Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1204626
Healthy Indoors | 43 Furnaces and air conditioners are supposed to be sized to heat or cool on the coldest and hottest days of the year in your climate. At these temperatures the equipment should run continuously or near continuously. Calculating this "worst case load" takes some measuring and some math. The problem is without using a blower door to measure your home's leakage, and looking at your energy bills, siz- ing your HVAC is a wild ass guess, with a broad variance of answers. For a LOT of reasons (fear, greed, urgency, ambivalence) people tend to lean way to the high side of the range. Almost every furnace we've seen is at least twice as big as it needs to be. So why does that matter? About 60% of human comfort is based on radiant en- ergy. Think about how pleasing a bonfire or sunny day is. Those are both radiant energies, and our bodies love it. In our homes, radiant energy is given off (or sucked in) by the walls, floors, and ceilings around us. Right sized HVAC washes the walls, ceilings, and floors of your house with a slow steady stream of heat or cool. Nice even surface temperatures give off pleasing radiant energy just like a fire or the sun. If you want to be comfy, we need to keep those surfaces within 2 degrees of the thermostat set point. To do that, we want our HVAC to be running almost constantly putting out exactly as much heating or cooling as our home needs at that moment - this is called "load matching". The trouble is, most home heating and cooling sys- tems are single stage, meaning on or off. Compare that to your car: (Chapter 2 of HCB page 77) With an oversized furnace or air conditioner all they can give you is a blast of heating or cooling. One speed means you don't get "subtle." It's the Ice Bucket Challenge instead of a nice cool shower on a hot day. To do "subtle" we need right sized, multiple stage HVAC. I dig deep into this in Chapter 3 of The Home Comfort Book. How do you get right sized HVAC? Imagine going into a clothing store and being told that a small, medium, large, XL, or XXL could fit you. But the clothing is $8000 per item, you can't return it, and you can't try it on. That's about what typical load calculations end up being. If you measure how much a house leaks using a blow- er door test, run an energy model and reconcile with how much energy the house is actually using per year, and your load calculations get much more accurate. You'll know within ½ ton what size the house needs. Yes, perfect is hard to achieve, but we see most equipment oversized by 1.5 - 4 tons Once you install right sized, multiple stage HVAC, you'll notice how the rooms are more even, and the temperatures no longer race up and down. (The exception to this might be a very leaky room like a bonus room over a garage, and that would be uncovered during a blower door test that includes zonal measurements.) Chapter 3 page 102 Oversizing HVAC equipment is about the single biggest problem we see. Once installed, it's nearly impossible to reduce its output, so if you choose wrong, the only way to fix it is to buy another HVAC system. HVAC is only part of the comfort picture. If your house is leaky, you will have to address the leaks if you want com- fort. HVAC will only fix so much. So, if your house is leaky and you find yourself in need of HVAC, we advise installing equipment for the house you plan to have, not the one you have now. The house isn't going to be fantastically com- fortable anyway, and chances are it'll be more comfortable 90% of them time if you undersize than if you oversize. You want to size to where you are going, not where