[ Read Responses | Return to the Index ]
[ ]

Posted by Corley on Saturday, 6 February 2010, at 10:25 p.m.,
in response to Re: Best way to install radiant, posted by BC

Can't say as how I can buy the 20' around the perimeter method at all. First, heat transfers through different mechanizms. Convection, radiant, and conduction. When I turn on my floor circulators and the floor is cold, before long, I start to feel warmth due to the radiant heat transfer, even though the slab is still pretty cold. Conduction and heating of the slab takes a couple of days.

Bleed off of heat to it's surrounding surfaces through conduction always happens. You can't stop it, no matter how much insulation you have, you can only slow it. The bigger the surface area that is bleeding off the heat, the more heat will be lost. Don't kid yourself with the notion of this huge heat sink being wonderful to store and hold all your heat, the bigger it is, the more surface area it has, and therefore, the more heat loss it has. You will be leaking heat, it's unavoidable, it's just a matter of how much surface area and the level of insulation that you have.

As far as storing heat, what good does that do anyway? If you could show me that you can generate heat at certain times for cheap, and at other times it's very expensive to generate, then storing enough to cover you through that expensive time period would be great. For example, if your source is the sun, then you may want to store enough to cover keeping you warm when the sun is not out. Any more, and you are just leaking away energy for no good purpose. (If your source is coming to you through wires, then you may find that your power is actually meterred cheaper at night than during the daytime when businesses are using it, as is done is many areas of the country.)

It's pretty much the same argument for/against large hot water storage tanks. If 100 gallons gets pumped through your floors in 5 or 10 minutes, (10 - 20 GPM) but it takes a day or more to heat the floor, what good is a storage tank of 100 or even 1000 gallons anyway? It just provides more surface area and leaks off heat. Of course if the big storage tank is inside the heated and insulated envolope of the home, nothing it really lost, except when it is a hot day and you don't want that heat leaking into your environment. Heat will always seek cold, and transfer to it. Storing extra heat in any form that is not inside the insulated envolope should not be thought of as wonderful, but instead as wasteful of the energy that it takes to generate it. Heating the earth under or around the insulated envolope is pure waste and you should never do it on purpose.

Of course, moderation is the key, and a well insulated 4" - 6" slab seems like enough heat storage, but not excessive to the point of being wasteful. If the slab is not insulated, then unless you can heat the entire earth to the desired temp, heat will continue to bleed off into the earth, be it down or out the sides, wherever there is a difference in temps, it will bleed off at the rate of the conduction of the materials involved. Heat seeks cold. Always and always. The earth is a pretty good conductor of heat, else why would anyone use it for ground source loops?

Although I did not install it in my new home, I have come to think that probably a layer of reflective metal foil on top of the under floor slab insulation is a good idea, to help capture the radiated heat and reflect it upward. It's not a substitute for good insulation that will slow down the conductive heat loss though.

Heat transfer from convection is pretty much not an issue here, only radiation and conduction.

JMHO
Corley

PS A great way to hold your floor loops in place while pouring the concrete is to put down the insulation, lay down the foil layer, lay down some light weight wire mesh 4'X8' flat panels, then tie the pex loops to the wire mesh panels. I used the big plastic staples into the foam insulation, and while it worked, I had a lot of them not hold too well, so would not use that method again. Don't forget you'll probably want more wire or rebar half way up into the concrete too, but keep the PEX toward the bottom so if you have to sink anchors you won't hit/damage it.

PS/2 After you have been away for a couple of weeks with the heat turned down low in the cold weather, you will appreciate having the slab come back up to temp in a day or two instead of the week or two it takes if you have no insulation and are heating mother earth as well. Insulate the slab, you will like it.

Responses

• Re: Best way to install radiant - [Radiant Heating & Cooling] - Brian -- 10 February 2010, at 4:38 a.m.

© 2000-2010, Phil and Lisa's relaxed lifestyle home. An exercise in Energy Smart, Not So Big living. www.OurCoolHouse.com - Ideas@OurCoolHouse.com

	Web OurCoolHouse.com

This site is all about building a cool, energy efficient house, that makes maximum use of earth sheltered design, passive solar heating and cooling, geothermal exchange energy management, and right sizing of the house for it's designated use. The home's placement is on a south-facing hillside in Deep Creek Lake, Maryland. This site describes the design process, the technologies used and the expected results. We also have a comprehensive Links Page for anyone who is also interested in designing a similar project.