it is claimed to be in year one...not one report has yet to cross my desk- zero - zip.
From the 2004 Canadian Mortgage and Housing Corporation study, "The bubble-pack insulation had a low insulating value compared to the polyurethane panels and the XPS board.The cost-benefit was the poorest of all insulating materials tested."<!---->
From U.S. Department of Energy /The Oak Ridge National Laboratory (ORNL) "Most of the field tests have been done with clean radiant barriers. Laboratory measurements have shown that dust on the surface of aluminum foil increases the emissivity and decreases the reflectivity. This means that dust or other particles on the exposed surface of a radiant barrier will reduce its effectiveness. Radiant barriers installed in locations that collect dust or other surface contaminants will have a decreasing benefit to the homeowner over time. <!---->
And probably the most entertaining of all reads comes from the the 2003 issue of Energy Design Updates where numerous manufacturers of the foil bubble products retracted statements with some making apologies for their claims. <!---->
Radiant heating and cooling is pure physics and engineering and when it is not treated as a popular science expiment can be the most wonderful way to do provide sensible space conditioning.
There is just so much solid information available today on radiant that there is no execuse for poor systems. I feel for those who have been hood winked.
Do your homework.RBean
http://www.healthyheating.com
http://www.healthyheating.com/blog