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Heat Pump Paired with Boiler

littlej
littlej Member Posts: 2
Hello. I've been in the residential HVAC business for a few years now, and recently have found myself thinking about dual fuel setups.

During my research, I read somewhere - sadly, I forget where - that one of the things you need to consider when setting up a dual fuel system is that, when the auxiliary fossil fuel kicks in, the heat pump turns off, so you're losing the cheap heat it can provide, meaning your system isn't as efficient as it could be.

My thought then was, if the house already has a boiler with radiators/baseboards, and if everything is wired in and set up right, what's stopping someone from installing a heat pump, and using the boiler as the auxiliary heat? That way, you can have you heat pump running at the same time as your fossil fuel system.

The only real issue I can think of is that the boiler must be sized and set up to heat the house as though the heat pump weren't there, in case it ever had to carry the load by itself. This would mean that during normal operation of such a system, the boiler may short cycle more often. This makes me think that a mod-con boiler with outdoor reset is a must for something like this, but even then, short cycling might still be an issue.

The other thing I can think of is a comfort issue if you're near an air vent. In cold weather, that supply air is never gonna be warm!

I haven't been able to find anything in my books or online about a system like this. I think it's worth thinking about, but I will admit I could be very wrong about it due to my lack of knowledge and experience: most of my time is spent with forced air systems. That is why I'm here for insights into this.

Thanks for reading

(as for defrost, I'd probably keep the electric heaters in to keep the air from getting too cold! would also work as an extra backup heat in a pinch)

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,151
    One of the tricks -- there are others -- is that you need to remember that the overall efficiency of the heat pump including the generation of the electricity to run it drops as the temperature drops. At some point -- and this will vary for each installation -- the efficiency of your fuel burning equipment will be greater than the heat pump. At some other point, again depending on the installation, the cost of running the heat pump will exceed the cost of running the boiler.

    Therefore... depending on whether the green your client is looking at is environmental or money, there will be some outside temperature at which it is better to run the boiler.

    So... you figure out what that temperature is, and set your controls to switch over at that temperature (with, of course, a sufficient dead band so that things aren't constantly switching back and forth).

    If you are looking at hot water heat, there is an additional consideration: if you have enough radiation so that a mod/con boiler will work well, that switchover temperature will be very different -- considerably higher -- than a conventional boiler, since a mod/con's efficiency increases with decreasing return water temperature -- and you can decrease the return water temperature as the outside temperature rises.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • HVACNUT
    HVACNUT Member Posts: 4,209
    Honeywell offers thermostats, the Prestige I think, that offers staging with programmed indoor temp differential set points or outdoor temp set point for stage 2. Some will need a fossil fuel kit.
  • Steamhead
    Steamhead Member Posts: 14,860
    We have one job where an outdoor thermostat in the HP outdoor unit stops the heat pump and calls in "emergency heat" (the boiler) at 40 degrees or so. All the switching is done in the 24V control wiring.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • littlej
    littlej Member Posts: 2
    edited August 2018
    Jamie - your comments appear to be about the "economic balance point," the temperature at which a heat pump costs more money to run than a fossil fuel. I have found a few resources for calculating that, and have found that, even at 80% efficiency and a heat pump with a very good COP, my best result in my area is a balance point of 50F. Ouch! I guess my line of thinking is only worth it in an area with cheaper electricity.

    For reference, the area is the Baltimore Washington metro area, and comparing with natural gas. I'd probably get a lower balance point if the boiler were propane or oil, but still, this was enlightening/

    Thank you for your post, this exercise has been a real eye opener for me.
    Rich_49
  • njtommy
    njtommy Member Posts: 1,105
    I’ve been running a dual fuel with a heat pump and hydro air for the last 4 fours. I absolutely love it. It’s very comfortable I get really long run times with low discharge air temps of 100-105f. Makes for a very comfortable house. I’m still hoping to do radiant floor and panel rads one day, but work is just way too busy.
    Rich_49
  • Topazy
    Topazy Member Posts: 3
    There's literally nothing you can find in books.
    Was searching as well.That is fine!
    I posted quite a lot about our Mitsubishi EcoDan 14kw heat pump some time ago. Nothings changed, still brilliant after several years. Location rural Scotland, old house with a total electric bill ~£800 odd pa. We do have solar pv which helps. Cheers