Air source heat pump vs oil boiler with tankless

mike3181

Hi all, 
I live in the Northeast, Connecticut in particular.
I have air source heat pump and oil fired boiler with tankless hot water. 

Currently have it set for boiler to recover at coldest part of the day 5-6am then switch to heat pump during the day while the sun is out etc. then set back at night, recover again with boiler next morning and so on.(when heat pump is running thermostats on boiler set back far enough they will not compete) 

Although a thought crossed my mind. Is running the heat pump essentially pointless since the boiler is still going to fire regardless to keep boiler at set temperature for hot water heating needs? 

So I guess my question is, is it worth it to run the heat pump? Or will the boiler fire a lot more if using it for heat/hot water?

MikeAmann

What type of control is on the boiler?
And do you need to have hot water during the day? Or is everyone at work?

JMWHVAC

On paper it is worth running HP when outdoor temps are mild. However, you have a valid point. If boiler is all hot anyway why not use it instead of putting hours on the HP. Add a HP water heater to the mix then you don't start boiler till it is cold. You didn't say how the oil heats your house. HP is a great combo with floor heat for those not so cold seasons, enabling a little quick heat without pumping a lot of heat in floor just to overshoot in the afternoon.

mike3181

Sorry the boiler feeds, baseboard heaters. House built 02’ so fairly well insulated etc.

i have Honeywell for control at the boiler, yes need hot water during day. 

mike3181

Aquastat controller*

hot_rod

Even if the boiler stays hot for the tankless coil, if you are not circulating water out of it, the oil consumption is only what is required to keep the boiler itself warm
If you calculated the operating cost of both systems, then you would have a better answer.
Some suggest the HP down until the COP drops below 1, or a low ambient temperature lockout prevents it from running

mike3181

hot_rod said:

Even if the boiler stays hot for the tankless coil, if you are not circulating water out of it, the oil consumption is only what is required to keep the boiler itself warm If you calculated the operating cost of both systems, then you would have a better answer. Some suggest the HP down until the COP drops below 1, or a low ambient temperature lockout prevents it from running

Exactly what I was looking for, so I am better off keeping it how I have it. Set back at night, recover early AM coldest part of the day with the oil then switch back to heat pump and so on. Then when winter really hits probably scrap the HP altogether and run the boiler. 

hot_rod

It depends on how old the HP is, what type of refrigerant. New models claim to work below 0 degrees. Your manual may have some performance data? My issue was the cold air temperature that they blow out in cold outside conditions.

If you are at work during the day or get passive gain, that may be comfortable enough for you?

mike3181

Mitsubishi h2i they claim -13°. Installed last March so first heating season with it, I do have a smart oil gauge and energy meter for the HP just trying to play around with both and find the most efficient combination. I have noticed the cold air you mention as the fan constantly runs at a low speed when not heating. Also sometimes will get a cold flow if the unit is in defrost. 

Robert O'Brien

What's your cost per Kwh?

Hot_water_fan

Mitsubishi h2i they claim -13°. Installed last March so first heating season with it, I do have a smart oil gauge and energy meter for the HP just trying to play around with both and find the most efficient combination. I have noticed the cold air you mention as the fan constantly runs at a low speed when not heating. Also sometimes will get a cold flow if the unit is in defrost. 

Constant blower is a setting, you can turn it off. At todays oil prices, run that heat pump! 

mike3181

Robert O'Brien said:

What's your cost per Kwh?

.255kWh 

mike3181

Hot_water_fan said:

Mitsubishi h2i they claim -13°. Installed last March so first heating season with it, I do have a smart oil gauge and energy meter for the HP just trying to play around with both and find the most efficient combination. I have noticed the cold air you mention as the fan constantly runs at a low speed when not heating. Also sometimes will get a cold flow if the unit is in defrost. 

Constant blower is a setting, you can turn it off. At today’s oil prices, run that heat pump! 

I was reading that somewhere although looking through the Mitsubishi manual I could not find anything about how to turn off the constant blower off. 

Jamie Hall

Ah... maybe not in Connecticut, @Hot_water_fan . The lowest price for residential electricity in CT at the moment is $.31 per KWh, and scheduled to go up about 10% in January. That works out to just under a dime for resistance, or perhaps 4 cents for a heat pump in colder weather, per mBTU. The OP should be able to get oil at less than 3 cents per mBTU -- I'm paying 2.4 this winter. Even with an 80% boiler, the oil is going to be cheaper.

That said, using a heat pump for spot heating does make sense -- the house Cedric powers has an apartment in it, and the tenant likes it a good bit warmer, so there is a heat pump for that space -- so Cedric does the heavy lifting, and the heat pump keeps the tenant more or less happy.

Hot_water_fan

@Jamie Hall that’s not what OP said. Anyway, let’s use $/MMbtu only for comparisons. Easier to comprehend. 

Hot_water_fan

I was reading that somewhere although looking through the Mitsubishi manual I could not find anything about how to turn off the constant blower off. 

I forget which manual it was, but the passcode is on the back of the thermostat. It’s not the normal user manual. 

mike3181

Jamie Hall said:

Ah... maybe not in Connecticut, @Hot_water_fan . The lowest price for residential electricity in CT at the moment is $.31 per KWh, and scheduled to go up about 10% in January. That works out to just under a dime for resistance, or perhaps 4 cents for a heat pump in colder weather, per mBTU. The OP should be able to get oil at less than 3 cents per mBTU -- I'm paying 2.4 this winter. Even with an 80% boiler, the oil is going to be cheaper. That said, using a heat pump for spot heating does make sense -- the house Cedric powers has an apartment in it, and the tenant likes it a good bit warmer, so there is a heat pump for that space -- so Cedric does the heavy lifting, and the heat pump keeps the tenant more or less happy.

I use alternate provide so my kWh is roughly .255kwh. Had my boiler cleaned the other day they paper work says 85% efficient how true that is I don’t know weil McLain gold. 

I’m not positive how to do the math maybe someone smarter than me could post the numbers per btu

Jamie Hall

Well, one kilowatt hour -- KWh -- is 3,400 BTU. One gallon of oil is about 140,000 BTU. So if your electric rate is currently $.255 per KWh, that is $0.075 per thousand BTU. If your oil is $3.25 per gallon, like mine, that is $0.023 per thousand BTU.

However, those are raw figures. For electric resistance heating those are also net figures for your heat, since electric resistance heating turns all of the energy directly into heat. For a heat pump, however, there is the "COP". Heat pumps have a somewhat magical air about them, but the bottom line is that through soem thermodynamic wizardry they extract heat from the outside air (usually) and raise its temperature, so it can heat the house -- and thus they use less electricity to provide heat than direct would. The COP is quite variable, but a reasonable figure to use in cold weather would be 2.5 -- that is, they deliver about two and a half times as much heat as they take electricity to run. Another way of looking at it is that the cost of a BTU delivered is the cost of the electricity divided by the COP -- in this particular case, then, your 0.075 divided by 2.5, or $0.03. On the other hand, not all of the heat in the fuel oil winds up heating your house -- so again we divide, in this case the cost of a BTU or oil by the efficiency of the boiler, which in your case is your $0.023 divided by 0.85, which comes out to $0.027.

Close to a wash.

mike3181

If my math is correct,
1M/btu oil 139,400/3.69 85% = $32.11
1M/bto HP 1M/3412x.255/2= $37.76

HVACNUT

Is it in the budget to pipe an indirect to the boiler and use a cold start aquastat, or HP water heater?

STEVEusaPA

A small indirect and switching the oil boiler to cold start may be the better option, and keep your same run strategy.

mike3181

I have thought about indirect although, both plumbers I spoke with said I could run into bigger problems with boiler leaking. Since the boiler has never really “cooled down” if I switched I could end up with leaking. 

HVACNUT

mike3181 said:

I have thought about indirect although, both plumbers I spoke with said I could run into bigger problems with boiler leaking. Since the boiler has never really “cooled down” if I switched I could end up with leaking. 

I call BS. What make and model boiler?

mike3181

HVACNUT said:

mike3181 said:

I have thought about indirect although, both plumbers I spoke with said I could run into bigger problems with boiler leaking. Since the boiler has never really “cooled down” if I switched I could end up with leaking. 

I call BS. What make and model boiler?

Weil McLain Gold P-Wtgo-3
Not sure maybe they were just not interested in doing the work, talking me out of it?

Hot_water_fan

If my math is correct,

1M/btu oil 139,400/3.69 85% = $32.11
1M/bto HP 1M/3412x.255/2= $37.76

@mike3181 I have a Mitsubishi hyper heat as well, and get around 2.8 COP when I compare kWh to the therms I used to buy, so I think 2 is selling the system short. 

Your strategy is well thought out - the average day’s low temperature is regularly between 4 and 6 AM, so this is when the heat pump COP would be lowest and also when the boiler run time would be highest (best for boiler efficiency). A lot of attention is paid to “shoulder seasons” but with how much variability happens day to day, “shoulder hours” probably makes more sense to be frank. 

HVACNUT

mike3181 said:

HVACNUT said:

mike3181 said:

I have thought about indirect although, both plumbers I spoke with said I could run into bigger problems with boiler leaking. Since the boiler has never really “cooled down” if I switched I could end up with leaking. 

I call BS. What make and model boiler?

Weil McLain Gold P-Wtgo-3
Not sure maybe they were just not interested in doing the work, talking me out of it?

I don't see a GO block leaking because of cold start. Circulator gaskets, maybe weepy relief valve. Both easy fixes. 
An indirect with a modern, boiler reset aquastat will save you $$.

mike3181

Hot_water_fan said:

If my math is correct,

1M/btu oil 139,400/3.69 85% = $32.11
1M/bto HP 1M/3412x.255/2= $37.76

@mike3181 I have a Mitsubishi hyper heat as well, and get around 2.8 COP when I compare kWh to the therms I used to buy, so I think 2 is selling the system short. 

Your strategy is well thought out - the average day’s low temperature is regularly between 4 and 6 AM, so this is when the heat pump COP would be lowest and also when the boiler run time would be highest (best for boiler efficiency). A lot of attention is paid to “shoulder seasons” but with how much variability happens day to day, “shoulder hours” probably makes more sense to be frank. 

You are correct I did a little more digging and reading replies here I came to the conclusion it makes more sense for me to use the oil with a set back at night down to 64° and recover early morning with oil to 69° then switch to HP during daylight hours. Night lows are mid to low 20s at the moment. advertised specs for multi head h2i are something like 3.5 @47° 2.5 @17° 2@5° 
so my break even point being my electric rate is on the higher side is low to mid 30°s 

mike3181

HVACNUT said:

mike3181 said:

HVACNUT said:

mike3181 said:

I have thought about indirect although, both plumbers I spoke with said I could run into bigger problems with boiler leaking. Since the boiler has never really “cooled down” if I switched I could end up with leaking. 

I call BS. What make and model boiler?

Weil McLain Gold P-Wtgo-3
Not sure maybe they were just not interested in doing the work, talking me out of it?

I don't see a GO block leaking because of cold start. Circulator gaskets, maybe weepy relief valve. Both easy fixes. 
An indirect with a modern, boiler reset aquastat will save you $$.

I will have to look into that, not only for the savings but tankless is horrible. Either the coil is gummed up and needs changing or just design flaw, but filling tub for kids takes forever having to start stop etc.