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horseonthefly 03-14-2012 11:37 AM

Heat Pump Efficiency Help!
 
I'm a noob so go easy on me :thumbsup:

Random information about my house:
It is 926 square feet, has R13-14 walls, R30 Ceiling, Insulated (1 inch) Slab foundation
The heat pump is the original 1987 Lennox, 2 ton, ~8 seer? (tag faded long ago), split unit with the air handler in the [unfinished] garage. All of the ductwork is in the attic and has about an inch of insulation. The thermostat is a honeywell programmable on a wall that is about 3 feet from the return duct.

The efficiency problem I have had this winter is that whenever the temps were below 40 or so, the blast of cold air stored in the duct work combined with the time it takes for the heat pump to warm up cools the house to where the thermostat calls for aux heat after only about a minute. So I spent most of January and February heating my house with the strips. If the strip doesn't kick in after about two or three minutes the heat pump will run until the house warms up. I keep the temperature constant in the winter since whenever the thermostat increases it, it also kicks in the strip heat (so much for saving energy).

Is there anything I can do about this (short and long-term)? I have thought about insulating the rafters, garage, and duct work. Has anyone tried this and how well did it work? All of the smaller ductwork is pretty well buried in the 14" of insulation but large ducts are exposed on 3 and 4 sides.

I have been thinking about replacing the heat pump if it doesn't fail before our warranty runs out. Would you recommend changing the layout of the system when I do? In a house that small, yes, I do need the 10 sq feet of closet space so it can't go in the house but I had thought about the attic. It would elminate about 30-40 ft of ducting, (probably over half of the duct air volume), two large 90 degree elbows, make fitting the car in the garage a bit easier, and make the system a royal pain to service :(. Doubt it could be done to code with the 2 X 4 trusses though, if they could even support the weight.

I have replaced the windows and doors which has really cut down on how often the heat pump runs (I didn't have drafts, I had a breeze). Is there any chance of getting the house efficient enough to drop to a 1.5 ton? I don't want to get undersized since we can get over 100 degrees with 80-100% humidity. The energy star site says I can save $600-700 a year by buying a 15 seer unit. Is that reasonable? Seems like a rather rough calc biased to sell energy star stuff. Though Carrier says it would cut my bills by 46%. I run the t-stat at 67 heating and 78 cooling.

The heat pump is on some kind of foam-looking pad right now. Would it be best to pour a concrete pad for the new unit?

Only other option is to burn the wood stove as much as possible and hope the free wood holds out!

Thanks for the help!

Marty S. 03-14-2012 12:09 PM

Get a more sophisticated thermostat. They can keep the second stage from coming on based on outdoor temperature or time. They also let you set the differential between first and second stage

HVACDave 03-14-2012 01:30 PM

A stat with an outdoor temp sensor would make it so you could adjust the temps the heat pump would switch over to aux. heat, and actually lock out the strip heat till a certain outdoor air temp is reached. It would also allow you to run on only aux. heat once the outddor temp drops as well.

Honeywell 8000 works weel for this with the outdoor sensor.

beenthere 03-14-2012 03:51 PM

Some of those 8 SEER where very slow to to stabilize and provide warm air. A new heat pump would do better.

But first, continue sealing. Seal your wall switches and receps. Also, seal all the penetrations in the attic where wire run up through.

Consider the Honeywell IAQ(YTH9421) thermostat. Its much better for what you want then the Th8000 series is. The IAQ gives you a wide range of lock out temps for the aux heat. plus it also allows you to choose balance point plus 2 droop for those real windy days when the heat pump can't handle the load only because of the wind.

horseonthefly 03-16-2012 06:26 AM

Thanks for the suggestions. Though for a little more than the price of the IAQ I could get a PLC and some thermocouples and go to town. Would it make more sense to control the strips off of the temperature differential between the ambient (assumed incoming) and outgoing air? So the logic would look something like this: If the heat set point - current temperature is greater than 2 degrees and heat has been running more than 2 minutes and output temp - current temp is less than 10 degrees then turn on strips. For safety I would add a line that turns on the strips if the temperature drops more than 3 degrees below the heat set point. AC would kick in if the temperature gets above the AC set point. Even more functionality could be added by interfacing with my alarm system.

I'm continuing to work on sealing things up but I've still got a few bigger holes to deal with before I get down to the outlets...

beenthere 03-16-2012 04:14 PM

That algorithm would have your strip heaters staying on after every defrost.

The IAQ already has the algorithms in it. Or you could move up to the prestige and access it through a network.


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