Extensive research into the state of the HVAC art has revealed more questions than it has answered. How do I set the thermostat in my new row house to maximize both comfort and efficiency?
My wife and I just bought (yay!) a row house in Washington, DC. While the "reclaimed swamp" thing is inaccurate
, we do experience high humidity from roughly April to September, with representative dew points this week above 70° F.
The house was built in 1924 and was renovated in 2009, with a 3.5 ton, single stage heat pump installed at that time. There are two floors above grade and a fully finished basement, with three bedrooms on the top floor and a guest bedroom in the basement. Our inspector noted that airflow in the smaller bedrooms upstairs seemed restricted, but it seems likely this was caused by sloppy duct design and not just a blockage that could be cleared. We don't have a zoned system, and the single thermostat is located on the first floor above grade.
Our first change in the house was to replace the existing, non-programmable thermostat with a Nest. We've already found that the first floor is comfortable set at 75° F, but the bedrooms are a bit warm and still with the thermostat set that way. The basement, of course, is relatively cool compared to the upper floors.
The Nest offers a great range of functionality, but I'm at a loss as to how best to set it for our purposes.
- It offers a "cool to dry" feature that will turn the AC on if the humidity passes a set point (55%, I believe), but the documentation just points out that this may be expensive, suggesting a dehumidifier might be a better choice (uh, maybe not so much). Since it says it will cool up to 5 degrees below your set temperature, I have turned it on (for now) and raised our away temperature a few more degrees, but I'm still unsure of the best advice here.
- It offers a feature called "Airwave" that will run the fan after the compressor shuts off, which (according to several sources) could result in re-evaporation of previously removed moisture. Doesn't that effectively contradict using the AC as a dehumidifier?
- It offers "every day" fan scheduling for set durations (e.g. run for 15 minutes every hour from 10 PM to 8PM), but they have a whole blog post pointing out how this could get expensive.
Further, the internet is a minefield of questions about closing off registers
and/or doors in unused rooms, with some received wisdom stating that you should never close registers because doing so could increase back pressure in your ducts and cause leaks, but with other received wisdom stating you should switch your upstairs and downstairs registers twice a year (cool the upstairs, heat the basement, let gravity help otherwise). And also, a well designed multiple floor system will supposedly have balanced returns and registers on each floor so each one gets an equal exchange of fresh air and they are "pooled" as they pass through the air handler … but we have no indication our system was designed well.
So, AskMe: what is best?
- Should we close registers in unused rooms?
- Should we open and close registers upstairs and down on a seasonal schedule?
- Since an air conditioner is an effective dehumidifier that doesn't add heat back into the house, is there any reason not to turn on the "cool to dry" mode on the Nest?
- How much, really, will running the fan without the condensor cause the humidity in the house to increase as it evaporates moisture from idle coils?
I know I'm supposed to put these questions to a licensed HVAC tech, but a good tech would tell me what I already know (that I'd be better off with a gas furnace and standalone AC), and a bad one is responsible for the poor duct design and choice of heat pump in the first place. So how do I make the best of it?