Another Efficiency Question

[Madscientist267]

I live in eastern VA, USA, and we have a dual 2 ton heat pump system that heats/cools the house. We recently moved into the house, and it is new construction, with new everything. Problem is, I used to be in HVAC, and a very interesting problem presented itself to me as of tonight.

A good portion of the time, the temp stays above 40-50 deg F during the winter, so a heat pump is plausible IMHO for use in this area. Problem is, occasionally it drops. I'm considering a sensor that disconnects the compressor contactor so that the compressors don't run if their operation is futile.

Over the last few days, the temp outside has dropped to 15 deg F. Now my understanding is, with an R22 system, one cannot expect the temperature differential between condenser and evaporator to be more than 30 deg F, even on the best of days.

This being said, its not possible for the heat pumps to develop anything higher than 45 degrees on the air handler side, and since I am trying to maintain the inside air temp above 70 degrees, does it really make sense to let the compressors run?

The aux and 'emergency' electric heat coils are kicking in on a regular basis as indicated by the thermostats, illustrating that the heat pump portion of the system just can't do it's job. Since the supply air going back into the air handlers is above 70 degrees, compressor operation seems totally pointless.

So, my question is, do you all think that it would be economical to disconnect the compressors when the temp outside drops below a given threshold to improve overall efficiency, and if so, at what temp do they really become ineffective at adding any considerable heat to the system?

My fascination with polls is also emerging, so I present one to you for my enjoyment.

Steve

[DanG]

First thing to do is get the manual out for your unit - there likely an internal low-temp lock out in place; long run times (low output) will not harm the unit, the lock out is from an economic point. The set point may also be adjustable...

It sounds like you mean well, but you'd be loosing by depending on resistance heat! The heat output from resistance coils is locked at a one to one ratio while the phase change cycle can produce output gains of three plus to one input power unit.

I am looking at a 5-ton unit output chart, at 40°F the compressor uses 4000~ watts to produce 47,300 BTU which is equivalent to 13800 watts resistance heat. At 0°F the compressor does less work so consumes less electricity, 2700w for 23,300 BTU output for a 6800w resistance equivalent. Even at -20°F even less power is used, but the BTU output drops to a loss, 2000w for 10,300 BTU output & 1900w resistance element equivalent.

Run the unit.

Really neat energy conversion calculator

[DanG]

I know this isn't really RE related with running 'Heat Pumps' being out of the reach of hobbyist RE and an excellent example grid power excels at keeping people dependent...

This post came up just as I've been studying housing-type split HVAC units for a travel trailer, a SEER rating of 21 and 9,000 BTU still requires 10A ampacity 240VAC (0.83kW rated, 1.8kW max draw) circuit so its out of well out of reach of average DIY RE types, no AC or heat pump for my trailer... yet : )

[Madscientist267]

I suppose I hadn't really thought about it in terms of raw heat movement. I had them disconnected last night, and I think you're right. The heat never cut off to my knowledge, but it wasn't with the compressors running either. The system seemed to maintain the set temp however, so I am not entirely sure.

Today however, the heat still wasn't cutting off, and outside hit the low 30s. I reconnected the control leads for the contactors and more heat did begin coming out of the vents. This was in definite contrast to last night, where there was no noticeable change after disconnecting the contactors.

I based the theory on heat transfer from air handler coil to inside air rather than heat displacement from outside to inside. Meaning, if the refrigerant coils aren't rising above the target temperature, and the air being supplied to the coils is already above the theoretical maximum that they can hit, how can they add heat to the system?

Somewhere, there's a threshold, but -20F sounds a little lower than where it really does seem to become impractical to run them, although numbers seem to suggest otherwise. I'll do a little looking as soon as I get a chance, but the temps are supposed to be somewhat higher for the next several days, so I'll leave them hooked up.

Funny you mention about providing power for AC and such being outside the average DIY RE scope... It was the countless hours of reading posts here that got me on that train. I've been looking for ways of saving energy all over the entire house with the intent on eventually powering a good portion of it from solar. The ultimate idea is to light the entire place and power things like small appliances and the like from the RE, and save the grid for the hungriest of the hungry (heat/ac, cooking since we have no gas here, and hot water, etc) where RE just isn't practical yet. I'd really like to build a windmill too, but that's not going to be practical I don't believe - I live in a wind-defunct area for the most part.

Thanks again for the input. The topic is somewhat off-genre for the board, and I definitely appreciate the help. The idea was to ask some of the most efficiency-minded people I could think of - and you came through for me.

Steve

[scottsAI]

Steve,

AC is totally possible for RE IF you live where temperatures go below freezing for a while. Our ancestors cut ICE and stored in an ice house, using it for refrigeration during the summer.

Using modern materials and new ideas, the same thing can be done for AC and Fridge.

My house needs block 13' x 13' x 13', can store for 200 days with 10% loss.

Have fun,

Scott.

[fcfcfc]

Hi Mad:

You need to get the exact model number for the unit, as already suggested. You then go online if available and get the submittal sheet for that unit. It contains the COP's, EAT's LAT's etc.. at given outside temps. as well as all the BTU info you want. Heat is one thing, but as you were in the beginning eluding to, if the LAT is lower than the set point, you are going nowhere. You can find out this "POINT" by looking at the sheets.... Be aware that you may have to pull some teeth to get this info...

.....Bill