Central air died…
bwsct
| Posted in General Questions on
Hi,
My Central air unit just died and I’m trying to decide what is best in replacement. My houses is 2 floors a 2038 sq ft with 2 zones. I’m in zone 5a in CT.
I’ll also be starting a house remodel soon and adding some more space (approx 400 sq ft) and a new roof.
I have an oil fired furnace which has some life left on it.
Should I consider a heat pump at this point?
Thanks
Brad
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>"Should I consider a heat pump at this point? "
Probably. Oil pricing is low right now, but it's volatile. Not many years ago when #2 oil was $4./gallon, even at CT's high electricity pricing cold climate heat pumps were a no-brainer investment with a fairly short "payoff".
Since you have a heating history on the place, run a fuel-use based load calculation to estimate the design heat load:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
If you're on a regular fill-up service that stamps a "K-factor" on the billing slips, a few wintertime fill-up K-factors would be enough information to ball park it.
Depending on the particulars of both the new and old space, and how much older exterior surface the new stuff is displacing, adding 400' can sometimes even LOWER the total heat load.
If you're keeping the oil burner and using the same ducts for both heating and cooling it may constrain the heat pump sizing, probably forcing it to be oversized. But with enough information we can probably figure out the best compromise.
How many tons is the (now dead) existing AC?
Hi Dana,
My dead unit is a 2 Ton Trane.
I had a sales person here to get an estimate on a new unit and brought up heat pumps and he said he wouldn't recommend it for my area because when temps are below freezing I'd still have to resort to oil. I think I've read in the past information against his statement.
I'll look for my oil delivery slips to see if I can get a k factor.
I'm keeping the oil burner for now as I think it has life left and yes I use the ducts for both heating and cooling. I wish I had the finances to take it all out an go with mini splits but at this time I don't.
I don't expect oil prices to remain at the $1.38 I bought it last.
>"My dead unit is a 2 Ton Trane."
Something 2 ton Fujitsu AOU/ARU24RGLX is good for about 22-24,000 BTU/hr at typical CT design temps, but you're probably looking at a design heat load of 30K, give or take. The 2.5 tonner might cut it (with strip heat backup), but you're probably looking at the 3 tonner in that series, if doing all zones with just one unit:
https://neep-ashp-prod.herokuapp.com/#!/product/25350
https://neep-ashp-prod.herokuapp.com/#!/product/25351
https://neep-ashp-prod.herokuapp.com/#!/product/25352
The 2 tonner modulates down to 5400 BTU/hr @ +47F, which is a decent low-end range if it's broken up into zones. The 2.5 and 3 tonners only drop back to 9200 BTU/hr, which is still fine if the zones are pretty large.
Whether a slim-ducted mid static air handler has enough cfm to work with your fossil burner remains to be seen, but probably not.
There are others.
>"I don't expect oil prices to remain at the $1.38 I bought it last."
Probably not. The 10 year average residential #2 oil price in CT is about $3/gallon:
https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=W_EPD2F_PRS_SCT_DPG&f=W
The average retail price for electricity in CT has been running 18-19 cents:
https://www.eia.gov/electricity/state/
Assuming 85% combustion efficiency on the furnace each gallon delivers 0.85 x 138,000 BTU/gallon = 117,300 BTU/gallon, so it takes (1,000,000/ 117,300 =) 8.5 gallons to deliver a million BTU (MMBTU). At $3 that costs $25.5/MMBTU, plus some electricity for running the air handler, probably closer to $26-27/MMBTU. At $1.38/gallon it's more like $13-14/MMBTU with the power use added in.
A modulating ducted mini-split will hit it's HSPF numbers in a CT climate if not grossly oversized/undersized. Assuming an as-used HSPF of 10 (= 10 BTU/watt-hour, or 10,000 BTU/kwh) it takes (1,000,000/10,000 =) 100 kwh /MMBTU which at 19 cents /kwh is $19/MMBTU.
That's a significant discount from the 10 year average price for oil, but considerably more than your recent nearly record-low priced fill-up.
Roof top solar electricity is now cheaper than the residential retail price in CT, and with federal state & local incentives can be substantially lower. Even Tesla's low-risk low-money down RENTAL solar comes in at around 13-14 cents (available in CT only for Eversource Energy or United Illuminating Company ratepayers: https://www.tesla.com/support/energy/solar-panels/learn/subscription-solar ) At the Tesla rental solar price a decent heat pump solution is already operating-cost competitive with $1.38/gallon oil, if you have enough optimally oriented roof area to cover most of your heating energy use.
I've recently been pitching a heat-pump + Tesla solar rental solution to an in-law in MA who has recently been paying close to 29 cents/kwh, and heating with electric baseboards. She doesn't want the encumbrance of a potentially cheaper solar lease which are a PITA to transfer to new owners, or taking the upfront hit to buy, since she's planning on retiring and moving out in less than 10 years. The severance fee to Tesla to just walk away from the rental deal is only $500, but the contract is easily transferred if a prospective home buyer wants to keep it (or buy it out and own the solar), much less messy than most solar leases. The rental costs $50/month for the 4kw system increasing by $50 per 4 kw increments in steps to a max system size of 16kw.
Give what I understand about the electric rates in CT it seems unlikely that a that twenty plus cent a kilowatt electricity and today’s low oil prices that the financial case for a heat pump could be made.
I say resist the urge to install a larger AC unit. An oversized unit will not run long enough the control the humidity in your home you will end up cold and damp not a good compo. If your old unit was cycling on and off in the afternoon of the hottest days it is likely big enough for the addition.
Walta
The salesperson I had here recommended a 3 Ton Trane 14 SEER unit model RX410A.
He gave me a printout of a manual J. He didn't actually go around measuring windows and doors. Just took the sq footage of the house. I've attached a screenshot of the document.
Is this the right way to do a manual J or more of an estimate?
>"He didn't actually go around measuring windows and doors. Just took the sq footage of the house. I've attached a screenshot of the document.
Is this the right way to do a manual J or more of an estimate?"
If he didn't measure anything, the Manual-J is crap. He just entered some numbers until the tool spit out something he found "reasonable".
Run the fuel use load calculation and compare that to the 41K heat load estimate. That was even at +5F, which is probably the 99.6th percentile temperature bin, not the 99th, which is already a thumb on the scale. Even using the 99% bin usually has margin in a properly aggressive Manual-J. The 99th percentile temperture bin at the Bridgeport Sikorsky airport is +15F, not +5F.
Most homes in New England have a square feet per ton 1% design cooling load of about a ton per 1500', so your actual cooling load is probably less than 1.5 tons, and a non-modulating 3 tonner would be simply ludicrous.
Take a look at Allison Bailes' graphic of square feet per ton on REAL Manual-Js, plotted against house size:
https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb
https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/Bailes_graph_for_Manual_J_blog.preview.png
Note, most of those homes were in the hot humid southeast, not New England. Even there a typical 2000' house would have a design load under 2 tons, and only the very WORST performing houses in that size range would need 3 tons.
I assume your house isn't the CRAPPIEST house in Bridgeport ( is it?).
Before it croaked did the 2 ton Trane keep up with cooling loads even on days well north of 1% outside design temp of 85F?
Also note, the 100F design temp used by the contractor, rather than the actual 1% outside design temp. Writing "Manual-J" on the printe report isn't the same as actually FOLLOWING THE MANUAL, (or is it? :-) ) Utter crap.
Am also in CT 5a (10 miles inland from LI Sound). For those of you who may not know it, much of CT is a SWAMP. Humidity all summer (except when the wind blows from North) and damned cold in the winter. With that said, consider any number of low temp heat pumps and consider PV panels on the roof. My $0.02 is that oil heating is dead. Or should be.
Lastly, find your local HERS (Home Energy Rating System) and have them do an independent review (Man J) of your home. They might get you some rebates and perhaps point your to a more efficient heating system.
HI David,
Do you have an example of a low temp heat pump?
I would love to go to PV and a heat pump but I'm not sure financially I can do that at this point. That's most likely 30k on top of what I'm already hoping to do with a small addition.
I'm also about 10 miles from LI Sound.
Ok- nothing against salesman but the printout you received could have been for your neighbors house as well as yours. Window size, solar orientation, insulation etc all figure into a proper Man J. Get yourself an expert; Cheap money. and PV panels should be paid for by the energy savings you get by reducing your Eversource bill.
Your cheapest option is probably a new compressor, which MIGHT be able to use the same A coil your current AC compressor was using. If the old compressor burned out, it probably contaminated your refrigerant lines in which case you probably don’t want to reuse that A coil since it’s probably cheaper to replace it than flush it (but ask your contractor). You would absolutely want an R410A unit, and you would need to make sure your existing A coil can handle the higher pressures involved.
That said, I agree with Dana that the heat pump is a better overall option, albeit at higher initial installed cost. You’ll need to go through the numbers to see which way you want to go.
Solar doesn’t have to be as expensive as you might think, and it will shave off load from your electric bill at the same time you need your AC the most. You DO NOT need batteries — you need only panels and a grid-tied inverter. Don’t try for net zero, try for “peak shave” — size your system to offset your AC load during the day, plus a little extra. If you have a good sunny area for the panels, you can cancel most of your AC costs out with the solar system. I worked out that I can buy the parts to install a smallish solar system (around 3kw or so) for about $1/kw, which I think is a pretty attractive option. For me, such a system will have a 3-5 year payoff and that is without any gov’t incentives.
You could potentially put in a solar system and a new AC now, or a heat pump now and solar later, or any combination you choose. Solar is expensive if you go for a huge net-zero sized system or an off-grid system. If you size your solar system for peak shave only, it’s not very expensive. You basically size your system for the most bang for the buck doing this. If you can get a “time of day” electric rate (which is what I do) the solar output will shave load off during the peak time of the day (11am to 7pm for me, weekdays), so your solar system is putting out the most power during the time of day with the highest electric rate which gets you the maximum benefit. The time of day rates have lower per-kWh rates during off-peak hours (7pm to 11am weekdays, and all day on weekends, for me). I’d consider the solar system if you have a good place to site it.
Bill
I figured I would update this because of all of the helpful answers.
The issue with the AC unit was actually just a bad capacitor. I got a 2nd opinion from a another company and that was the result. The unit has worked fine since the capacitor replacement and it costs me $150 and not $8k.
I will explore the option of a heat pump more so I'm ready to make that decision when the system does fail.
Thanks to all.
That would probably be a bad motor run or motor start capacitor. Those are maybe $5-20 parts usually and are pretty easy to replace when needed. Typical expected lifetime of those capacitors is about 5 years, although it's not uncommon for them to last much longer.
Note that sometimes premature capacitor failure can occur due to compressor operation on lower than normal line voltage. This can SOMETIMES be helped with the installation of a "hard start kit" on the compressor, which is usually around $50-100 or so. Another thing to check is to make sure you have properly sized power cabling to the unit from your electric panel, and no loose or corroded connections along the way. Make sure to include a check of the contactor that operates the compressor motor.
It's easy to do voltage checks, but contractors will sometimes skip this step. It's a good idea to check to be sure there is nothing going on that will result in the same failure down the road.
Bill