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Insight to optimizing a hybrid heat pump and oil furnace set up

Loya1ty | Posted in Energy Efficiency and Durability on

First post here, so go easy on me :). I’ve been lurking since February when I bought my home (built in 1977), and have now lived here for 1 full month. Time to get stuff done!
Short, no context question: Should a 3 ton 15 Seer unit heat pump be able to heat a 2800 sqft 2 story home (w/unit blowing from the basement)?

Longer, more detailed set up in question: Just had an old, leaking oil tank (500gal) dug up from outside (shouldn’t have been buried), and a new 165gal roth tank installed in my basement. We thought the tank was only 275gal, and I planned on downsizing due to efficiency upgrades I’ll be making and the fact that the house was purely oil heated before a hybrid system was installed in 2013. But I may be having to spend some $$ sooner than I thought. During the 1.5 days without oil in the new tank, the system kicked into Aux heat (with no success obv). Meaning the heat pump couldn’t keep up. Thermostat was only at 68F, and it was about 58-60 degrees outside. Is that pretty poor performance for the pump? I’ve read aux systems being needed at 35F or below, which is what I based my tank size on. It was 62 in the house this morning, 48 outside right now, and after bleeding the oil pump and getting it started this morning after oil delivery, it took 20-30 min of aux heat and now the heat pump has taken over to maintain at 69 inside. After reading through pages of manuals that thankfully the prior owner left, I can’t even control when aux heat goes into effect (i.e. the system knows best for optimization). I don’t like this and will eventually install a smart thermo and want to control more closely via app since I will adjust based on oil rationing and/or electric rates (planning on some solar in the future).

The reason I ask, is that there are definitely inefficiencies in the house. If my heat pump should be able to manage, then I will address those sooner than later (I’m losing a lot of heat/ac into the basement and into the attic). If it’s just out-sized for the job, then I will either expect more oil costs for the short term, or potentially install the set up I was planning for 2-3 years down the road when I finish the basement and need to condition that space (remove 2nd level duct system from the attic, route current hvac system to basement in addition to 1st floor, and install a ductless mini split system for upstairs bedrooms… $$$).

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Replies

  1. Walter Ahlgrim | | #1

    In my opinion in a duel fuel setup the heat pump is unlikely to get much use.
    People can and will get use to a heat pump a blowing 90° air to heat the house but not if an oil burner ever comes on blowing 130°air. You would need to be be one tough cookie to after feeling the warmth of the oil heat to turn it off..

    With that said my heat pump alone will heat my house when it is 6°F but it has a variable speed compressor and electronic expansion valves your system is unlikely to have. Without the electronic you will need to find a very good HVAC guy to tune your system. It can be done but it takes time and they need to do it while it is cold to get it right. It is all wasted effort if the system has a leak even a tiny leak before long the system not be working right.

    I think the best way to know if the system is working right is the study the supply and return temps calculate the difference You should see about 20° when it is in the 40s outside.
    Also when heating with a heat pump I say forget about using a setback thermostat. Set it at one temp 24/7 and let it run.

    Walta

  2. Ben Bray | | #2

    Homeowner/non-professional efficiency hobbyist perspective:

    You are correct most thermostats are pretty dumb about aux heat, even some of the "smart" ones. On mine the default setup is to call for help from aux anytime the temperature set point is different by 2 degrees (i.e. raise from 68 to 70 when you get up in the morning).

    I have an all-electric house with a pretty basic 2.5 ton 14 SEER heat pump in NE Washington state. It's one story with mostly finished daylight basement (2400 sq ft total). Not a ton of windows. The house was built in 2007 but hasn't had any air sealing work done yet and the insulation in the attic is barely R-30. I have had the auxiliary electric heat disconnected all winter (never got really cold here). The heat pump will keep the whole house at 70 down to about 15 degrees F outside by itself (runs constantly in this situation). But this morning it still took about 90 minutes to raise the inside temperature from 69 to 70 (about 35 F outside). Heat pumps work best to maintain a set temperature - don't expect it to handle recovery from a big overnight set back without help from (more expensive) auxiliary heat.

    To me it seems like yours is under performing if it cannot maintain 68 when it's 48 outside. But if you are trying to go from 62 to 68 that is going to require the heat pump to run for a very long time (multiple hours) to make up that temperature difference by itself. I don't have any experience with oil fuel, but I believe the heat pump is going to be more cost effective to operate as much as possible by itself. At some point it's not going to cover the heat load when it gets really cold and your oil furnace will need to kick in. Depending on your insulation and air sealing that might be somewhere between 15 and 30. If you have the option on your thermostat, you may be able to lock out aux heat so it stays off above this temperature to maximize your savings.

    Hopefully this helps and gives you something to chew on... :-)

    Edit: As Walter said there's also the comfort factor to consider. At 15 F outside my system is blowing 83 degree air. Works for me and my family but not everyone finds that comfortable depending how drafty your house is and where your vents blow.

    1. Loya1ty | | #3

      Definitely helps. Good point about the variety of setups for aux heat, the manual didn't indicate what the constraint is, just that I couldn't do it manually. I have an HVAC tech coming in a week for Spring check up, so may ask if they can get that setting info for me. I would imagine my heatpump *could* get me from 62-68 (albeit cost me in electric) but whatever the aux setting is wouldn't allow it. If it was similar to the 2 degrees you mentioned, I suppose I could increment the thermo every time it shuts off lol I just want this control.. if oil issues arise, I'd rather have expensive heat than no heat.

      And to clarify, it can *maintain* at 68-70 fine. It just took 20-30 min of aux oil heat to get up and then the heat pump took over. Been managing ever since, although I have no way of knowing if the furnace has been kicking in during night time lows. I'd like a 'smart' thermo that records that switch. I'm a data nerd and all about efficiency. Slowly turning this old colonial into a smart home :)

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