Proposed furnace and AC system for a townhouse
I’ve enjoyed this site as a homeowner and as my parents are getting proposals to upgrade the AC and convert to NG furnace, I figured I’d take a look at the proposals. From what I gather it seems massively oversized, as usual.
1650 sq foot end unit townhome (3 walls exposed, shared other wall)
Decent sun exposure throughout the day but trees help for low elevation sun
1987 construction, probably higher end quality for it’s day
Mechanicals are in below grade basement, additional 200sq ft of finished basement space I believe that is tied in to the duct system
1st floor living room say 250sq is open to the second floor with cathedral ceilings
Osterville, MA
HVAC company with decades of experience on Cape is proposing:
Carrier Model 59SP5A080E21-20 80,000 Btu 96% (AFUE) Gas Furnace
Carrier Model 24ACC636 3-Ton, 16 SEER Outdoor Condensing Unit
Carrier Model CNPVP4221 Matching Puron Evaporator Coil
46 btu/sqft heating
21 btu/sqft cooling (550sq ft per ton)
Seems grossly over sized just off the rough numbers, right? I couldn’t find design temps for Osterville but they have to be lower than Boston’s for cooling at least.
More importantly, how do I politely tell them they can’t size a system properly? Manual J numbers can be tweaked to their liking, so how do I get them to go back on their own recommendation? It’s July, it’s hot, and it’s hard enough to get someone to even come out or return a phone call for an estimate. For $10k I thought it was pretty reasonable.
Do I have a third party do a manual J and tell them to install it that way? Doubtful they would provide service warranty if it’s not “their way.” Anyone on this site helpful?
They are on the books to install in the next couple weeks.
Thanks,
Kevin
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Replies
Kevin,
Allison Bailes says that new homes in Georgia are usually 1,000 square feet per ton. So your parents' proposal (550 square feet per ton) could be oversized by a factor of 2.
No way to tell for sure, of course, without performing a Manual J.
It never hurts to ask a contractor for a copy of their Manual J calculations. Either (a) they performed the calculations, or (b) they didn't.
If they didn't, they won't share the calculations because they don't exist.
If the did perform them, they may share them -- or not. Hard to say.
If they share them, you can post a copy of the paperwork here, so we can scrutinize the inputs.
Thanks Martin. Safe to assume they didn't use a Manual J. What are the energy penalties for being 2x oversized? Are we talking double the energy usage or less than that? What's worse is the home will be vacant most of the winter (not that that is an excuse to skimp on sizing). My parents will probably keep it at 50 degrees most of the winter and maybe we'll be down there a couple weekends throughout the winter months...so they really won't see the energy bill hit.
Multi-stage AC has benefits even if load matching is perfect.
You can estimate loads based on previous usage.
For a 1650' town house that sounds like a LUDICROUS level of oversizing!
Since this is an upgrade from existing systems you can probably run a fuel-use based heat load calculation to firmly bracket the heat load using this methodology:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
I would expect a tighter than average 2 story end unit townhouse to come in with a heat load of ~17-20,000 BTU/hr or a bit less @ +13F (the approximate 99% outside design temp in Osterville). If it doesn't have a massive expanse of west facing glass the likely cooling load would be between 12-18,000 BTU/hr @ 82F (the approximate 1% outside design temp). It may be possible to do the whole thing with a single ducted Fujistu -18RLFCD (and NO furnace.)
If there is an existing AC system it will be possible this weekend and next week to measure the cooling load using the existing equipment by measuring it's duty-cycle whenever it's between 80-85F (which would be just about every afternoon this coming week.)
https://www.greenbuildingadvisor.com/blogs/dept/building-science/how-tell-if-your-air-conditioner-oversized
A couple of years ago I saw a credible Manual-J on a single family house TWICE that size in Vineyard Haven that came in
at about 31K heating, 18K cooling (including 4 of duct loss for ducts outside of conditioned space, so it really was about 14K cooling for the ductless solution they eventually went with). It was well shaded with very little west facing window, but still it's TWICE the house, with no common-walls to other conditioned space,and the older half of the house was pretty leaky 2x4 construction with thin clear-glass double-panes (not high end Cape condo type construction). They had already gotten quotes for 100K propane burner w/ 4 tons of cooling equipment and a 4-ton Greenspeed heat pump + 1 ton Daikin mini-split before I stepped in and made them put on the brakes, and get a Manual-J.
That room-by-room Manual-J was performed by Heyoka Solutions in Falmouth (at a not-cheap but also not outrageous price for the service- less than a grand):
http://www.heyokasolutions.com/
866-389-8578
[email protected]
Seriously- they will be better off waiting for the Manaul-J even if it can't be done until October!
BTW: For reference the 1% and 99% design temps for East Falmouth/Otis AFB are 82F and +14F. (Vineyard Haven's are 81F & +13F, pretty similar.)
https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
Thanks Dana, this is what I had feared. I did reach out to Heyoka but they only do Manual J's off of plans and not room by room analysis. Any other suggestions on the Cape?
They are recently new owners so I'm not sure if they can pull historical energy records. The AC unit is currently non-functioning.
I also was reading a post from you back in 2012ish about how a furnace even 2-3x oversized has negligible efficiency hits...this still true?
For less than the cost of some Manual Js, you could install two window AC units. They aren't equivalent, but my guess is that the load data derived from the latter would be more accurate. And make you comfortable while exploring options.
Air sealing verified with a blower door test is a good idea.
Funny you should say that- we are bringing some down for next weeks heat. A 10K for the first floor (only have crank out windows for 2nd floor loft area), and 2x 6Ks for 2 second floor bedrooms. Should be a good test.
If the current system is operational I say wait a year so you understand how well the current system works.
Do not let them up sell you into a larger system than the current one unless it is working correctly and can not maintain temps.
Understand if you need larger equipment then you need larger ducts for the bigger equipment to work properly. If they do not bid the upgraded duct they will turn up the fan so you get a noise drafty system.
People that did not have propane tend to forget the shortage a few years ago if you could get any it was over $7.00 a gallon.
Walt
The efficiency hit from a 3x oversized gas furnace is indeed negligible (this one is more like 4-5x oversized), but the COMFORT hit is real. The room temperatures fluctuate more, the oversized air handler is noisier and imparts more wind-chill.
An oversized air handler will force the oversizing of the AC coil too. Most 80K gas furnaces can't be matched to a 1-ton or 1.5 ton AC, and are better suited to 3-4 tons of cooling.
It doesn't take a degree in architecture to measure up the rooms & windows & walls, make floor plan drawing and describe the construction type, etc, which is essentially what they take off of plans. (The Vineyard Haven folks had just put on an addition, and had the plans for roughly half of the house, and I believe had somebody else draw up the rest.)
How is the place currently heated?
If you can't find anybody willing to run a Manual-J, run an IBR type load calculation using a spreadsheet tool. See:
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-2
Most '80s vintage double-panes were good for U0.6, If the windows are wood framed double panes with 3/8" between the sheets of glass use U0.5. A 2x4/R13 wall with most siding options run about U0.9, 2x6/R19 runs about U0.65. Use this set of tables for figuring out what values to use:
http://www.energy.ca.gov/title24/archive/2001standards/residential_manual/res_manual_appendix_i.PDF
If you don't know how much insulation is in the ceiling, just assume it's no more than a U-factor of U0.05. (That would be about right for R19s in 24" o.c. rafters.)
For figuring out the air leakage & ventilation just punt: Assume it can hit 100cfm, maybe even 120cfm in a high wind, even though the place is probably tighter than that. So with a 68F indoors, 13F outdoors you're looking at a 55F delta-T
120cfm x 60 min= 7200 cubic feet per hour...
...x 55F x 0.018 BTU/cf/degree= ~7000 BTU/hr of ventilation & infiltration.
Reality could be less than half that- this is your fudge-factor margin- be aggressive on all the U-factors.
With the raw IBR plus air leakage fudge factor, and no adjustments for plug loads and occupancy there is no need to upsize the equipment from there- it'll already be padded by at least 15%, sometimes by as much as 35% from what an aggressive Manual-J would deliver.
Thanks Dana, I'll see if I can put some numbers to this over the week while I'm there.
There were electric baseboards throughout the house, as well as ducting already in place. I'll take a look at the unit but I it is currently forced hot air. It's slightly confusing because the AC condenser is rusted and not working, pipes froze over the winter so I'm not sure if that was due to the homeowner or faulty equipment. They recently got natural gas piped to the house so I'm not sure if they converted or if it was a heat pump to begin with. Sorry, don't have all the answers at the moment.
I do have to thank you for assisting in one of my other threads about mini split sizing. You were correct. I left the 2nd floor 12K split running all day today, and my house is sitting at 71 degrees without even trying. 1900+ sq feet and for a couple hours today when it was 90+ degrees, I had 2 off cycles of 2.5 minutes each over 95 minutes, mostly running at very low speed. Didn't even have to use the swing mode to push it down the stairs. Set it at 72 and forget it!
Kevin
Kevin
Now that I'm at the condo I have some more details.
NG was installed some years ago. Currently a Comfortmaker ii RPJ forced hot air. Couldn't find the size it must be on the side next to the chimney.
I measured up all the windows and drew up a floor plan of the first floor. Second floor is essentially the same, except the front bedroom is a little shorter with a porch instead of finished space, and there is a lot of space lost due to the cathedral ceiling.
These measurements were absolute worst case- I didn't account for the cathedral sloped ceiling, it's really just a perimeter foundation view of the unit x 2 stories, but does at least put it in the ballpark. We'll see if we can get the HVAC company to come out and do a true manual J.
I'm completely ignoring the partition wall with the neighboring condo, I'm assuming that's what you do.
Kevin,
The HVAC companies usually aren't the best resources for securing an accurate Manual J. See Martin's article on "Who can Perform My Load Calculations?" for more info: https://www.greenbuildingadvisor.com/articles/dept/musings/who-can-perform-my-load-calculations