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-   -   Electricity usage - A/C vs HP Mode (http://www.diychatroom.com/f17/electricity-usage-c-vs-hp-mode-83008/)

kb3ca 10-04-2010 10:36 AM

Electricity usage - A/C vs HP Mode
 
I recently had a new Trane XL15i heat pump installed. It works great but I've noticed that it uses about 25% more electricity in the HP mode than it does in the A/C mode. It this normal? I checked it by observing the rotation rate of the electric meter. It's a dual fuel system (gas and Hp) so there's no electric heat strips. I've checked to make sure that nothing else was running when observing the electric meter. I recall that my old HP did something similar but wanted to check with the Pro's on this.
Appreciate your comments.

beenthere 10-04-2010 05:30 PM

25% is a bit of a jump.

What size heat pump. What temp are you maintaining your house at?

kb3ca 10-04-2010 06:46 PM

The unit is a 3.5 ton condenser with a 5 ton evaporator coil. The temperature in the house was 68 when I checked the electric usage in the heat mode and the outside temperature was 52. When I checked it in the A/C mode the outside temp was 85 and the inside temp was 75. The unit is rated at approx 15 Seer and HSPF of 9. I believe there was a difference with the old system as well but not as much. I was just surprised at the difference in the rate the meter revolved with this system. It works very well and am not complaining. Just trying to understand why there would be a difference. Can't measure the difference exactly but it's between 20-25 percent.
Thank you for responding.

HVAC_NW 10-04-2010 06:55 PM

The compressor is not running at a constant load and it depends on the load.
In summer, you have indoor at 75 and outdoor at 100, the difference is only 25, but in the winter, you have indoor at 68 and outside at 30 or 38 degrees of diff.

Colder evaporator temperature reduces coefficient of performance.

beenthere 10-04-2010 06:57 PM

At 52 outdoor temp. its grabbing a lot of heat. Which will make it work a bit more then when its 30 outside. But, you should have been getting a lot more heat from it then you did from your old unit.

kb3ca 10-04-2010 07:09 PM

Yes, heat output is much better than old unit which was approx 12 Seer or lower.
Appreciate the insight to how these things work. Mine seems to be doing quite well even with the 5 ton evaporator coil.

HVAC_NW 10-05-2010 06:24 AM

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Quote:

Originally Posted by kb3ca (Post 511432)
Yes, heat output is much better than old unit which was approx 12 Seer or lower.
Appreciate the insight to how these things work. Mine seems to be doing quite well even with the 5 ton evaporator coil.

The condensing temperature plays the biggest role in electrical power input.
Also, lower the temperature gradient, higher the COP.

Here is a chart showing input vs output for an example 3.5 ton 8HSPF heat pump (I really do not know why COP of HP can ever be below 1.0 though, since you'd think all the energy going into compressor will be dumped into condenser)

You'll see that when it's 30 degrees outside and 60 degrees inside, power consumption is 2.8kW, but if you were to run it when its 50 outside and 80 inside, such as at night during heating season right after its cooled outside, it will use as much as 3.8kW

You'll also see that indoor temperature has a great impact on power consumption per degree than outdoor, but power consumption isn't as important as COP.

On a 20 degree day, if you keep the stat at 60, you'll use 2.8kW, you'll enjoy a COP of 3, but if you raise the indoor setting to 70, COP drops to 2.56.

On a 0 degree day and 70 degree indoor, COP is down to 1.25, or just a tad bit better than resistive heater and heating capacity drops to tad over 12,000 BTU.


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