amp draw on a heat pump

oreo123 · February 15

I have some questions about current draw on a heat pump..

If a heat pump were running 1 of its 3 heads on a mild day do the amps drop to a third or even lower?
when a manufacturer says they want 220 volts at 40 amp yet they mention minimum current required. In a perfectly running system are they saying that it could actually operate at the min current draw when at full load? What I am trying to figure out is the actual watts for say a 3 ton unit on a wicked cold or hot day.
Have one of you placed an amprobe at the electrical panel to see the total load when a system is running 100 percent to see what percentage of the breaker's load its running at?

Just installed a new boiler. I can see its efficiency at 94 - 95 percent at full load and 1 percent load. I want to compare the efficiency between both units.

right now my area is 38 cents per kilowatt (generation and delivery).

my area is now $2.49 per therm of nat gas. a therm is 100k btus

Thanks from Boston

pecmsg · February 15

With 1 of 3 heads, it depends. some gas bypasses to maintain a minimum flow.

The only way to know is to run all 3 heads at full power.

Hot_water_fan · February 15

this info will be on the outside unit but I think the question you’re asking is more easily answered by comparing COPs, which are available (sometimes more easily for some brands) online.

oreo123 · February 16

On a high efficiency boiler it can roll down to 1/8th output. What I am wondering is IF a heat pump drops in current proportionally under very low load. For instance if a 4 ton unit had 4 head at 1 ton each - if only one head is working does the current draw drop to a fourth.

pecmsg · February 16

high efficiency boilers are not inverter technology.

Jamie Hall · February 16

As @Hot_water_fan says, the number you are looking for is the COP for your unit under various conditions. It is not a single number — it will change depending on the overall demand on the system, the delta T from cold deck to hot deck, and the temperature of the cold deck, among other factors.

The current draw of the system is easy enough to measure, provided you have a way to do so — but I don't recommend poking about in your switchgear with a clamp meter unless you are experienced with normal voltage/high current power. If you do measure the current draw, you will find that it is quite variable, with brief (or sometimes not so brief) spikes to near the rated maximum current.

Another possibility is to get an electrician to install a watt meter on the connection to the unit.

DCContrarian · February 16

If you have a variable speed compressor, in general at lower outputs the COP is higher. So not only does the electricity usage drop proportionate to the output, it drops further as the COP improves.

As an illustration, check out the data sheet for the Mitsubishi M-Series:

https://ashp.neep.org/#!/product/34581/7/25000/95/7500/0///0

At 47F, at maximum output the COP is 3.6 and at minimum output the COP is 4.65. At maximum the electricity usage is 2.28 kW, which corresponds 9.5A at 240V. At minimum the usage is 0.63 kW, which corresponds to 2.6A.

pecmsg · February 16

https://forum.heatinghelp.com/discussion/comment/1842983#Comment_1842983

The OP is trying to measure the current with only 1 head of 3 operating, that COP # will not be accurate under those conditions.

hot_rod · February 16

20 bucks at Harbor Freight and you could measure the current draw yourself.

120 bucks buys a top quality Fieldpiece or others.

Screenshot 2025-02-16 at 8.44.08 AM.png

oreo123 · February 16

https://forum.heatinghelp.com/discussion/comment/1842988#Comment_1842988

Thanks. I posted the chart below. o

When it gets colder say 47Fthe kwatts drops off to .63 for 10k btus of heat

At 17 F the kw is .86. But its not outputting 10k btus of heat - instead its 6.6k btus. To obtain 10k btus would require more kw. Do you think this would include the pan heater?

the reason why I am trying to figure this out is I have a space I want to condition. I have a 2 level house. 1100 feet per floor. a zone for each floor. Downstairs is forced hot water baseboard. I just roughed new kitch and bathroom and buried 3/4 pipes above the first floor ceilings-individually for each of the 5 rooms above.

boiler is a few months old, high efficiency and I have seen the 95% efficiency ratings. I am debating whether to do a heat pump for heat in second floor OR stay with the forced hot water. We do not want to heat all of rooms in second floor when they are unoccupied.

second floor has 2.5 ton central a.c. on R 22. Over the 22 years only had one break down - cap in outside unit. And a lot of filters replace.

I am hoping to figure out what is cheaper for heating right now. Electricity is 37 cents per kw. Nat gas is $2.49 per therm. I know that both will increase in price going forward and the idea of 2 different systems sounds like a plan. I know in my area a lot of people are complaining about higher utility costs.

Performance Specs

Heating / Cooling	Outdoor Dry Bulb	Indoor Dry Bulb	Unit	Min	Rated⁺	Max
				10,000	24,000	24,000


Cooling	82℉	80℉	Btu/h⁺	11,000	-	26,000
			kW	0.61	-	1.74
			COP	5.28	-	4.38
Heating	47℉	70℉	Btu/h⁺	10,000	28,000	28,000
			kW	0.63	2.28	2.28
			COP	4.65	3.6	3.6
Heating	17℉	70℉	Btu/h⁺	6,600	18,000	25,000
			kW	0.86	2.1	3.45
			COP	2.25	2.51	2.12
Heating	5℉	70℉	Btu/h⁺	4,800	25,000	25,000
			kW	0.87	4.19	3.83
			COP	1.62	1.75	1.91
Heating	-13℉	70℉	Btu/h⁺	5,900	-	20,000
			kW	0.37	-	5.1
			COP	4.67	-	1.15

pecmsg · February 16

2 stories

2 systems

Kaos · February 16

At your costs the heat pump would have to run at a COP of 4 to break even. Possible, but not easy.

Probably the best investment in your case is a PV array. At that silly high electricity costs, the ROI is very short. If you can get a bigger array with excess PV, then the math for the heat pump would be much better.

DCContrarian · February 16

@oreo123 : "Electricity is 37 cents per kw. Nat gas is $2.49 per therm. "

One therm is equal to 29 kWh. If you're seeing 90% efficiency, that's 26.1 kWh. At $2.49 per therm that's equal to 9.5c/kWh. That means your break-even COP is 3.87. In Boston you'd be lucky to see 3.0 as an annual average COP.

Agree with @Kaos that at those electricity prices solar is a good investment.

pecmsg · February 16

https://forum.heatinghelp.com/discussion/comment/1843093#Comment_1843093

That COP is only going to happen at 40+°F!

Kaos · February 17

You just need an oversized unit and run it at low load. For example, this one is COP 4 at 5F at min modulation:

https://ashp.neep.org/#!/product/25343/7/25000/95/7500/0///0

The thing you have to be careful with is not to oversize too much and run into cycling issues.

COP4 of is possible, but not easy.

hot_rod · February 17

if PV is a consideration act fast, here are programs available in MA

https://programs.dsireusa.org/system/program/ma/solar

oreo123 · February 20

PV is not gonna do much. Surrounded by 100 foot pine trees. Also think that the non leaking 23 year old roof would have to be replaced.

Jamie Hall · February 20

Not only that, but… even a pretty big roof won't have enough room for a big enough array to power a decent sized heat pump, except in full sun in spring or summer.

But not at night. Or on a cloudy day. Or under a foot of snow…

ethicalpaul · February 20

Jamie I know you know the PV hooks into the grid and what you generate offsets against what you consume.

These are not off-grid systems. Let’s not fight with straw men.

hot_rod · February 20

https://forum.heatinghelp.com/discussion/comment/1843746#Comment_1843746

I have 6200 W on my roof, 2300 sq ft home. Getting around 4800W any sunny day this winter. That more that covers my A2WHP.

My summer power bills have been as low as $35.00, the monthly minimum service and connection fees.

Jamie Hall · February 20

Oh I quite agree, @hot_rod . We have a 10 KW system (ground mounted) for Cedric's home — a farm — and expect the total bill from the power company for the year to be right around break even. Even with the apartment's heat pump. I'm all for it.

But…

As @ethicalpaul noted, also accurately, you still have to be connected to the grid, and that is something which I mention from time to time — not that most of us could be misled, but there are those out there who don't recognize that…

hot_rod · February 20

https://forum.heatinghelp.com/discussion/comment/1843867#Comment_1843867

I would expect the % of off grid PV systems is very small? And would they be running heat pumps of they are off gridders, Y2K survivalists :)

Jamie Hall · February 20

I would think so — very small, and for rather special situations. The fundamental problem for renewables at this point isn't the power supply — if you don't mind (to misquote Joni Mitchell) paving paradise and putting up a PV array, taken on a yearly average basis it's not hard to generate all the power needed for a year. The problem is… you also need to store, for all practical purposes, that much power when it is available (a sunny summer day, for PV, for instance) to be used for when it is needed (at oh dark hundred on a cold winter night). To really cover the need, the amount of energy storage required is staggering: a review of our own records, for instance, shows that to make it through a typical New England winter we would need to store about half the power we generate in the summer — something on the order of 10 megawatt hours. And that's for one farm. And I don't have space for 1,000 Tesla Powerwalls…..

DCContrarian · February 20

https://forum.heatinghelp.com/discussion/comment/1843888#Comment_1843888

On a personal level, sure. On utility scale, not so much.

I recommend this site, which shows the generation mix for the UK:

https://grid.iamkate.com/

In the past year, they've gotten more electricity from wind than from all fossil fuels. They get significant storage from pumped storage, where water is pumped upstream at hydroelectric facilities during slack times and released during peak times. The reported numbers under-report the degree of storage, because a lot of off-peak generation is exported to Norway where it is stored in reservoirs there.

ethicalpaul · February 20

It's a process, Jamie. The tech keeps improving and will continue to improve, and it will improve faster as the takeup increases. As for storage, I'll leave that to the utilities, except for maybe enough battery to run my stuff for a day or two.

I don't have PV yet, but I suspect I will within a couple years. My house faces south with a 10:12 roof pitch in that direction which I think is nice for my latitude.

Kaos · 3:15AM

https://forum.heatinghelp.com/discussion/comment/1843740#Comment_1843740

I would get a PV installer out to check. Even with some shading you can get surprising amount of PV. At your electricity costs and the low price per kW for PV, oversizing will still save you money.

A 23 year old shingle roof doesn't have that much life left in it. If you get your ducks in a row, you can do roof replacement and PV install at the same time when the current roof fails.

amp draw on a heat pump

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