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Comparing Efficiencies Between Gas-Fired 83% Atmospheric Boiler & Heat Pump

Member Posts: 1,849
edited March 2023

Lot of push out there on converting to heat pumps for both heating and AC. Con Ed requires de-commissioning the old boiler to get the rebate. I will not dump my fairly new boiler for any rebate, but I would like to know the numbers.

In the previous 12 months we used 500Therms for heating only. If I convert those therms to KW-Hours (14,664kwhs!) and multiply that by our 33¢ per kw hour I get \$4,836. But that leaves out the higher efficiency of the heat pump and other things I'm probably not even aware of.

How would I compare apples to apples here? Of course I would have to go month by month and calculate what those 500 therms cost me including all charges and taxes. Last month price was \$2.68 a therm incl all charges—very high, compared to a few years ago. If that price was constant then those 500 therms cost me \$1340. My guess is that regardless of heat pump efficiency, my correctly sized boiler with ECM circs, zone valves and reverse indirect does better. Though I haven't really specified which heat pump or factored in the electric charges for running the boiler….

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• Member Posts: 22,120

I think the heat pump would need to be in the 120 degree range to get the numbers to work out

If you don't have or able to get a heat emitter to work in that temperature range the boiler will pencil out better.

The building envelop drives the load.

Idronics 27 takes you through the math for the heat pump efficiency prediction based on all this

you have data on the boiler already

Ac can be a part of a heat pump option, if you want to factor that in

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,840

Oil \$/MMbtu: \$gallon x 1000000/ (138,000 X COP)

Heat pump \$/MMBtu: \$/kwh x 293 / COP.

• Member Posts: 1,849

@Hot_water_fan Thanks—what would the formulas look like if 'Oil' was changed to gas, which is what we use?

• Member Posts: 1,840
My bad! It’s \$/therm x 10 / cop
• Member Posts: 587
But the COP changes depending on the outdoor temperature, doesnt it ?
At some point, if electric resistance was used as primary when it got too cold for the ASHP, then the COP would be 1 ?
30+ yrs in telecom outside plant.
Currently in building maintenance.
• Member Posts: 16,832
And don't forget- much of our electricity still comes from fossil fuels. The efficiency of generating electricity this way is something like 30-35% by the time the electricity gets to the house or other point of use. So the boiler is way ahead.

This will change as more clean energy comes online, but as of now, switching would be even worse for the environment.

Then there's all the upgrade work the grid will need to move all that power.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting
• Member Posts: 23,275

But the COP changes depending on the outdoor temperature, doesnt it ?
At some point, if electric resistance was used as primary when it got too cold for the ASHP, then the COP would be 1 ?

It does indeed -- which makes the arithmetic rather messy. How much of a hassle that is depends on where you are -- anywhere warmish, say metro New York/Chicago and south -- it's not that big a deal Anywhere north of that it gets to be quite significant.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,840
The COP of a boiler also changes as temps do !
• Member Posts: 1,840
And don't forget- much of our electricity still comes from fossil fuels. The efficiency of generating electricity this way is something like 30-35% by the time the electricity gets to the house or other point of use. So the boiler is way ahead.
Steamhead, this is incorrect. The CO2/MWh is lower using the grid vs a boiler. The reason is that the 30-35% number is incorrect - it includes nuclear!
• Member Posts: 16,832
edited March 2023
@Hot_water_fan , I did not include nuclear in that posting, since its future is uncertain at best. "Fossil fuels" are coal, oil and gas.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting
• Member Posts: 1,840
Well that’s what the grid is right now. You reduce carbon emissions, on average, with a basic heat pump. Even your numbers prove it!
• Member Posts: 23,275

Well that’s what the grid is right now. You reduce carbon emissions, on average, with a basic heat pump. Even your numbers prove it!

Too many variables, friend. Best not to make statements like that without a lot of qualification. But we've talked about that before.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,245
Giveaway is that electric company wants you to trash your burner so that if you're unhappy with electric then tough noogies. QuebecHydro did that before an ice storm blacked out Montreal. During Canadian winter having heat is more important than \$\$\$.
• Member Posts: 1,849
Just had a combustion test and the tech echoed those sentiments. Says the heat pump units have a 2% tolerance for voltage reductions so if there's a power shortage in the hottest months their ACs won't work during that time.
• Member Posts: 4,842
D107 said:
Just had a combustion test and the tech echoed those sentiments. Says the heat pump units have a 2% tolerance for voltage reductions so if there's a power shortage in the hottest months their ACs won't work during that time.
BS

All motors can tolerate 10% above or below name plate.
• Member Posts: 22,120
A few graphs to help show efficiency. Remember also, output of the HP drops as temperature drops. Graph 2.6

Graph 2.9 shows COP at different SWT temperature

Does you boiler in fact run 83%? If it is zoned and short cycles it could drop into the mid or high 70%. You need to look at cycle efficiency, not the number one the label.

Next you could look at historic weather data to see how many days you are at various outdoor temperatures. Fig 3.4 for upstate NY

Graph 2.11 shop EER, cooling performance, if that matters.

Compare energy cost here.
https://coalpail.com/fuel-comparison-calculator-home-heating\

So if you could run 3 COP or higher, Gas at \$2.68, 83%.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,840
edited March 2023
@Jamie Hall sure, variables are involved. If a 95% efficient gas boiler emits 116 lbs of co2 per MMBtu input that’s 122 lbs of CO2 per MMBtu output. For the sake of moving this along, let’s just call that the gas side of the equation. In 2021, the EIA pegs US CO2 emissions at .855 lbs/kWh. Multiply that by 293 and divide by COP. Anyone else can now fill in the COP of their choosing to compare modcon vs heat pump.
• Member Posts: 23,275
Provided that pick the COP which is relevant to the temperatures at which the unit is running, I'll buy it. Unhappily, just like the mileage sticker on your car, most people will pick the advertised figure -- and completely overlook the real figure when you really need the heat.

If you are to be an engineer or a good tradesman, you will work with the specific application at hand. If you are a salesman or an advocate, you work with broad averages. Take your pick.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,840
edited April 2023
A seasonal COP is appropriate if you’re comparing annual emissions between two systems. Let’s be real - most people won’t even bother to do that. If you want more granularity, you could monitor real time COPs of both systems and also the grid generation mix. Obviously, all three of these variables are constantly moving. The name called is truly uncalled for.
• Member Posts: 529
To almost everyone not reading or posting on The Wall, none of this matters. Beyond reliability, the only thing important to the vast majority of people is how many dollars it costs per year to heat and/or cool. The price of different energy sources (natural gas, propane, oil, electricity) going forward is a complete unknown. We can analyze till the cows come home, but tomorrow all those numbers have every chance of being completely irrelevant. Efficiency is but one of many factors to consider when selecting HVAC equipment.
• Member Posts: 1,849
edited April 2023
@Sal Santamaura Agreed about future fuel sources and choices being very hard to foretell. As a homeowner I look at longterm cost/efficiency/reliability. The question came up here because as I--and many others--look to replace an aging central AC system I am being bombarded with heat pump deals with incentives and tax credits without any kind of backing for the claims. I don't like the bandwagon or stampede approach to the latest flavor. Despite the energy market changing beneath our feet, I do think it's possible to make an informed choice for our house given current circumstances. If cold fusion somehow happens in five years, we'll all have to deal with it I guess.

So since we are on the Wall--small minority we may be--trying to figure these things out can be useful and for whatever ripple effect this site may have. Having a five year old solid heating system with fine system efficiency that could easily last my lifetime makes me a tough target for the contractor promising how my bills will plummet if get the heat pump for both heating and AC. If the utility really thought that heat pumps will save so much money over all other systems, they would have already done the studies to prove it for us to see. I'm not even mentioning comfort.....
• Member Posts: 1,840
@D107 often this is confused as a binary. It’s not! You can install a heat pump and keep the existing system too. If an AC is at end of life, it’s a easy sell - you add flexibility for cheap. Or add AC, which is popular. I don’t personally care about the rebates - they can keep them. I want what’s most comfortable.
• Member Posts: 23,275
I aopologise, @Hot_water_fan , if I gave offense. I didn't intend to. And I quite agree with @Sal Santamaura ''s point about none of it mattering to the vast majority of people -- who simply want something which works and doesn't cost a fortune My point is this -- perhaps best summarised by the old maxim "horses for courses". Our job, as responsible people, is to not only satisfy that need for something which works and doesn't cost a fortune, but to do it in the best way we can find. If you were to ask me for a new build or replacement for a hot air system for the Baltimore area, for example, I wouldn't hesitate to recommend an air to air heat pump; for an over-radiated hot water system, an air to water unit (but I'd have to see what they had in the way of radiation vs. heat loss). For San Diego (do you even ever need heat out there, @Sal Santamaura ?!) the same. For my next door neighbour? Oil fired hot water -- but probably gas fired in those more urban areas which have natural gas, but with an air to air heat pump if they needed air conditioning.

And so on.

My beef, and yes I do have one, is for a one size fits all solution, either promoted or mandated, when that one size simply doesn't work with present technology.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,849
edited April 2023
@Hot_water_fan Understood. But many contractors always push for complete replacement. I already know I'll stay with my current heating, but I wanted to know the numbers nevertheless. Turns out comparison is not so easy and utlimately for me it's academic.

What the contractors seem to be saying is that if you opt for straight AC replacement, it will be more difficult or expensive to get the features that will provide greater comfort and economy by matching variable speed fans to the AC load of the moment. So my plan is to either get the straight AC or add the heat pump (installing it but not using for heat) which will get me a partial rebate/credit) and greater comfort but allow me to continue using my existing system.
• Member Posts: 1,840
What the contractors seem to be saying is that if you opt for straight AC replacement, it will be more difficult or expensive to get the features that will provide greater comfort and economy by matching variable speed fans to the AC load of the moment. So my plan is to either get the straight AC or add the heat pump (installing it but not using for heat) which will get me a partial rebate/credit) and greater comfort but allow me to continue using my existing system
Can you explain this some? Are you saying they aren’t offering variable speed air conditioners but are offering variable speed heat pumps?
• Member Posts: 1,840
edited April 2023
@Jamie Hall I understand and respect your stance. However, can we expect to individually answer questions about the ~100 million residential systems in the US? Or does this do a good enough job: if you have ductwork, perhaps consider a heat pump when your AC breaks while keeping the furnace/boiler if necessary/desired, and if you don’t have ductwork, a heat pump is substantially harder  but may be technically possible if you’re especially motivated and are open to a backup system? Most systems in the US are not bespoke - they are either heat pump or furnace + AC.
• Member Posts: 2,245
Is it a plus or minus? A heat pump is powered all year so the oil heater is never turned off by disconnecting electricity.
• Member Posts: 587
I think we're in a gold-rush moment for HP makers, and this has potential for future headaches.
If everybody starts tripping over themselves and begin sourcing sub-parts from new (unknown quality) vendors, and putting new hires onto the mainline building..

Would be a good time to be a young(ish) hvac guy with the will to put in the serious hours though.

30+ yrs in telecom outside plant.
Currently in building maintenance.
• Member Posts: 22,120

I think we're in a gold-rush moment for HP makers, and this has potential for future headaches.
If everybody starts tripping over themselves and begin sourcing sub-parts from new (unknown quality) vendors, and putting new hires onto the mainline building..

Would be a good time to be a young(ish) hvac guy with the will to put in the serious hours though.

From the same manufacturers making components for mod cons, home appliances, etc?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 2,646
edited April 2023
To get the rebate you must decommission the boiler.........

That is a very very scary thing if given much thought. Ol' Con Ed is really looking at the bottom line ain't they? Can you say POWER OUTTAGE?

Here in the 4th grid of the US (Norther Maine) the utility is pushing for a 34% increase, that is after we had a 30% increase January 2021. Hmmmm, all those heat pumps aren't so economical anymore for heating.
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 2,646
D107 said:
Just had a combustion test and the tech echoed those sentiments. Says the heat pump units have a 2% tolerance for voltage reductions so if there's a power shortage in the hottest months their ACs won't work during that time.
All of the Mini-splits I've installed (I'm not pushing them) are rated for 200-240 volts. Your residential voltage is 240 nominal, and the unit will tolerate a very wide dip in voltage, much more so than a typical PSC A/C unit. So that isn't a worry.

Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 236
@Solid_Fuel_Man - looking at the ConEd website right now, it shows a \$2500 rebate if the heat pump is set to priority with an 'integrated control', and an \$8000 rebate for decommissioning of the previous system. ConEd is both the gas supplier and the electric supplier for most of their service area, and both gas and electric are relatively expensive - \$0.296/kwh and \$2.42/therm for my last bill. Breakeven COP vs an 80% boiler is like 2.9 right now - operating costs would probably be a wash for most of their customers.
• Member Posts: 1,849
@Hot_water_fan
You wrote: Can you explain this some? Are you saying they aren’t offering variable speed air conditioners but are offering variable speed heat pumps?

They'll do either but they push where the rebate is highest as you can see from some subsequent posts. The highest rebates require de-commissioning the boiler. The big push up here now is geothermal and ashp. There are other types of (air source?) heat pumps which I believe can transfer the heat to an electric hydronic boiler but I never hear of that up here in Westchester. And most of them don't mention a Manual J even if we've insulated substantially since the AC was installed 30 years ago. They just ask if we're happy with the way our existing AC works--like heating companies who just replace boilers with same-sized units.
• Member Posts: 236
@D107 - We're in the same area. I haven't found a place advertising air-to-water heat pumps up here yet, although I'm hoping Taco's new System M might gain some traction. Mini-split installers seem to be common.
• Member Posts: 529
edited April 2023

...For San Diego (do you even ever need heat out there, @Sal Santamaura ?!)...

San Diego Gas & Electric is our electricity utility and Southern California Gas Company supplies us natural gas, but I'm not in San Diego. We live in San Clemente, in a valley 2.5 miles from the coast, halfway between Los Angeles and San Diego. To answer your question, yes, we do indeed need heat here. In our typical 2,250 square foot "heap of stucco" as Little Richard used to call tract houses in this area, with its absurd high ceilings and 1993 original builder's furnace, this has been by far the coldest winter season in the 30 years since moving in. Typical summer months, during which an occasional outdoor grilling session is the only thing consuming gas beyond a water heater, stove and clothes dryer, sees us purchasing around seven therms. In winter we set the thermostat to 69 degrees F daytime and 65 setback at night, wearing warm clothes and placing a down comforter on the bed. In 2021/2022 we peaked in January/February at 41 therms. This heating season, which was not only colder but longer than any of the previous 29, extending from November through March (and there's still a cycle now and then in April), we've averaged 73 therms per month. Growing up in the Bronx and Yonkers, with cast iron radiation (standing iron first and Burnham semi-recessed "radiant" second), it always felt warmer than scorched air does here. Some of that might be attributable to the contrast of coming back inside after being out in really low ambient temperatures, while even the lowest we've had here is in the mid to high 30s F. Most of the comfort difference, however, is likely wind chill from the single-speed blower of our Bryant. It has usually not been much of an issue, since your perception of "heat, what heat?" was pretty close to accurate. I've often felt wasteful replacing the filter given how little air passed through it in a season. This year, anthropogenic global warming has upset weather patterns so much that I no longer feel guilty referring to this time of year as "winter."
• Member Posts: 1,356
...For San Diego (do you even ever need heat out there, @Sal Santamaura ?!)...
San Diego Gas & Electric is our electricity utility and Southern California Gas Company supplies us natural gas, but I'm not in San Diego. We live in San Clemente, in a valley 2.5 miles from the coast, halfway between Los Angeles and San Diego. To answer your question, yes, we do indeed need heat here. In our typical 2,250 square foot "heap of stucco" as Little Richard used to call tract houses in this area, with its absurd high ceilings and 1993 original builder's furnace, this has been by far the coldest winter season in the 30 years since moving in. Typical summer months, during which an occasional outdoor grilling session is the only thing consuming gas beyond a water heater, stove and clothes dryer, sees us purchasing around seven therms. In winter we set the thermostat to 69 degrees F daytime and 65 setback at night, wearing warm clothes and placing a down comforter on the bed. In 2021/2022 we peaked in January/February at 41 therms. This heating season, which was not only colder but longer than any of the previous 29, extending from November through March (and there's still a cycle now and then in April), we've averaged 73 therms per month. Growing up in the Bronx and Yonkers, with cast iron radiation (standing iron first and Burnham semi-recessed "radiant" second), it always felt warmer than scorched air does here. Some of that might be attributable to the contrast of coming back inside after being out in really low ambient temperatures, while even the lowest we've had here is in the mid to high 30s F. Most of the comfort difference, however, is likely wind chill from the single-speed blower of our Bryant. It has usually not been much of an issue, since your perception of "heat, what heat?" was pretty close to accurate. I've often felt wasteful replacing the filter given how little air passed through it in a season. This year, anthropogenic global warming has upset weather patterns so much that I no longer feel guilty referring to this time of year as "winter."
I'm sorry, I got a chuckle from your 73 therms. I can burn that in a little over a week. :P
• Member Posts: 1,356
edited April 2023
One thing I haven't seen mentioned is the savings if one can eliminate gas service altogether. For my area the base charge is now \$57 I believe. Or will be shortly. While the puco(public utilities commission of Ohio) didn't approve what Columbia gas was asking for, they did approve a pretty steep increase.

What this means is that even in the middle of July when I use maybe 2ccf, I'm still paying almost 60\$. If you can decommission all of your gas appliances and shut off the gas entirely. There is that. 60\$ can buy a good amount of electricity.

Or put another way, in my neck of the woods that is 684\$/year, every year regardless of outside temp and system efficiency until prices go up again in energy savings.
• Member Posts: 1,356
And I personally am real close to going all electric. I'm seriously looking into replacing my gas dryer with an electric. With the biggest benefit being that I won't have to have the gas line relocated for me to finish my mancave, which offers substantial savings(I can do the wiring my self 240v 30a is easy peasy). That will leave me with only the 40 year old boiler.
• Member Posts: 2,646
@JakeCK can you have propane where NG is available?

That \$60 a month would push me away from gas too! I know some utilities (poco) do the same thing with electric meters.

Thankfully, our POCO only charges us for 50kW of electricity if we use it or not. That way if you use 55kW a month, you pay for 55kw. If you use 40kW a month you pay for 50kW. They do this in lieu of a base charge, which I think is more than fair, even though our electric rate is now over \$0.20 a kW......
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 529
JakeCK said:

...I'm sorry, I got a chuckle from your 73 therms. I can burn that in a little over a week...

That was the average. Our January 2023 bill shows 83 therms. As a result of Sempra (SoCalGas' parent company) manipulating transactions between its wholesale and retail holdings, the cost to us for those 83 therms was \$334.66. Please note that I'm not whining; we're fortunate enough to just pay it and move on. But many who aren't ought be provided with adequate support so they don't suffer in the cold. While those in rigorous climates might chuckle at low ambient temperatures "only" in the 30s F, people around here who simply didn't have the money found their homes becoming close to that inside. Not healthy or appropriate.