Normal Hybrid Heating System Operation ???

saltidawgApril 1, 2011

Hi all. I was an active poster about a year ago and got lots of help in deciding what I should do about replacing my ancient oil furnace and aging A/C system.

I ended up getting a Carrier system including a 25HPA6 Heat Pump and appropriate air handler, electronic filter, humidifier, and a Carrier Oil Furnace.

I intend to come back here in a few months and provide a detailed assessment of my energy savings (substantial during cooling season) and to provide some info on my new GE Hybrid Hot Water Heater.

I do have a question for you now, however. We had a not too severe winter here in the suburbs of Washington, DC. That said, my oil consumption was quite high. I recently called my installation contractor and reported that when my thermostat called for increasing house temperature, the heat pump would only run for 15-30 minutes and then heating would be shifted to the oil furnace. After verifying that my heat pump was operating correctly based on refrigerant measurements, he is telling me that it is normal for a step change in thermostat set temperature to see the heat pump run and then shift to the oil furnace.

I did not see this happen last winter in Dec and Jan when outside temp was warm "enough." (I have the heat pump lockout set to 35 deg outside temp.)

My question is as follows. Is the following scenario proper and normal?

With neither the furnace or heat pump running, I nudged the thermostat temperature setting up ONE degree. The heat pump ran for ten minutes in Single Stage. After that ten minutes, the heat pump shifted to two stage and ran for 15 additional minutes. At that point, the indicated thermostat temperature had not increased and the heating shifted to Aux heat... given time the house would see the increased temp.

If that is normal, I don't know why I paid for the heat pump as far as heating.

I do have a call in to the owner of the installation contractor and I have called Carrier.

Please educate me as to what is normal heating operation.

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That is normal. If oil is your backup, you really need to not touch the thermostat and not turn it down at night. I'd also set your switchover much lower unless your electric rates are really high.

Just a quick calculation would show that even at $.20 a kwh, an average heat pump would be cheaper to run than oil down to 10 degrees.

In my area, electric baseboard is a lot cheaper than oil right now.

Don't ever change your stat. Now thank me for the literally hundreds of dollars you will save.

    Bookmark   April 1, 2011 at 1:20PM
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Thanks Mr. C. My electric is indeed very nearly at 20Cents per Kwh.

I just changed my thermostat temp setpoint to 64 deg 24/7. My wife and I will see if we are comfortable with that.

I really am upset with my research last year - you guys were complimentary about my research, BUT I didn't ask anything about how to operate the new systems.

My electric operating costs are way down since the heat pump and HW heater HP install, but my oil consumption remained about the same or is even higher.

I was hoping for a decrease in oil usage in exchange for an increase in electrical usage during heating season. We'll try the constant temperature approach and next year will again evaluate things.

    Bookmark   April 1, 2011 at 8:53PM
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Maybe you do have a problem, but there is not enough info to make a determination. What is the outdoor (OD) temp when these switchovers occur? Have an Infinity controller? Temperature rise at low and high stage during these switchover events? Temperature of OD coil and OD when HP active?
My experience with my dual-fuel system is that when the outside temperature is within 2 degrees of the lock-out, the heat pump will start a cycle, but will abandon it if the OD temperature starts to fall. I'm guessing that the controller estimates that the HP will not complete the cycle before the temp drops to the lock-out. When the OD temperature is rising, I never see that behavior. If the OD temperature is at least 3 degrees above the lock-out, I can raise the thermostat as much as 3 degrees and the system never switches over to gas. If your OD is at least 3 above your lock-out and you still get that behavior, then I agree that you have a problem. But I'm just a home owner, not an HVAC pro.

    Bookmark   April 1, 2011 at 9:23PM
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Zircon - I'm surprised you can move 3 degrees and not reach into aux.

That being said, I reread the details and maybe you do have a problem. Changing the stat manually is not the same as programming a setpoint of course but you know that.

It is 45 here. My downstairs was 66 when I got up (setback to 64). I bumped it to 68 because baby decided to get up. In 30 minutes I was at 68 (maybe less time). I then bumped in to 70 - and aux still didn't kick in. After going to 74, I realized that I must have locked out my gas furnace at 40 or 45 so it won't go on. It took an hour to go from 66 to 70 although it would have been shorter if I bumped the whole way from the beginning. (Just giving an example for comparison)

So maybe things would be helped by locking your oil furnace out when it is above 40 (or 45). And maybe there is something wrong with your heatpump. How was the summer performance?

The speed which your house heats up depends a lot on your sizing relative to load. For NC, the a/c dictates the sizing so I have excess heating capacity. I believe it is 2:1 downstairs (4 tons for 24,000 btu heating load). You might have a smaller system, although DC seems to get as hot as here. But your windows might be smaller or not exposed as much as mine.

So what are your capacity and load numbers? Try locking out the furnace. Almost $.20? Time to go solar and drill for gas. I am surprised that no one mentioned solar hot water as that payback would be very quick with oil or high electric. I did it with gas and it still pays off pretty quick - 10 years or so. With oil/high electric, it might be 3 years depending on incentives. I had federal, state and utility which made my total cost just under $3k.

    Bookmark   April 2, 2011 at 4:52AM
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The GE Hybrid Hot Water Heater I mentioned is a hybrid electric and Heat Pump water heater. I operate it the most efficient mode - heat pump only. If my grandchildren and their parents come to visit and increase the hot water usage significantly, I switch it over to regular electric heating elements only or a mode that uses the heat pump unless it can't keep up in which case it energizes the electric elements. When the water heater is operating, it actually cools my basement and also dehumidifies it. (I have a separate condensate pump for it.)

The unit is not cheap - ~$1500 as compared to ~$450 for a standard 50 Gal electric heater.

The hybrid hot water heater saves me over $50 per month on my electric bill - year round. Here in MD, in order to encourage reduced electric usage, MD does not collect Sales Tax on the unit. Lowes had the unit on sale for 10% off. I am a retired Navy Submariner, so Lowes gave me another 10% off for Retired Military. The State of MD gave me a $300 Rebate and the power company gave me $20. Montgomery County also gave me a $250 Property Tax rebate and, did I mention, it qualified for the Federal Tax Credit had I not already reached the limit because of my qualifying heating and cooling unit?

I have a payback period of under two years and I have only half jested to my neighbors that I would buy them a unit if I could be assured that they passed on any electric savings to me. If I could match the return on my money in the market I would be wealthy indeed.

    Bookmark   April 2, 2011 at 10:12AM
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Heat pump hot water is great. It is just that at $.20, solar would still be beneficial. The usual downside is cooling your basement which inevitably raises your heating requirement. Depending on how the house is set up, it can be an issue. The very best situation is in Florida where heat is rarely needed.

Your electric bill may be lower because of it but your heating load (ie oil) is higher. There is no way around this fact. If your basement is usually 50 in the winter and now is 45, then you lose more heat to the floor and if your ducts and furnace is in the basement, then you lose more heat there. Could be minimal. Could be 1/2 your benefit.


    Bookmark   April 2, 2011 at 11:44AM
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Location for the Hybrid Hot Water heater can be a problem in some cases as there is some noise associated with its heat pump operation. Also, there is the need to carry away up to a gallon a day of condensate.

As far as temperature change of the basement, it is miniscule in a below grade basement. (It is, after all, a closed system operating within the basement space. The only heat removed from the basement is the heat associated with the mass of hot water out and cold water in when using hot water. That heat would be transferred from the air, the bsmt contents contents, earth mass at the wall, etc, etc.) The sub grade basement walls are a highly effective virtually constant-temperature heat sink.

I meant no negative vibes about solar. I was just extolling the virtues of a hybrid electric hot water heater which has the same foot print as the regular electric heater it replaces, can be installed or replaced by any plumber or homeowner , if permitted, and has a great payback on investment.

I am one of those people that never purchases an extended guarantee, but Lowes and some other sources have fixed prices for such policies based on SIZE and not COST of the heater. In the case of Lowes, at the time I purchased mine last year they charged a price for 50 gallon heaters that appeared to be based on the $400-$450 replacement costs. (~$50) I got the 11 year extended warranty (Now $80) which essentially will double the time that my initial investment will provide me a return on investment.

In any event, I was looking for help in understanding the operation of a hybrid heating and cooling house system and I've received some great help here. I suspect that when I talk to Carrier next week I'll simply be explaining that I did not understand the algorithm that controls the heat pump and oil furnace when a step change in temperature is desired.

    Bookmark   April 2, 2011 at 12:20PM
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what kind of thermostat do you have? Is it programmable? My thermostat has something called 'selective energy management recovery' or some such name. It 'learns' how fast the system can adjust temperature and activates the unit before the call for a temperature rise (program change). The amount of time beforehand changes and can be perhaps as much as 90 minutes before the program change. (I think it keeps track of how much time it takes to raise/lower the temperature by 1* and then multiplies by the number of the degrees to change.) That way it never goes into Aux mode unless it is so cold that it cannot raise the temperature very much at all.

If I raise the temperature 1* manually I don't think it ever goes into Aux. More that 2* and it will.

So I think what is going on is your thermostat. It sounds like it is a model that uses fixed time intervals for stage 1, 2 and Aux activation. These time intervals MAY be changeable through a hidden setup menu if the thing is electronic.

What David suggests about setting the cutouts is a good idea. Also, does this have an outdoor sensor? You really need one for things to work well. Depending on the efficiency of the HP and the cost of oil, you can set points of HP cutout and oil furnace shutout to where you need them to be. I might suggest the HP cutout around 25-28* (if efficient) and the oil shutout around 35-40*. You can check the temperature of what is coming out of the vents to see if these numbers work for you. If the vents are too cold, then you can raise the HP number a bit.

    Bookmark   April 3, 2011 at 9:45PM
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I believe this is a thermostat/setting issue. Oil furnace should be locked out at a specified outside temperature on recovery from setbacks.

what thermostat do you have?


    Bookmark   April 4, 2011 at 7:41AM
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Thanks for the great suggestions.

I have a Carrier Heat Pump 25HPA6 . The Thermidistat is a Carrier Edge TP-PRH-A. The oil furnace is also carrier. All installed last May. Additionally, yes there is an outdoor thermistor.

My installer on Friday called me back and told me that he had talked to Carrier and suggested to me that I do what David and weedmeister suggested... that I change the Aux Heat Lockout setting from its default value of "Function Disabled" to, say 35 degrees.

Additionally, he said that Carrier has been traditionally closed lip about the specifics of the algorithm that controls Heat Pump vis-a-vis Aux heat usage in response to a call for increased room temperature.

With respect to what weedmeister called 'selective energy management recovery' my thermostat calls that "Smart Recovery." and it is set at the default value of 90 minutes. As was pointed out to me on Friday, Smart Recovery does not come in to play when one manually adjusts the temperature set point up. Further, I was told that while the specifics of the algorithm for furnace vs HP usage to satisfy a time PROGRAMMED setback temperature are not available to us, it could be said that for a user temperature change that the algorithm apparently calls for the fastest possible response in order to satisfy the user. In my case, that means the oil furnace. The installer went on to say that this is not at all how some of the control systems he is more familiar operate. He is used to being able to manually raise the temp set point 2-3 degrees without forcing the aux heat to be used. (He is mainly a Trane Dealer????)

Today I am going to observe the response when a PROGRAMMED temperature increase is called for. Smart Recovery will come to play and there is a different time-sensitive part of the control algorithm that also will be in-play.

Looking back, last winter, due to my ignorance in this area, I frequently would go to the thermostat an hour or so before a programmed early evening temperature increase and manually upped the programmed temp 3,4 or 5 degrees. I must assume that invariably that would have called for Aux heat and defeated Smart Recovery.

    Bookmark   April 4, 2011 at 10:25AM
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A fascinating read about the heat pumps and hot water heaters.
I too may have a system of this nature, a heat pump and a oil fired boiler.
The price of oil is man reports electric being less expensive that oil...which I have suspected.
Maybe I will convert to all-electric with no oil.
The man giving the cost estimate is also the fuel oil man, so its natural that he favors oil.
I favor economics, most of the time.

    Bookmark   April 4, 2011 at 10:48AM
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Dawg: I was reading the instructions for my tstat (WhiteRogers). It has two additional settings called 'Heat Fast' and 'Cool Fast'. When set to 'off' they delay the onset of 2nd stage (or Aux) for up to 30 minutes. The default setting is 'ON'.

You might want to check and see if this Carrier unit has a similar setting.

    Bookmark   April 4, 2011 at 6:18PM
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The minimum time of 10 minutes 1st stage operation before 2nd stage is not adjustable. The minimum 2nd stage operation time is adjustable up to 25 minutes. The default - its current setting - is 15 minutes. My sense is that this additional ten minutes will not satisfy my desire.

With the info I've received here, and from my installer, I think that I'll have no problem taking care of the problem next fall. (Temp here in MD today was in the 80's. Too hot to be running the furnace. :-) )

As I said, I do hope to speak with Carrier and will let you know if anything different comes up.

    Bookmark   April 4, 2011 at 7:32PM
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CJ Mechanical of North jersey llc.

I had installed bryant oil furnace,evolution heatpump,thermostat T6-PRH control here in jersey.
I set up the homeowners control per tech support to option 8 50*(oil lock out) option 9 heatpump lock out 30*
I want only heatpump operation down to 50* then only oil heat at third stage. and below 30* all oil heat.
The homeowner asked to change instead of 50* put to 40*
because of exspensive oil prices.
It is working very nicely very iortant that the outdoor sensor is working properly.

    Bookmark   April 9, 2011 at 8:54PM
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heating with oil has gotta be expensive. I would run the heatpump down to 30 if you have oil as your backup. How much is your electric rate? I know electricity isn't cheap, but its gotta cost less than oil unless the heatpump is old/inefficient.

    Bookmark   April 10, 2011 at 7:34PM
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I did set it up so that above 3o degrees it will be heat pump only, and below 30 degrees it will be oil.

My electric rate is very high - just under 20 Cents per Kwh.

All equipment is new - less than a year old. (Carrier)

    Bookmark   April 12, 2011 at 1:00PM
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ouch - .20 cent electric is expensive. Still probably cheaper than oil above 30 with a efficient heatpump.

    Bookmark   April 12, 2011 at 3:36PM
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