Will a heat pump compresser draw the same amount of power no matter what the temp?

[Bretts]

I'm into home automation and right now I'm trying to use my home automation system to help control my energy usage and also get an idea of how much energy I'm using. I was looking at several devices that can read the electric usage of my house and/or a large appliance like my heat pumps, but they are all fairly pricy and I really don't want to spend that much right now.

I was thinking about it, though, and I realized that since my thermostats are already tied into my automation system that I can tell how often and how long the heat pumps run for. I was thinking that if I took an ammeter and measured the current draw of the heat pump while it was running and the current draw of the air handler while the fan was on, then by multiplying it out based on how often and how long the system runs for I could determine approximately how much electricity it was using. I imagine that the current draw of the blower motor in the air handler is pretty much static, but I didn't know if the compressor used the same amount of electricity whenever it runs or if it will use more when the AC is on and it's 100 degrees outside rather than 80 degrees.

Do you guys have any idea? Just to be clear, I'm not asking how to measure the current draw of the compresser, but simply whether the current draw will be the same all the time or if it will vary depending on the outside (or inside, for that matter) temp.

Thanks much,
Brett

[4l530]

The compressor's amperage will tend to increase as the outdoor temperature increases from what I've noticed.
The amperage will also increase as the condenser coil progressively loads up with grass clippings, cottonwood seed, etc.

[beenthere]

The condenser amp draw varies with teh OD temp, and teh ID temp.
If you have a VS blower, its amp draw increases as teh filter loads up.
A standard blower the opposite.

[Bretts]

The condenser amp draw varies with teh OD temp, and teh ID temp.
If you have a VS blower, its amp draw increases as teh filter loads up.
A standard blower the opposite.

Oh well... It seemed like a good idea, at least.

Thanks

[beenthere]

It was on paper.

Just not as good when applied.

[mark beiser]

Hmm, my reply to a different post ended up here for some reason...

[a0128958]

I'm into home automation and right now I'm trying to use my home automation system to help control my energy usage and also get an idea of how much energy I'm using. ...

... I was thinking that if I took an ammeter and measured the current draw of the heat pump while it was running and the current draw of the air handler while the fan was on, then by multiplying it out based on how often and how long the system runs for I could determine approximately how much electricity it was using. ...

Brett

You can look here for ideas on monitoring electricity consumption (and other HVAC related stuff): http://welserver.com/WEL0043
This is what I have set up for my residence.

One of the charts shows KW consumption and specifically breaks out the heat pumps. The chart shows steady current draw as a function of time of day, and without regard to OD temp. In my case, though, since my heat pumps are ground source geothermal units, it makes sense that current consumption is steady, as my incoming water temp doesn't vary significantly during just one day.

Looking at the specs for the GSHPs I have, KW varies significantly as a function of incoming water temp. For my 5 ton unit, operating at low speed, for example, power consumption ranges from 1.48 KW (30° water) to 4.21 KW (120° water).

This matches up with posts above that confirm that for an air-source heat pump power consumption will vary as a function of OD temp.

Best regards,

Bill

[travlinjohn]

In both the heating and cooling mode a heat pump's amperage increases as the temperature increases. the amperage on the fan decreases as the filter or any air restriction increases.

[cem-bsee]

if you want to save money, get a 7d programmable t-stat --
set the temp back when not at home & when family is sleeping --
the more setback = more savings!
the amount depends upon what your family will tollerage --
I like but 10F cooler than OD in summer --
wider in the winter -- just wear sweater | hooded sweat --

DON'T try to start | stop hp with automation!!!

[Bretts]

DON'T try to start | stop hp with automation!!!

My thermostats are connected to my automation system, so I can start and stop the system by adjusting the thermostat through the automation system. That way it behaves like a programmable thermostat, but I don't have to worry about always getting up or going to work at the same time every day. When the last person leaves the house in the morning they just have to push a button by the door... that arms the alarm system, shuts off any lights that were left on in the house, and sets the thermostats back to 85 degrees. When we're on our way home we can send an email to the house from a cell phone to lower the thermostats to 78 degrees so it's nice and cool when we get there.

Same thing at night... there's another button by the bed that will arm the alarm, shut off the lights, and set back the downstairs thermostat (That button won't set back the upstairs thermostat, though, so we stay comfortable as we sleep).

I tried using a regular programmable thermostat for a while, but the problem was that I don't always get up or go to bed at the same time... especially on the weekends. I found that I left the programmable thermostat in manual mode most of the time because it was too inconvenient when it tried to change the temp when I was still home or awake.

Brett

[beenthere]

cem bsee was refering to the heating season.
Unless you have the aux heaters locked out. Temp set back with a heat pump can cost you more then letting it maintain one temp.

[Bretts]

cem bsee was refering to the heating season.
Unless you have the aux heaters locked out. Temp set back with a heat pump can cost you more then letting it maintain one temp.

Really? That's good to know. I don't think there's much of a heating season here in Central Florida, but I just moved here from upstate NY, so I've never had a heat pump before. Out of curiosity, why does it cost less to maintain the temp? Because the heat strips are expensive to operate?

Thanks,
Brett

[beenthere]

Yes.
If the heat strips come on. They can use 2, to times 3 more electric then the heat pump does. Sometimes more yet.

[jcameron]

I too have an automation control system on our electric heat pump. I have learned that if I set back the thermostat at all more than 4 degrees in any zone, it costs me $$$. (Unless I am gone for more than 24 hours.) Now we have a Carrier Infinity Zone System designed specifically for our equipment, sounds like you might have a whole house security/lighting/etc... type, or what I call a load shedding type system.

We use our "set back" for more of a comfort setting, than an efficiency setting. We like it cooler at night for sleeping.
Taking into account that todays systems are alot more efficient than the systems produced pre-1992. It is my feeling that it costs you a lot more to set back a thermostat more than 2-4 degrees than what is worth. In theory it sounds good to set a thermostat back, but by the time your new "High Efficient" system heats up or cools down all internal walls, furniture, etc.... (let alone the space) you have used up all those $$ you saved for that 6-8 hours you were gone from your home with a lower thermostat setting.

Our control system lets us program what outside temperature that the electric heat strips are allowed to come on to help supplement the heat pump. We have set this at our "Balance Point", (The point at where the heat pump no longer is able to heat the home) which is at 22 degrees, in our case.
In my opinion, if your system is sized properly, your balance point should be somewhere between 15-25 degrees outdoor temp.

As for you initial question, the hotter outside it gets during the summer, the higher the amp draw is on the compressor. As for the winter, the same is the norm, but after a certain outdoor temperature, the compressor should maintain a given amperage. Keep in mind that at your balance point the compressor should run continuously with the electric resistance heaters going on and off with a call for 2nd or 3rd stage, unless your control system keeps the 2nd (and/or 3rd) stage on until the call for heat from the thermostat is gone. (then the compressor will also shut off).

Good luck, and hope this helps!

[teddy bear]

Here is Kw for a 2 ton HP compressor only at different outdoor temps.

Outdoor Kw
85^F 1.84
95^F 2.08
105^F 2.35
115^F 2.66
This current efficient equipment.
Regards TB

[Bretts]

Here is Kw for a 2 ton HP compressor only at different outdoor temps.

Outdoor Kw
85^F 1.84
95^F 2.08
105^F 2.35
115^F 2.66
This current efficient equipment.
Regards TB

Thanks TB. My HP's are new (3 or 4 months old) 2.5 ton 14 SEER Lenox ones, so I imagine the numbers should be a bit more than that, but it's good to see at least ball park numbers.

[beenthere]

Those numbers may be at an indoor temp of 80*F

[beenthere]

A 14 SEER York, 2.5 ton.
OD temp/ID temp/KW
75/70/2.07
85/70/2.3
95/70/2.54
105/70/2.8