Saturday, March 15, 2014

Heating and Cooling


I think we're going to use a hydronic underfloor radiant heat system. Underfloor radiant heat is supposed to be considerably more efficient than forced air, because it provides better heat distribution. As the image shows, forced air heats rooms unevenly, and the hot air tends to accumulate at the ceilings, particularly in homes with high ceilings. I'm not sure the image is completely accurate; other resources I find on the web indicate that heat still accumulates near the ceiling, which makes a lot more sense thermodynamically than what the image shows.

However, radiant floor systems do produce a much smaller difference in heat between ceiling and floor. While forced air systems often have 15+-degree differences between the cool floor areas away from heat registers and high points in ceilings, radiant systems generally see no more than 2-4 degrees difference. This makes radiant heat systems more efficient, since you're not overheating one area in order to achieve a comfortable temperature elsewhere.

They're also, by all accounts, simply more comfortable. Since the heat comes from below, radiant head means that your feet will be warm. I tend to have problems with perpetually-cold feet, so that sounds really nice to me. Plus, imagine never stepping onto a cold floor in winter time!


For cooling, hydronic floor systems are also useful, but only to a limited degree. Cooling the floors below about 68 degrees is said to cause in-floor condensation, which is bad. I'm not sure that is as great a problem in Utah, given how dry the Rocky Mountain air is, but it's probably a good guideline. Still, hydronic radiant systems can do a great deal to cool a home as long as the air temperatures don't get too high. Given that Morgan isn't that hot in the summer, and given that the air cools down significantly at night, I think we may actually be able to avoid using air conditioning most, if not all, of the time.

A key component of making summer cooling work well is another high-tech element I'm planning: automated windows.

Linear chain-drive motors like the one shown above cost about $200 each, but should last for decades with no maintenance, and are easy to integrate into a home automation system. My theory is that if I can reliably open the windows in the summer evenings, as soon as outdoor temperatures drop below indoor temperatures (assuming indoor temps aren't already too low), then we should be able to cool the air and all surfaces inside the house each evening. Combined with the cool floors -- particularly with floors that have high thermal mass (more on that in a bit), we should be able to have comfortable temperatures throughout the bulk of the day.


Of course, a key to keeping the house comfortable year-round, inexpensively, will be insulation. Based on my research, I think we want to shoot for wall insulation factors around R-30, with a bit more in the ceiling, about R-50. In practice it appears there is very little difference between R-30 and R-40, or higher.


The hard part of insulation, though, is the windows. And we have lots of windows. Big windows. Kristanne likes windows. In particular the great room has windows 13' long and 18' high. That's 234 square feet of window, which is going to lose a lot of heat in the winter and will also produce significant solar heating in the summer. Even the most efficient triple-pane, argon/krypton-filled windows are only about R-6.

The insolation (solar heating) factor will be reduced by the fact that the window is facing northwest, and there is also a mountain range nearby to the west, which reduces the duration of direct sunlight. An open question is whether we should get high, medium or low solar heat gain coefficient (SHGC) windows. SHGC is a measure of how much solar heat and visible light the windows allow to pass through. Low solar gain (LSG) windows have multiple layers of reflecting films that reduce the amount of solar energy that passes through.

Medium Solar Gain Windows

I think we probably want low or medium-gain windows. It would be attractive to use HSG windows to get a little solar heat during the winter, but I don't think we'll get any significant amount of direct sun to the big windows in the winter.

Unrelated to heating and cooling, there's one other thing I'd really like to do with a lot of our windows, if we can: get German-style tilt-and-turn windows. If you've never seen them, they're awesome.
I don't know how available they're going to be in the US, or what they'll cost, though.

Heating Zones

I originally thought I wanted to have per-room heating zones. Basically, each zone in a radiant system consists of a thermostat, a pump and a set of in-floor tubes. My theory was that per-room zones would provide great flexibility, but everything I read says that small zones just add cost and complexity without really making that much difference. Radiant heat is slow and pervasive so you can't really do things like focusing the heating on the areas where people are.

What does make sense is to separate out zones that may need more or less heat on a longer-term basis, such as rarely-used guest rooms (which we don't have), or areas which need more heat input because they experience more heat loss, like rooms with lots of glass. That we do have.

So I think we'll mostly do just one zone per floor, but with the great room as a separate zone, and probably a zone for the master suite, mainly because of the sun room which has the potential to be a heat-sucking area in winter.


Saving the most interesting for last, a really important question is what energy source will be used to heat (and maybe cool) the hydraulic fluid? There are lots of options. In fact that's a great thing about radiant; any approach you can think of to heat or cool water can be used. Some options:
  • Electricity (heat/cool)
  • Heating oil (heat only)
  • Natural gas (heat only)
  • Solar (heat only, unless it's photovoltaic, which means electricity)
  • Geothermal ground loop (heat/cool)
Further, it is even theoretically possible to use multiple energy sources, mixing and matching as appropriate. For example, you could use solar to pre-heat the water that goes into a gas-powered boiler.

I need to see what the costs will be but I'm leaning strongly towards a geothermal ground loop as the primary source. I'm not a green freak, but I really like the idea of drawing most of the heating / cooling energy from the ground, which means ultimately from the sun. There are some electric water pumps that have to be run, and an electric heat pump, but even though electricity is far more expensive than, for example, natural gas. From what I read, return on investment is generally pretty good, only a 4-8 years, assuming you are already spending on a radiant system.

If it's not geothermal, I think I'll probably want to go with a gas-fired boiler (or high-efficiency water heater), plus maybe some solar pre-heating. With that approach, I'm not sure how you'd do cooling. I need to do more research. I'll follow up with another post when I know more about that.

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