Effect on mean radiant surface temperature by air heaters
I’m weighing up the pros and cons of hydronic heating versus multi split heat pumps.
Obviously heat pumps work by heating and circulating air within a space. Hydronic heating, depending on the type and position of emitters will radiate heat to people and room surfaces as well as (to some degree) heat some air and cause convection loops.
My question is about the effect of heat pump air based heaters. If they are kept on indefinitely over the colder months and keep the air at a pretty even temperature, won’t the room surfaces in contact with this air eventually warm up via conduction to something like what a radiant based system would have achieved? I know air holds very little heat, but given enough time, the walls would have to heat up, no? If the wall surfaces don’t heat up, how does heat get lost through the walls, given the very little heat radiation that air absorbs or gives off. And if the walls do heat up, won’t they then be providing radiant based heating for the people in the space, just like hydronic / radiant systems do?
The thinking behind the question relates to human factors and thermal comfort. I’m just trying to understand why modern heat pumps anectodally provide great thermal comfort, despite the proponents of radiant based systems selling the importance of mean surface radiant temperature to the world. Any thoughts on this very academic and less pragmatic topic? Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
George,
I don't think it is a "less pragmatic" topic at all. It's a very interesting one.
Any constant air-source heating in a well insulated and air-sealed dwelling should keep the surfaces in the room, including the walls and floors, pretty close to the ambient air temperature. That means they will all be cool to the touch - and they will vary in how cold they feel depending on what they are made of. For them to heat up to a point where they radiate heat back that the occupants will feel, they and the air would have to be above body temperature.
The last point is true of radiant heat systems too. So while you may feel heat from a point source system like a wood stove, a more diffuse source, like in-slab heating, would only feel warm to the touch if it was hotter than 98F. And in a well insulated house you don't need anything like that temperature in the slab to maintain a room at 70F. So from the point of view of the occupant you are left with the air being a very similar temperature in either case and the surfaces being slightly warmer with radiant systems.
The benefits in comfort of a radiant system appear to be in two situations. A poorly insulated and air-sealed house where the radiating surface can be kept very hot, or small point sources, like a heated bathroom floor, where again the temperature can be above body temperature.
Does that sound fair?
I think that "why modern heat pumps anectodally provide great thermal comfort" is because they are usually installed in buildings with excellent envelopes, and the envelope provides the thermal comfort.
Around here, heating systems are usually hydronic and are usually installed by plumbers. One of them told me me encourages all the builders he works with to go to the Vermont state workshops on designing and building better envelopes, with the result that his customers think he did a great job on the heating system, even though his work had little to do with the comfort they experience.
George,
Discussions of "mean radiant temperature" are basically discussions about windows during the winter. In a house with code-minimum levels of insulation, the drywall will be at a comfortable temperature. If you specify good windows, and you pay attention to air leakage, it doesn't matter what type of heating system you have.
For more information on this issue, see Comfort Problems Related to Radiation.
Thanks everyone for your input.
Malcolm - Thanks for clarifying in your detailed answer. My question was a bit vague. When referring to surfaces radiating heat, I meant with respect to negating some of the heat loss from our bodies (decreasing net heat loss) rather than actually being a source of point heating. The comment about low temperature radiant heating surfaces being cool to the touch makes complete sense (the walking on warm heated slab and house overheating problem).
Charlie - interesting point. I might've fallen for the same human tendency as his customers. It's easy to focus on the active process fighting for you, than it is to focus on the passive process fighting against you. Maybe the mean surface temperatures have more to do with the R value of the material beneath the surfaces (as Martin has alluded to with the window discomfort issue) than wih how the room is heated. Thanks all for the basic physics refresher. Simple stuff in hindsight!