How much HVAC do I need for a big multi-zone house?
September 14, 2016 8:25 AM Subscribe
When sizing an air conditioner for a multi-zone house, do you size it for the full house or for the largest zone? How much cooling do I really need?
I'm replacing a 5 ton HVAC system in a 3500 square foot house in Grass Valley, CA, USA. I found an HVAC company I like and trust. They're recommending a fancy modulating 5 ton furnace paired with a 4 ton variable speed AC (Carrier Infinity). They say the 4 ton AC is enough capacity because the house is zoned. Is that true?
According to method 2 on this site I'd need more than 5 tons to cool the whole house. However, we have a zoned system: one 2000 sq ft zone and two 750 sq ft zones. We almost never cool one of the small zones. So really it's more like 2750 sq ft in regular use, for which 4 tons is adequate according to the chart.
Currently we have a fifteen year old 5 ton AC that has worked well. It never labors all day to keep the house cool. Our house is very well insulated. And where we are isn't super hot; a lot of folks get by with no AC at all because it cools off enough at night. The system designer is certain that 4 tons of AC should be enough. I asked about a 5 ton variable speed AC, but apparently Carrier doesn't make a variable speed that big. So the alternative would be a 5 ton two-stage AC (proposed by a second bidder). But the two stage presents the problem of too much cooling for just the smaller zone, requiring some sort of bypass or dump. (We currently have a bypass.)
I've mostly talked myself into believing the 4 ton AC is enough, but I don't really know anything about HVAC so am looking for second opinions.
I'm replacing a 5 ton HVAC system in a 3500 square foot house in Grass Valley, CA, USA. I found an HVAC company I like and trust. They're recommending a fancy modulating 5 ton furnace paired with a 4 ton variable speed AC (Carrier Infinity). They say the 4 ton AC is enough capacity because the house is zoned. Is that true?
According to method 2 on this site I'd need more than 5 tons to cool the whole house. However, we have a zoned system: one 2000 sq ft zone and two 750 sq ft zones. We almost never cool one of the small zones. So really it's more like 2750 sq ft in regular use, for which 4 tons is adequate according to the chart.
Currently we have a fifteen year old 5 ton AC that has worked well. It never labors all day to keep the house cool. Our house is very well insulated. And where we are isn't super hot; a lot of folks get by with no AC at all because it cools off enough at night. The system designer is certain that 4 tons of AC should be enough. I asked about a 5 ton variable speed AC, but apparently Carrier doesn't make a variable speed that big. So the alternative would be a 5 ton two-stage AC (proposed by a second bidder). But the two stage presents the problem of too much cooling for just the smaller zone, requiring some sort of bypass or dump. (We currently have a bypass.)
I've mostly talked myself into believing the 4 ton AC is enough, but I don't really know anything about HVAC so am looking for second opinions.
Also, bear in the mind the consequences of a slightly undersized AC (which still sounds unlikely, but just to put your mind at ease). The consequence is that you might be a degree or two warmer than you'd like on the very hottest days. So, once every few years, you are a degree or two warmer for part of a day or a few days. Not exactly the end of the world.
posted by ssg at 9:22 AM on September 14, 2016
posted by ssg at 9:22 AM on September 14, 2016
I'd agree that you've got room to spare with the current AC's capacity, though you'll want to look at how far the AC has to push that air along your ductwork.
Based on somewhat similar recent experience, I'd suggest having one additional conversation with a company that works with mini-split systems (e.g. the Mitsubishi range) to see if it's a feasible supplement for one or both of the 750 sq ft zones, thus allowing you to focus the main AC unit on the main zone. Maybe it'll suit your space, maybe it won't. (Caveat: where we are, Carrier installers tend not to work with mini-splits and vice versa, so if the same applies for you, that could complicate long-term maintenance.)
posted by holgate at 9:26 AM on September 14, 2016 [1 favorite]
Based on somewhat similar recent experience, I'd suggest having one additional conversation with a company that works with mini-split systems (e.g. the Mitsubishi range) to see if it's a feasible supplement for one or both of the 750 sq ft zones, thus allowing you to focus the main AC unit on the main zone. Maybe it'll suit your space, maybe it won't. (Caveat: where we are, Carrier installers tend not to work with mini-splits and vice versa, so if the same applies for you, that could complicate long-term maintenance.)
posted by holgate at 9:26 AM on September 14, 2016 [1 favorite]
Sizing AC is not an art - get more bids if you're unsure about the ones you got. Worse case scenario you hear the same thing over and over a couple times. Best case, you find someone who's competent and interested and they come up with a great solution or persuade you the solution you've been offered is adequate.
All that said, I agree with ssg's comment above.
posted by From Bklyn at 9:56 AM on September 14, 2016 [1 favorite]
All that said, I agree with ssg's comment above.
posted by From Bklyn at 9:56 AM on September 14, 2016 [1 favorite]
In my experience, HVAC contractors "size" your house by selling you the exact same capacity unit as you already have, so if this contractor is recommending going down from existing, at least they've done some calculations.
posted by hwyengr at 11:05 AM on September 14, 2016
posted by hwyengr at 11:05 AM on September 14, 2016
Best answer: Looks like something you're not taking into account is humidity. HVAC affects in-house humidity, and getting that wrong can condemn you to a poor-smelling, cold-and-clammy house.
Get your trusted HVAC contractor to do the ACCA Manual J load calculation on a room-by-room basis. This can be time-consuming and will add some expense, but it will provide all the information your preferred contractor needs to determine total capacity, zone configuration, capacity per zone, dehumidification capacity, etc.
Source: Born into a family of Lennox dealers and HVAC installation and repair contractors.
posted by infinitewindow at 1:09 PM on September 14, 2016 [2 favorites]
Get your trusted HVAC contractor to do the ACCA Manual J load calculation on a room-by-room basis. This can be time-consuming and will add some expense, but it will provide all the information your preferred contractor needs to determine total capacity, zone configuration, capacity per zone, dehumidification capacity, etc.
Source: Born into a family of Lennox dealers and HVAC installation and repair contractors.
posted by infinitewindow at 1:09 PM on September 14, 2016 [2 favorites]
The variable speed units are nice - a few years ago I got a two speed outdoor unit and last year my neighbor got a Carrier Infinity and his is better. Mine goes ON & OFF loudly, and his just hums along at a constant (low, quiet) speed. The infinitely variable speed units are also better at dealing with humidity.
For California residents, I would recommend pricing a single heat pump unit (which both heats and cools) along with solar panels, rather than having a separate natural gas furnace and an Air Conditioning unit. If you are on a year-round net metering plan, and have a well-insulated house, you may find it costs about the same, and ends up with a much lower CO2 footprint in the long run.
posted by soylent00FF00 at 6:44 PM on September 14, 2016
For California residents, I would recommend pricing a single heat pump unit (which both heats and cools) along with solar panels, rather than having a separate natural gas furnace and an Air Conditioning unit. If you are on a year-round net metering plan, and have a well-insulated house, you may find it costs about the same, and ends up with a much lower CO2 footprint in the long run.
posted by soylent00FF00 at 6:44 PM on September 14, 2016
This thread is closed to new comments.
Ideally, your AC would be sized so that it does run all day on the very hottest days. Otherwise, it is oversized and it sounds like your 5 ton AC is oversized.
posted by ssg at 9:17 AM on September 14, 2016 [1 favorite]