Posting on behalf of a friend building an addition. The addition will turn a home of approximately 1800 sqft + fin bsmt into a bit over twice that. The new section will rise from exposed basement to partial third floor.
The existing furnace is older. Two builders, so far, have advised replacing the existing furnace during construction AND adding a new furnace for the addition. Home-owner wants to know likely cost differences between one larger furnace and two. And what about the AC unit(s)? What would this gang recommend for the quietest, most effective options system?
Climate facts: Toronto = temperature extremes. Forced air gas. The addition part and top floor will require a fair bit of AC in summer, while the front half and top floor will tend toward chilly in winter.
Replies
Just a homeowner, but I would recommend two systems.
As you mention you have two areas of drastically different requirements.
I would not base them on old building and addition. But as much as reasonable I split them based on the differenent HVAC needs.
Now I only have an 1600 sq ft, 2 story home, but the requirements for the two are very different. It took a while to find an HVAC contractor that would split it up. That was in 79. If I was building now I would serious look at single system with zoning.
However, I am guess that the size of that house zoning would not be the best option.
Also with 2 system I would guess that the duct work would be greatly simplifiied.
I am in Georgetown ON and am adding a 1200 sq ft addition to a 1600 sq ft house. One HVAC company recommended two gas furnaces, another recommended using one. Before I had to decide my existing unit broke down so I put in one unit for both. The new single unit was not much larger in capacity than the original. I went with the 96% efficiency, variable speed motor etc. I've had a considerable drop in gas and electricity use.
The duct work in the addition has yet to be installed and I expect to have to spend some time balancing the system. I also have hydronic in the basement slab and in the office and new bedroom floors, just to complicate things.
I am going with one AC. The point is that with the addition we are still dealing with about 3000 sq ft. The cost of purchasing two heating and AC units was greater than buying one larger unit and I'm betting that the cost of operating two sets of fans and two compressors would be greater than the aggravation of balancing the single system.
I'll cast my vote with Bill.
Many time adding a larger unit means creating a hybrid duct system, or replacing much of the existing system. Cost and efficiency become competing factors, and comfort gets left out.
A large zone system would be good, but agian, without load calculations and some serious design work, it is just a crap shoot.
Two system may mean that, if the existing unit is working properly, you only need to design for the addition. Operating cost could be a little higher, but you expected to double that cost anyway.
A good HVAC contractor will do the load calcs. and figure both ways for you. They may also do a cost projection.
My definition of a good HVAC contractor is one that puts the clients interest and comfort first, not his over selling unneeded or inefficient design and equipment.
Dave
I design HVAC systems for a living, though not usually for the residential market. For this situation, I would not design a system around a single furnace. On the other hand, I have a 2-story, 4500 sf house with a full basement (50% conditioned) and I have a single 125 MBH, 92% furnace and a 4 ton AC system. I am willing to tolerate the compromise of a single system in my own home, and I don't mind adjusting dampers from season to season. But that's me.
The climate in Toronto is not that extreme and the house could be heated with a single furnace. Dividing the house into zones by using two separate systems makes the most sense from a comfort and control standpoint. One 125 mbh furnace with a 4 ton coil/condenser (my SWAG) will be cheaper than say 2 - 75 mbh furnaces, each with a 2 ton coil/condenser. Two systems would cost 20-30% more, than a single, depending on particulars.
So my recommendation would be two systems. The quitest, most effective systems in common use for residences (in the standard, forced air category) is a 92 or 94% variable speed, two-stage furnace. In Toronto, any AC unit over 10 SEER would be a waste. My furnace is a Heil, and I would recommend that brand. High quality units are also made by Carrier, Trane, Lennox and Armstrong. The quality of the contractor is way more important than the name on the box.
Thanks Tim (and all) I'll pass that along.
PS, I only said 'extreme' lest the pros in the Southern US think that AC wasn't necessary at all in frosty Canada - but your local expertise is welcome and relevant. They run their current AC unit much of the summer and will have a lot of unshaded South-facing windows.
Thanks for brand recommendations.
I work in northern Illinois and the climate is very similar. In this and that part of the world, a properly sized AC is important. No matter what your friend does, the loads must be calculated accurately and the AC sized accordingly. In my home (I have a lot of windos, heavy shade and and over 1000 sf/ton) on a 95+ day my AC runs continuously. At 100 degrees, I cannot cool my house below 80, but the air is dry, and therefore, quite comfortable. At 95, I can get to 75, which is my design criteria.
Our southern MN house has two furnaces, but one is for downstairs (split-entry -- half below grade) and one for up. It works out well with AC only on the upper unit. However, if we ever replace (and at 28 years that's coming up) I'll probably get a single unit with dampers, if only to save the space in the utility room.
It has been nice a few times in the past to have a "backup" furnace, especially when a fan bearing goes out when the temps are 25F below. But the space/bother/cost of having and maintaining two furnaces is a problem (twice as many fan bearings to go out, eg).
Go with two ACs. Maximum flexibility and redundancy. Each can be sized for the zone it is handling and will run only when that zone needs to be cooled. A common problem with a too large unit is that it may cool the air down TOO fast, which means that it doesn't run long enough to draw the humidity out of the air. This is why some homes are cold and have condensation on the windows. When the unit comes on it chills the house in just a few minutes, then shuts off, leaving the humidity in the air. If your HVAC contractor doesn't tell you about this or doesn't know how to figure the calculations, get a different contractor or at least a second opinion.
AG