Less expensive radiant heat
I’m a developer in the design stages of relatively affordable homes for the urban infill. I’m trying to find ways of keeping the heating cost down. I think everyone deserves the comfort of radiant heat, but no developers are providing it, since it usually adds too much cost. Dry sinuses from scorched air heat is a common complaint.
My idea is similar to Ultra-Fin, which heats the air in the joist space below the floor with fins on tubes. A potentially less expensive method would be a fan coil and blower moving warm air beneath the floor surface. Instead of floor joists, open floor trusses would be preferable if not required. You couldn’t have separate zones on a particular floor. This isn’t a problem in due to the small house footprint.
Ultra-Fin uses up to 180 deg. water. Probably creates 130 deg. air in the joist cavity.
IRC disallows the joist cavity to be used for “supply air”, but this isn’t really supply air, since it never leaves the joist cavity plenum.
A similar system is in the “Passive Solar Handbook” but the plenum is made of concrete block laid on its side.
I’m thinking this has probably been done before, has anyone seen it?
Replies
I can't answer your question about the practicality of heating through a wooden floor with hot air below.
But in your initial statement you said the reason you wanted something other than forced air is because of dry indoor conditions being uncomfortable. The moisture content of air has nothing to do directly with the type of heat. It is affected by the number of air changes in the house, so is indirectly affected by the choice of heating system. Living in a house produces humidity from bathing, laundry, cooking, etc. If there is a lot of outside air coming in the moisture is carried away and the house is dry. (I'm talking winter in a cold climate.) If there are few air changes then moisture builds up. If the air changes are insufficient moisture can go too high, leading to mold and wet windows.
If either hydronic heat or forced air heat are used, and the boiler or furnace are fired with a fuel, as opposed to electricity, then the amount of air exhausted from the heating plant would be roughly the same in either case. So, humidity would be about the same. If either used outside air for combustion the amount of air changes would be lower, so all other things being equal, the humidity would be higher.
If the house is built tightly, the normal moisture from the daily activities is often sufficient, sometimes even excessive, for proper indoor humidity. It would not matter what type of heating plant was used, if it were standard oil or gas combustion. Wood heat is properly called "a dry heat", because of the very large amount of outdoor air draw in into homes heated by wood.
One advantage of forced air heat is that humidity is easily added with a humidifier installed at the furnace. I suspect that low end homes are dry with forced air heat because low end homes are not equipped with humidifiers. With radiant heat, adding humidity is difficult. I think you would better solve your problem, at a lower cost, with a forced air system with a humidifier. I don't think switching to radiant heat will accomplish what you desire.
Wayne,
You make some good points, but humidity is only half the issue. A radiant system will have lower room air temps. That also helps reduce dry sinuses. At all temps, radiant is more comfortable for the occupants. This "extra comfort" is a growing market. If radiant cost = forced air cost, everyone would choose radiant as soon as they knew about the advantages.
Forced air systems always draw a lot of new dry air into the house. The reason is that some rooms have supply ducts and no returns, and other areas have returns but no supplys. Even a well designed and installed system with well matched supply and return ducts still can't be balanced. Therefore almost all the rooms in the house are above or below atmospheric pressure. That can drive a lot of air in and out (less of a problem as homes get tighter).
Speaking of tight homes, either type of system would need an HRV. Codes I'm sure will require them soon.
The claim that, with radiant floor heat thermostats are set lower because it feels warmer is often made by the radiant floor industry, and repeated around as if it's general knowledge, but research does not show that to be the case. A few years ago there was a small study in Canada to determine if the claim has merit. They found that, in actual homes with ordinary families people did not set the thermostats any lower in homes with radiant floor heat compared with matching homes equipped with forced air heat.
Besides the ability to control humidity, another advantage of forced air heat is the ability to clean the air. Especially with an electronic air cleaner, the amount of dust can be significantly reduced compared to a system that does not filter air.
I've lived with both types of systems, in fact, one home had a split system with part of the home as forced air and part with radiant floor heat. The radiant heat was comfortable on the feet. But I found the long heat up time with radiant heat and the overshooting of temperature when kicking the thermostat up 5° to be bothersome.
Many people find radiant heat comfortable and prefer it over forced air. For me, giving up humidity control, electronic air cleaning, quick response and precise temperature control, and installation cost isn't worth the advantage of warm feet.
Radiant heat would be wonderful in a shop. There, the lack of filters and a heat exchanger where sawdust could gather is a real advantage. Bringing in a vehicle in the winter and working on a nice warm floor is a pleasure.
"Even a well designed and installed system with well matched supply and return ducts still can't be balanced. " Only a well designed system can be balanced and all well designed systems can be balanced. If the system cannot be balanced, then it is NOT well designed or properly installed.
"Forced air systems always draw a lot of new dry air into the house. " BS. Poorly designed/installed systems might. Most air inleakage is caused by no makeup for exhausts, no combustion air provided to gas/oil burning appliances and by differential pressures due to wind.
"At all temps, radiant is more comfortable for the occupants." If you had experienced a decent or even good forced air system, you might reconsider this as an absolute. Most uninformed people will agree with you. Well designed and properly installed/balanced and controlled forced air systems are rare in the commercial world and non-existant in the typical residential market.
"If radiant cost = forced air cost, everyone would choose radiant as soon as they knew about the advantages." Unless of course, they wanted air conditioning, dehumidification or humidity control as well as heat.
"Well designed and properly installed/balanced and controlled forced air systems are rare in the commercial world and non-existant in the typical residential market."
So all residential systems have rooms above or below ambient air pressure, which drives air in or out of the house through the exterior wall.
"So all residential systems have rooms above or below ambient air pressure, which drives air in or out of the house through the exterior wall."
No. Inleakage/Outleakage is primarily caused by wind induced differential pressure and lack of makeup air being provided for combustion and exhaust.
The slight variations in pressure from area to area within the envelope (with a few notable exceptions) even in a poor system are insignificant in terms of forced leakage. One exception, not applicable to msot residences is the use of a return plenum ceiling.
If the 'budget' homes that you build are of such poor quality that a few hundredths of an inch of static pressure causes significant leakage, then the design/installation of the HVAC systems is the least of your victims' concerns.
Timbo,
After a little more research, I find your right! Check out this article: http://www.energy.state.or.us/res/tax/Radiant.htm
So I think if I spend $200 per bathroom for that electric infloor heat, the tenants and buyers will love it.
KD,
I apologize for the tone.
I have discussed this at length many contractors and builders. In my previous life, I spent a lot of time troubleshooting poorly functioning HVAC systems in residences, commercial buildings, schools, etc., and providing plans and/or specifications and recommendations to remedy what usually turned out to be half-a$$ed installations or what I like to refer to as "design by installer" meaning some barley high school educated, union tinner, did whatever was easiest, so he could get to his breaktime quicker.
Good design of these systems is very basic, as is decent installation.
BTW, the electric heat mat in the bathroom floor, is a very simple and wonderful touch to make a place more comfortable. I have 4 installed in my home and love them.
Tim
Edited 8/4/2004 8:43 am ET by Timbo
Tim-
Can you share the name of the electric mat heating manufacturer? I have installed embedded radiant heating (water)in concrete in about half the house and was intending to continue with the existing baths. Obviously floor thickness is an issue to be dealt with.
The baths currently have SlantFin copper baseboard which does nothing for the bare feet, and is dependent upon a thermostat not located within the bath.
Thanks-
Windy(:-)
Windy Wood
From the Helderberg Mountains
I have installed two different versions.
One is manufacured by Infloor and consisted of a roll of wire held in place by clips and came with a simple thermostat that works off of air temp and has an internal high temp limit (adjustable) for the floor temp, air temp settining is adjustable and it has an on/off switch. The wires required about 3/8"of thinset to be covered completely. I have this one installed in the master bath floor.
The other one, which I have 4 installed in my house (only one of the former) , one with two mats on one t-stat, is made by NuHeat. This version consists of a fibrous mat with much smaller wires imbedded, comes with a programmable thermostat and controls soley on floor temp. Easier to install, but more expensive.
Both have their better features. Both are "required" to run on a dedicated 20A, 120v circuit, though even the one with two mats on a single control draw no more than 10A. The NuHeat stats have built-in GCFI, the Infloor system requires this be done separately. Infloor was less expensive, but was more work to install. The programmable stat is nice in certain applications. For a bathroom, I like the air temp control. When my furnace goes on night set-back, the floor heat comes on. Really nice in the middle of the night with bare feet on tile. When the furnace warms up the space enough, the floor heat will go off, depending on how its set.
Suntouch - Lowe'sLes Barrett Quality Construction
Here's a very interesting way to do radiant heat:
http://www.sipweb.com/monitor/bc_08.25.2004.asp
Shouldn't cost more than a forced air system.
It is interesting. I've seen a similar design that uses concrete blocks on their sides with the cores line up north-south and a manifold at each end then covered with concrete slab. I specified that for a house I designed for a competition to make a better Habitat for Humanity house. (Didn't win.) I coupled mine with tanks of water that the sun heated and tied that in with a heat pump (for North Carolina climate). The only problem I've heard of is recirculating air through this eventually results in moisture in the comcrete blocks and then they can get musty. I guess with proper humidity control it could work. I like the concept, and it is simple and easy to understand.
Interesting, but looks complicated to construct, setup, and balance.
I just wonder how much you can really save over a conventional hydronic system.
In some ways, what you are proposing is similar to a suspended tube system, in the sense that the heat is transfered to the air, then transfered from the air to the floor.
Some of the problems include:
1) Difficulty of estimating heat transfer.
2) Difficulty of insulating below to keep hot air trapped very well.
3) High temps could cause expansion and contraction problems in framing and subfloor.
4) Difficulty of controlling and directing airflow within cavities.
While I appreciate your out-of-the-box thinking, I suspect that by the time you put the whole custom air system together, I'm not sure that you would come out ahead in terms of cost. If you want to stay underfloor, consider conventional staple-up. Heat transfer by conduction is more efficient and predictible. Prices for materials have fallen in recent years.