Lockout setting for Heat Pump to Furnace

kypattiOctober 18, 2009

We are installing a new 4 Ton HP and Furnace this week. What outside temp should we set the HP to transition over to Furnace?

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the main idea behind hybrid or dual fuel residential systems is to leverage relatively inexpensive electric rates vs nat gas or oil rates. my suggestion is to find the balance point temperature-that's the temperature while operating the heat pump where the heat loss and heat gain are equal and your inside temperature can be maintained at that outside temperature. I like to take that temp and add 10% as a cushion. Keep in mind that the balance point temperature will vary from home to home primarily based on your HVAC equipment and your home's insulation qualities.

for example if your home's balance point is estimated at 27 degrees, it would be my suggestion to set your HP lockout at around 30 degrees.

a good dealer can calculate your balance point. however very few can or will take the time. if you don't know your balance point, I suggest setting lockout at 32-35 degrees as a starting point and adjust up or down as necessary.


    Bookmark   October 19, 2009 at 8:07AM
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I had a similar question as kypatti and have the same dual fuel setup, thanks for your answer.

By the way, if you're a Gator fan, as a Hog fan "we was robbed"!

Base upon what conditions and results would you adjust up, or down? Not even sure if I'm asking the question right. If heat pump alone doesn't maintain desired temp, one should raise the lockout point right?. If the heat pump runs too long per cycle to maintain desired temp, is that an indication the lockout point is too low? Or as long as the desired temp is maintained by the heat pump itself, is the lockout point at least high enough?

When you talk about balance point, you mean calculating comparative costs based on electric rates, COP and gas rates right?

I also had other thoughts you might address.

Does a 4 ton setup need the blower speed set to 1600 CFM for best efficiency, or would a lower setting be more efficient?

    Bookmark   October 19, 2009 at 5:53PM
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for parkview

I hope you have verified qualification of your York Hybrid system for the tax credit.

not a Gator or Hog fan, but regularly attend college football games. I did attend the 1960 Gator Bowl with my grandfather and father. It was a trip I will never forget. Arkansas beat GT. The hotel was full of loud "suey" noise following the game.

On dual fuel/hybrid systems, I recommend that all homeowners know how to adjust the switchoff temp for HP to gas. This can vary based on your HVAC and controls. You are right. If HP is not maintaining or struggling to maintain inside temp, then your switchoff temp is too low. If HP seems to be easily mainting inside temp, then switchoff temp is most likely too high.

Keep in mind there are two balance point temperatures. As mentioned in my post there is the temperature balance point and then there is the economic balance point which takes into consideration the costs of your fuel. I don't want to get too involved with the economic balance point. you may want to "Google" these terms for more information.

for heating with HP, on a four ton system I suggest 1600 CFMs(400 CFMs/ton) blower speed setting.

Good Luck!

    Bookmark   October 20, 2009 at 9:26AM
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I hope this is not inappropriate to jump in like this but I think from what I have read hear that you might be able to help me.

This is all new to me and if I am in a position where doing a lot of base-line education is needed prior to asking the question I understand.

We have a new home with a dual fuel heat-pump and LP furnace. I also have 4 solar panels to supply hot water for the house and to pre-heat the return air going to the furnace.

The contractor that installed the heat-pump has never worked with solar before. He has the balance point set at 40 degs (outdoor temp hits 40 or less and the LP furnice kicks in). I am being told by 2 knowledgable people that this is too high. I am also told by the solar people that with the addition of the solar help the set point should be lower still.

The solar people recommended setting the temp on the low side...say 20 degs. We keep the thermostat at 66 (sounds low but the house so far is comfortable...and we don't change it). They suggest, on a colder day when the temps are dropping to keep a thermometer over the registar and if the air comming out is about 95 to 100 deg's I'm good. When it drops below that, that sould be my "balance point" temp to switch to LP.

Does this sound like a reasonable approach to determine that number? Would you have any suggestions or can you point me in a direction where I can learn how to determine this stuff?

Thanks so much for any advice and your time


    Bookmark   November 1, 2009 at 2:32PM
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a couple of questions.

what area of country do you live?

what is electric rate? what is propane price?

what size is your home, what size is your furnace, what size is your HP?

I can not speak to the solar panels but the idea of determining your balance point from your supply temp is nonsense.

The temperature balance point is the outside temperature where heat loss and heat gain is equal at your inside comfort temperature setting. When the heat pump can not keep up, that is the temperature that you should switch over to propane from HP.

I would lower your switchover point down to at least 32 degrees and then make further adjustments up or down as indicated.

Good Luck!

    Bookmark   November 1, 2009 at 3:48PM
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I appreciate your help.

I am in western North Carolina, about 30 miles west of Asheville.

Our electric is currently $.1185/kWh.

I think I bought the propane at 1.35 a gallon but I can't recall at the moment.

The home is a two story hill-side walk-out and is about 4000 sqft.

I'm embarressed to say that I have no clue at the moment about the size of the two units. I can find out if needed. It's new construction and if the rest of the house is any indication, the set-up should be good.

I suspect I didn't explain myself well in my previous post...it was recommended that I take the temp of the heated air being pumped into the room, not at the supply side. He suggested this because I was looking for something that I could objectivly measure...I guess I just don't know what it means, or what to look for to see if the HP is not keeping up... I think this was what he was trying to explain to me.

Is it possible for a neophyte to tell when the HP isn't "keeping up"...if so, what is it that I am supposed to look for.

Again, I know that dealing with questions that are so rudementary can be frustrating...so thanks for putting up with me!


    Bookmark   November 1, 2009 at 6:38PM
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you live in a beautiful area that I know very well. and a dual fuel system particularly since you have propane is certainly a plus. I will assume you have excellent new home insulation qualities.

it would be nice if you could post mdl numbers of outside condenser, inside evap coil, and furnace mdl number.

I was referring to inside supply temp which can vary from room to room and again the suggestion that one can determine the switchover point by that temperature is ridiculous. no offense intended.

how do you know if "the HP is not keeping up"?

you know if your outside condenser is running continuously plus your inside comfort temp is not being maintained.

I have dual fuel system. I have determined my BP is around 27 deg. I give myself a little cushion and set my changeover at 30 degrees. This works well for me at inside comfort temp of 68 degree.

you will be better off financially/operating cost to run your HP as long as it can maintain your home's comfort setting.

BTW, you have relatively inexpensive propane cost.

Good Luck!

    Bookmark   November 2, 2009 at 7:58AM
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Thanks for the information....I will start to experiment.

For what it's worth
Furnace is Rheem classic 90 Plus premium efficiency model # RCFL-HM6024AC
HP is Rheem classic series model # PPNL-060JAZ

Evaporating coils are above my pay grade! I can tell from it's name what it is but wouldn't be able to find it without busting something I'm sure!

Appreciate the above information.


    Bookmark   November 2, 2009 at 9:24PM
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