Whole-house fan, residential economizer, or low ambient kit?
I’m on the southern edge of Zone 5A. As I am nearing the point where I will start construction, I’m working with a contractor and subcontractors on some of the details.
A common issue with prior houses was having a house that was too warm, when the temperatures were cool outside in the spring and especially the fall. This was an issue when it was cooler than the recommended temperature for running the air conditioner. I had a whole house fan that dealt with that quite well, and even reduced the number of weeks I needed tor run the air conditioner, particularly in the fall when the dew points were relatively low.
On the new house, I could ask for them to install an insulated one like the Tamarack, and it wouldn’t need to be a very high CFM unit because of house I would use the whole house fan. However, it seems like there may be other options, but I’m not sure if they are better.
They don’t seem that popular, but I have read that residential economizers exist. That would filter the incoming air.
Since the house will be insulated significantly better than minimum code, I wondered if a low ambient temperature kit for the air conditioner might even be a better option as my cooling load is likely to be quite low.
Before someone asks, “why don’t you open a window?” this never really seemed to work that well, especially if I had been using the oven or the daytime temperatures were high. While this house will be very well insulated, which might make this issue less significant, I’ll have about 5-6% south facing glass which could heat up the house on warmer days in late September and October when the overhang isn’t shading the south side of the house. Aside from the whole house fan, economizer and low ambient kit, there another option I should consider?
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Replies
User-7027358,
First of all, can you tell us your name?
Of the three options you mention, a whole-house fan makes the most sense. It's the simplest.
The cost of installing so-called "economizer" equipment (basically, a large outside air duct connected to a large grille, with motorized dampers and controls) is usually too high to justify in a residential application.
I've done some Googling to learn more about low ambient kits (used to improve air conditioner performance when air conditioners are operated during low outdoor temperatures), and I understand the principle. That said, such a kit shouldn't be needed with a well designed residential system.
It looks like my name is showing up correctly now, I've posted here before so I'm not sure why that was. In any case I appreciate the response. Whole house fans worked well for me in the past, and I never minded the activity of making sure plenty of windows were open and turning on the fan when dew-points were low in the spring and fall. The fresh air always seemed nice too.
As for the whole house fan, other than being insulated, is there anything else I should look out for? Are they given an air infiltration rating? Given that it would only be used when it was significantly cooler outside, not as an air conditioner replacement, do you think 1000 CFM would be sufficient for about 1800 sqft of space? Are there any other special considerations given that I will have about 2 feet of insulation in the vented attic?
I can tell you that 1 air change per hour at 5C difference doesn't do much. It reduces the temp 1-2 degrees in 8 hours.
You need bigger temperature differences and/or more air flow. That said i have not found high cfm fans for very cheap but i do know the Vornado cfm numbers are garbage (or perhaps just the model i bought and exchanged twice)
I don't know what a low ambient temperature kit for the air conditioner or an economizer is.
Shane,
You should read this GBA article about whole-house fans: "Fans in the Attic."
Here are two quotes from the article:
1. "The traditional recommendation is to choose a fan that can move between 15 and 20 air changes per hour (ach). If you’re aiming for 15 ach, that means you need to divide your home’s volume by 4 to obtain the cfm rating of your fan. If your ceiling height is between 8 and 9 feet, just multiply the floor area of your house by 3 to obtain the cfm rating of your fan."
2. "You need one square foot of net free vent area for every 750 cfm of fan capacity. The vent area can be made up of a combination of soffit vents, ridge vents, and gable vents. If the vent has insect screening, remember to make the opening 50% larger than the rule of thumb dictates. It’s better to have too much vent area than not enough."
There is more information worth reading in the article, as well as equipment suggestions.
Generally a low ambient kit is needed in office building with lots of workers and computer in small cubical, they produce enough BTUs that they end up cooling 10 months of the year.
Mostly they are heaters that will keep the compressor warm enough that any refrigerant in the compressor remains as a gas when the unit is off. Liquids are uncompressible if you try compress them steel parts tend to end up broken.
Read your equipment's installer’s instructions, but I think you will be fine if you want to cool when it is over 50° outside without the kit.
If this is a heat pump it should not need a kit.
Walta