Replace old central AC with heat pump?

MikeC555

I have a 2000 sq ft house with how water baseboard heat and a central AC system. The AC system is over 20 years old and is undersized for house (2.5 ton condenser, system struggling on hot days) and I'm getting estimates to replace the system. One salesman mentioned I might want to consider a heat pump system. Because of existing rebates on a SEER 16+ system, I could get a 3 ton Heat Pump system for less than a 3 ton central AC system (air handler and condenser). The salesman says I can save oil by using the heat pump down to 38 F, as it is more efficient down to that temp. I'm not sure that I would necessarily save money since electricity isn't cheap in southern New England. Since I'm replacing the AC system anyway, the cost to install isn't really a factor. How might I evaluate if going to heat pump would reduce my heating costs? Is this commonly done, using a heat pump with boiler back up/ DHW?

EBEBRATT-Ed

The heat pump will probably save you money run in the spring and fall without having to run the boiler. In really cold weather their efficiency drops, and they need to be defrosted. Since you're replacing the AC anyhow a heat pump is probably the way to go.

And you will have a backup heat source

Hot_water_fan

The switchover point is constantly changing, so it could be 0, could be 55. The salesman pulled 38 out of thin air. Generally, heat pumps are very competitive vs. oil heat though, so through some trial and error you’ll dial it in. 

The math is simple:
Oil $/mmbtu: $/gallon x (1000/138) / COP. So $3.5 oil in an 80% boiler is $31.70 $/MMbtu. 

Heat pump: $/kwh x 293 / COP. So $.25 juice and a heat pump with a COP = 3 would be 23% cheaper than $3.50 oil. 

The tricky part is the COP is constantly changing, as is the price of oil and the price of electricity. 

Jamie Hall

In your situation, I'd get the heat pump and keep the baseboard heat for the cold days or when you want to warm up quickly.

SuperTech

Before you get the heat pump have your duct system evaluated. Sometimes ductwork designed for cooling doesn't work as well for heating. Are your returns all in the ceiling? For heating you want your returns near the floor where the air is coolest.  Also it's really rare to find existing systems be undersized.  Usually the systems are oversized and the ductwork is undersized, this leads to poor performance that's misdiagnosed as the system not being large enough.  Then a larger system is installed on the existing ductwork making the problem even worse. 

Start with a heat loss calculation to determine what size equipment you need. Bigger is not better.  The size of the equipment determines the size of the ductwork.  And I would also strongly recommend keeping the boiler and baseboard heating. 

HVACNUT

MikeC555 said:

I have a 2000 sq ft house with how water baseboard heat and a central AC system. The AC system is over 20 years old and is undersized for house (2.5 ton condenser, system struggling on hot days) and I'm getting estimates to replace the system. One salesman mentioned I might want to consider a heat pump system. Because of existing rebates on a SEER 16+ system, I could get a 3 ton Heat Pump system for less than a 3 ton central AC system (air handler and condenser). The salesman says I can save oil by using the heat pump down to 38 F, as it is more efficient down to that temp. I'm not sure that I would necessarily save money since electricity isn't cheap in southern New England. Since I'm replacing the AC system anyway, the cost to install isn't really a factor. How might I evaluate if going to heat pump would reduce my heating costs? Is this commonly done, using a heat pump with boiler back up/ DHW?

That's what I'm doing when the time comes. A 2 stage high efficiency heat pump. Third stage baseboard. 
I'm staying away from inverter systems for ease of service and readily available parts.

I'll set the baseboard to kick in at 35° because I had a frozen loop in the past. Keep the water moving. 

pecmsg

MikeC555 said:

I have a 2000 sq ft house with how water baseboard heat and a central AC system. The AC system is over 20 years old and is undersized for house (2.5 ton condenser, system struggling on hot days) and I'm getting estimates to replace the system. One salesman mentioned I might want to consider a heat pump system. Because of existing rebates on a SEER 16+ system, I could get a 3 ton Heat Pump system for less than a 3 ton central AC system (air handler and condenser). The salesman says I can save oil by using the heat pump down to 38 F, as it is more efficient down to that temp. I'm not sure that I would necessarily save money since electricity isn't cheap in southern New England. Since I'm replacing the AC system anyway, the cost to install isn't really a factor. How might I evaluate if going to heat pump would reduce my heating costs? Is this commonly done, using a heat pump with boiler back up/ DHW?

Is it struggling or just sized properly?
A 3-Ton system will require more air flow than the 2 1/2-Ton, can the ducts handle the extra flow?

Agreed get the heat pump. Mine changes over at 25°F. Thats just for comfort not energy.

JUGHNE

Heat pumps live on air flow, both inside and outside.

If your AC is struggling because of undersized ductwork, then things will be worse with upsizing to 3 tons,

especially a heat pump.

As mentioned you need a heat loss/gain survey done.

There are many variables in house structures to consider.

I have well over 2000 sq ft and a 2 ton AC takes care of the cooling here in 90 to 100 degree Nebraska.

MikeC555

The existing ductwork is a good point to consider. When I originally moved in, I found that the system had only a single 16" flex duct (to hallway) for air return to air handler in attic. I knew this was too small for the existing 3 ton air handler. The condenser was only 2.5 ton and I suspect this system was sized before screen porch was converted to living space and bathrooms (2) had no AC vents (it was maybe 1800 sq ft at that time). I added a 2nd 16" duct return on opposite side of house, so I'm confident that I'm OK as far as ductwork, but it is all in the ceiling, which isn't ideal I realize.

I've also been told that to qualify for the rebates, the existing heating system must be 'disconnected' or a control must be added to heat pump that will automatically shut it down and turn on back up heat when temperature gets below 'X'. This seems to assume back up heat source is off until this switchover occurs. How that works if your back up heat source produces your DHW, I don't know but I suspect increased complexity that I do not want.

As far as 'disconnecting' the existing heating system to qualify for rebate, sales tells me disconnect is not well defined and as such, simply turning off the boiler qualifies as far as rebate is concerned (and you can turn it back on as soon as installer leaves).

"I have well over 2000 sq ft and a 2 ton AC takes care of the cooling here in 90 to 100 degree Nebraska." You must have a very well insulated house. Right now I have R-30 in ceiling but plan to add another layer of R-30.

ChrisJ

@MikeC555 There's quite a few on here that either have very well insulated houses, or like living in a 78F oven.

I use 3 tons in a 1600sqft house and it runs continuously quite often in July and August, but I'd call it sized about perfect for my uses. I like my house at 72F during the day and the bedrooms at 68-69 at night and the house lacks insulation.

If you feel you need more cooling, than by all means go for it. But keep in mind, like I said about my own system, it runs continuously during very hot weather and that's a very good thing. It's actually better for equipment to run continuous than cycle. A LOT better. Assuming it's keeping you comfortable and you're not sitting there soaked and miserable of course.

BUT

The points about duct work are very real. When I designed my ductwork I went oversized according to most and it gave me a very balanced and quiet system. If your ductwork is undersized then it's best to stay where you're at, OR upgrade the insulation and windows and go smaller if possible. Either that, or upgrade the ductwork appropriately.

Don't try to shoehorn a bigger system onto bad ductwork, you'll be sorry.

MikeC555

Thanks Guys.

JUGHNE

I made this statement mainly to discourage you from oversizing your new AC.
I do have a very well insulated house.

It is designed as a passive solar house. This design also helps you for the cooling in the summer.

The most glass is on the south with a designed overhang to allow max sun on winter solstice and all glass shaded on summer solstice.

There are only 2 west windows and one on the east.

In the summer the wind blows often from the south thru those open windows and across the house exiting out a smaller north window. So there is almost a constant breeze thru the building. It helps that we live on 2 acres of land and the south is open to the wind.

This is an old concept from farmhouse design of the 40-50's before AC was in use.
So we are comfortable until outside gets above 85.

On Dec. 21 the sun penetrates into the sun room heating the tile laid on concrete with radiant tubing in it.

I actually have two 2 ton AC's. They are zoned for pretty well what you would call daytime and nighttime areas.

And to complicate the story, the return ducting from daytime zone is connected to nighttime zone air handler.
And the nighttime zone return ducting is connected to the daytime zone air handler.
All return airs grills are high wall design. Most supplies are floor registers.

So for quick cool down both units come on with 4 tons. After catch up is done then the daytime unit might be set to cooling and the nighttime zone has fan "on".

This keeps the nighttime zone within 2 degrees of the daytime system, in the evening we switch the functions on each tstat.

Jamie Hall has expounded here on passive solar design. It is really over looked in today's housing design.


This is June 21 at noon so the shading is obvious for all the first floor glass.

bio_guy

There is a lot of excellent advice offered, as usual. I have a couple of comments. Nobody is kidding when they say that efficiency at lower temperatures is a moving target. Flash technology started, I believe, coming in 10-15 years ago and really extended the efficient use of heat pumps so much that ground source got pretty much blown up due to its much higher installation costs. The investment could never be recovered with the reduced differential of operating an air source HP. At that time, some heat pump manufacturers offered a like of low-temperature capable heat pumps apart from the regular ones.

One thing that often eludes my ability to calculate and add in to costs is the cost to operate the appliances outside of fuel. In this post, how does one know the lifetime cost of the boiler vs. a heat pump? Sure, it is easy [Purchase price + repair costs + maintenance costs] if the data is available, but who has it. I suspect that most boiler and furnace costs over the lifetime are lower than heat pumps, but I don't know.

Back to low temperature performance; have dual-stage heat pumps developed to the same level of low-temperature performance? Any purchaser of a new system is just going to have to look at performance vs (low) temperature and judge from that. Apart from flash, variable speed systems have a sweet spot much like boilers and furnaces do and that is not at flat-out, full speed. That can argue for somewhat over-sizing the systems. Arguing against that is the fact that when you run out of their turn-down space, you start to cycle and, in cooling mode, lose humidity control. Honestly, it seems like it is very difficult to account all the variables including the users' lifestyle.


Here are some web sites that might be useful:

https://ashp.neep.org/#!/product_list/

https://www.nyserda.ny.gov/Featured-Stories/US-Heat-Pump-Sales
(Note that "New York" mean the real New York, the state, not the city on the costal plain. At one time I did a heating degree day survey from central NY to across NH and VT to Maine and there was not much difference until well into Maine.

https://resstock.nrel.gov/factsheets/ It looks like this is a bit dated, maybe the data and analysis goes back to 2017, but much of the methods and data description are available.

https://www.energystar.gov/sites/default/files/asset/document/ENERGY STAR Version 6.1 Central Air Conditioner and Heat Pump Final Specification (Rev. January 2022).pdf Has anyone noticed that Energy Star will have standards for low temp heat pumps going into effect in January?

https://www.energy.gov/eere/buildings/articles/building-science-based-climate-maps-building-america-top-innovation#:~:text=The zones are hot-humid,, very cold, and subarctic.

Judging from what I passively absorb, and I don't look much these days, the only places in the USA where heat pumps are not likely to work are in zones 7&8.

Hot_water_fan

One thing that often eludes my ability to calculate and add in to costs is the cost to operate the appliances outside of fuel. In this post, how does one know the lifetime cost of the boiler vs. a heat pump? Sure, it is easy [Purchase price + repair costs + maintenance costs] if the data is available, but who has it. I suspect that most boiler and furnace costs over the lifetime are lower than heat pumps, but I don't know.

@bio_guy the comparison is flawed because a boiler or furnace can’t provide AC. I think it’s got to be…furnace + AC vs. heat pump costs. I think in that scenario, they’re even or the heat pump is cheaper. When I got them quoted, heat pump was cheaper than furnace + AC. YMMV.

WDGIBBS_M_E

No one on this thread has mentioned air-to-water heat pumps and I wonder why. Are there already hot water baseboard heaters in the house? Might need to update emitters but it seems ATW heat pumps should be considered.

bio_guy

@bio_guy the comparison is flawed because a boiler or furnace can’t provide AC. I think it’s got to be…furnace + AC vs. heat pump costs. I think in that scenario, they’re even or the heat pump is cheaper. When I got them quoted, heat pump was cheaper than furnace + AC. YMMV.

Hot_water_fan said:

One thing that often eludes my ability to calculate and add in to costs is the cost to operate the appliances outside of fuel. In this post, how does one know the lifetime cost of the boiler vs. a heat pump? Sure, it is easy [Purchase price + repair costs + maintenance costs] if the data is available, but who has it. I suspect that most boiler and furnace costs over the lifetime are lower than heat pumps, but I don't know.

@bio_guy the comparison is flawed because a boiler or furnace can’t provide AC. I think it’s got to be…furnace + AC vs. heat pump costs. I think in that scenario, they’re even or the heat pump is cheaper. When I got them quoted, heat pump was cheaper than furnace + AC. YMMV.

Hot_water_fan, Consider that the question I am asking is what are the LIFETIME COSTS of operating a furnace + air conditioner that would mean not operating the compressor in the heating season saving wear and tear on it. With a heat pump alone, you get a lot more hours on it instead of on a furnace. I don't know how it would break down, but as an oversimplified example consider one very made-up model. Excluding fuel costs entirely, how about a furnace that costs half of what a heat pump does and lasts twice as long. Heating mode hours are about equal to cooling hours. Say a heat pump operating alone will last 10 years and that is the baseline cost X. If you install a furnace for 1/2 X and it lasts twice as long, you have a cost of 2x for the heat pump alone if you have to install a replacement while the furnace is still chugging along costing 1/4 of the heat pump for the same period in equipment costs. You do have to then add the equipment cost of the cooling equipment which is X since you are only operating it half the hours. Given the assumptions about cost and lifetime, over 20 years the relative costs are X + 1/2X = 1.5X for the furnace-AC combo vs. 2X for the heat pump alone system.

Hot_water_fan

@bio_guy the issue I see is that compressors last a long time so I wouldn’t assume a 10 year life. Then you get to a point where interest rates and life events make a potentially longer lasting furnace mostly worthless and if it costs more to run? Ouch. 

The way I find helpful to think about operating hours is this way: I’ve lived in the Deep South with very old air conditioners which clock many annual hours. An air conditioner in Maine should last at least 5-10x longer than one in the south if operating hours determined lifespan, but they don’t. 

WMno57

@Hot_water_fan My brother just bought an 80 percent furnace and basic central AC. No heat pump. His degree is in Finance. He started with nothing and retired at the age of 40. He is good at math.
He and I both drive 20 year old V8 SUVs. Low repair bills, and if you drive 5000 miles a year, who cares if you get 18mpg. As Wallies know, my gas boiler is 75 years old and less than 80 percent efficient.
We are two of the cheapest, thrifty, whatever you want to call it, people you are likely to encounter. I will electrify anything if it makes financial sense. In most cases, I don't see electrification making financial sense. For the Electrify Pioneers that want to try new stuff and electrify their lives, Great! I wish them well. Just like I wish LeonZ well if he wants to build a Gravity Hydronic system.
Environmentalist's are going to have to SHOW ME THE MATH if they want to make their case.
Here is an example of a biased environmentalist, pretending to make an objective case for Heat Pump over Gas Furnace. He cooked the numbers. Check out the price for the "PK Wadsworth 97 % gas furnace". Yeah right. Furthermore, he did not include an 80% furnace and basic AC in his comparison.
https://decarbonizeyourlife.com/home/f/cleveland-duplex-the-all-electric-side-costs-less-than-gas-side

Hot_water_fan

@WMno57 breathe, brother. I’m not looking to determine if you and your brother are the most thrifty, more virtuous people. Not interested in that at all.

Electrification is absolutely the cheapest option sometimes. It will not be every time. That’s fine with me. The case when it’s makes the most sense is when you’re replacing an AC. Then it’s basically free to replace the AC with a heat pump. If you’re the nervous type, keep the furnace too, doesn’t hurt anything. This doesn’t seem controversial. 

pecmsg

The cost of a heat pump is not even 10% additional. In a few years that's all that will be available. So YES go with the heat pump but keep the backup!
Now as the numbers change you can too!

bio_guy

Hot_water_fan

I would not assume a 10 year life either. The question is, where can real numbers be found so meaningful calculations can be made?

fentonc

@bio_guy - 'real numbers' can only exist either in retrospect or as a model/estimate of the future, as the real numbers will depend on both future weather and commodity prices (oil/gas/electricity). You can make a model by getting quotes for equipment installation, and then guessing about what gas/electricity and weather prices will do in the future, along with an estimate of equipment reliability and future labor costs, and comparing the total cost of ownership to move the required BTUs over whatever time frame or estimated lifespan you choose. Electricity, gas and labor costs vary significantly from location to location, and even year to year for the same location.

In my area (outside NYC), a heatpump would have supplied cheaper BTUs than gas for most of the last several winters, but labor costs dominate almost all other considerations. If the only people you can get quotes from want to double the installation price for an equivalent capacity heat pump vs A/C-only, the numbers will rarely work out, but in the many places where it's a relatively small cost difference, it will frequently work out favorably. You can make your own guesses about future fuel/electricity costs and weather for your location, and work out what kind of installation costs and reliability you would need in order to save money.

woobagooba

I can confirm that Mass Save awards whole home heat pump rebates with a promise to use the boiler hydronics only in emergencies. Heat pump has to qualify as Cold Climate heat pump.

Definitely have the duct work evaluated versus your heating/cooling loads. Delta T is bigger for heating in MA than cooling.