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Heat Pump Guideline

  • 25-03-2022 11:01am
    Registered Users Posts: 861 ✭✭✭


    I'm looking into replacing my oil boiler with an air to water heat pump. The house has UFH only, no rads.

    I've done the technical assessment and the house is suitable in terms of heat-loss, but I'd like to check a few items:

    1) Currently have oil, and it heats 2 300L insulated tanks - DHW and UFH. Each tank is tripple coil. Solar thermal and oil boiler feeds the DHW tank.

    2) DHW tank heat is supplied by a coil, UFH tank heat is also supplied by a coil. UFH water itself flows through the tank

    3) My preferance is to use the heat-pump on night rate to warm the house to ~21-22C, and would have stats turned back during the day to 20C. Based on current heat loss the house will not call for heat during the day - only loose 1-2C over 24 hours.

    4) I'd heard that heat pumps often drive the UFH directly. Is this required?

    5) I'd heard heat pumps must never be turned off. Does this mean they consume a few KW constantly, that would see crazy - or are they always on, but running in some low-power mode unless heat is needed? Even if this is the case, how much parasitic power is used in this case?

    In my case I'm hoping it would be a case of cutting off the boiler and connecting the heat-pump to the existing plumbing, so not a very involved install. Given 16KW HPs can be gotten for around 5K, I'd hope the install wouldn't be too much over the 6.5K grant...but would be interested to hear any thoughts on that.

    Finally - any other 'things to know' before I pull the trigger on this?



  • Registered Users Posts: 12,119 ✭✭✭✭KCross

    4) I'd heard that heat pumps often drive the UFH directly. Is this required?

    That would be typical. You only need the run the UFH pump when the HP is running so it makes sense for the HP to control that rather than have the pump running 24/7. That would be a waste of energy.

    5) I'd heard heat pumps must never be turned off. Does this mean they consume a few KW constantly, that would see crazy - or are they always on, but running in some low-power mode unless heat is needed?

    Correct, always on but not always running. The HP is basically powered on and monitoring various sensors so that it can decide when to start heating. The alternative is that you are manually turning it on/off yourself each time. They dont work like oil burners where you put them on timers and they run at full tilt. The HP will come on/off as it sees fit.

    When they are in monitoring mode they wouldnt be using much... just powering the electronics.

    You can shift some of the usage to night rate but you shouldnt shift 100% of it. It will be a false economy. It will have a larger deficit to make up if its disabled during sunlight hours and it wont be as efficient when running at night as the air is colder and will have to go through more defrost cycles.

    You should shift some of the load to night but then let it do its own thing after that which will mean it will come on during the day as well, but it shouldnt be that much if the house is as airtight as you suggest.

    Given 16KW HPs can be gotten for around 5K

    Thats a rather large HP. Are you sure you need that? I'd get some advice on that before getting one that size.

  • Registered Users Posts: 2,528 ✭✭✭bullit_dodger

    Can't speak to the heat pump thoughts as I don't have one, but you might want to consider getting a large'ish battery (8-10Kwhr or above). Not only can you heat the house as you plan with night rate, but you can also charge that battery and use it during the day to "top the house up".

    Bear in mind that day/night rate will be replaced with some other "smart policy" in a year or two. Hopefully they will continue to have a similar day/night tariff with the new meters. Just something to think about.

  • Registered Users Posts: 12,119 ✭✭✭✭KCross

    but you might want to consider getting a large'ish battery (8-10Kwhr or above)

    The economics of that are very questionable unless you can do DIY.

    He would also need to buy the inverter as he has solar thermal, not Solar PV at the moment.

  • Registered Users Posts: 2,756 ✭✭✭Captain_Crash

    16Kw is massive… more than twice as much power as is needed. Maybe that’s why you can pick one up at that price? I’ve just installed a 7.3Kw pump, which is more than suitable for most houses in this country.

    Regarding the grant, I can’t speak for your individual situation but on my side the extra work required to do to the house to even be eligible cost a hell of a lot more than what the grant would have given back! So we didn’t bother.

  • Registered Users Posts: 861 ✭✭✭tails_naf

    Thank - a few comments:

    1) The UFH pump does not run 24/7 today, basically the setup is: Stats in each room -> UFH controller to start the pump, and send a flag to boiler controller it needs heat. On the UFH tank are stats to say if the UFH tank is warm or not, and only if the UFH controller flag is set, and the tank stats flag is set does the boiler begin to heat.

    So the same would be ideal for the heat pump - only when UFH needs heat and the 'buffer' tank is cold would the heat-pump be required to provide heat.

    Good to know that always on does not mean always running. In a way my existing oil boiler is 'always on' in that the controller part is always on, but only actually uses significant energy when the flags mentioned above are set.

    In terms of sizing - how is this computed? I assume 16KW, but have no clue. Smaller heat-pumps are << 5K.

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  • Registered Users Posts: 861 ✭✭✭tails_naf

    I actually have both, solar thermal installed 10 years ago, solar PV installed a year ago. PV is great, and have a 4.8KWH battery, and wish it was larger.

    Don't think heating directly from a battery on night rate is very cost effective though, even DIY. 1 Unit at night rate is ~8-9c, while 1 Unit of oil (last few months excepted) is around the same - so big cost for the battery but no payback.

    That said I would like a 10KWH batter for my PV, as 4.8 is just a little too small and end up importing. Could be fully self sufficient for general electricty for 8 months of the year with a big battery.

  • Registered Users Posts: 861 ✭✭✭tails_naf

    What sq ft does the 7.3 KW HP work for? Also, I assume 7.3KW out means 2.4 - 1.8 KW in, depending on COP?

    My house qualifies for the grant as-is, so things are fine that way - but hope installers dont see $$ signs and just jack up the prince and pocket the grant. You'd imagine an install that does not need significant re-work should be a few K (3?) so HP prince for 5K+3K = 8K - 6.5K= 1.5K outlay. I'd be happy with that - but I've heard some folks charing up to 13K for install, which seems like profiteering...

  • Registered Users Posts: 2,756 ✭✭✭Captain_Crash

    The house is a shade under 80sqm, I’ll have to check the COP numbers tho, I’m not 100% on them. Regarding the install cost, yeah it’s gonna be high! Mine cost a bit more than the number you quoted, but I needed new rads too and so that would have jacked it up a couple of grand.

  • Registered Users Posts: 12,119 ✭✭✭✭KCross

    The HP will work in much the same way but it wouldnt typically work on individual internal room stats. It would use a combination of an internal and an external stat and then use "weather compensating" control to decide when to come on. When its colder outside it would run for longer to maintain comfort in the house. It should leave the house at a constant temp 24/7.

    The room stats will just open/close the valve for that room in the manifold but wouldnt actually trigger the HP itself. Thats typically how it works.

    The reason for that is that you dont want one stat calling for heat, the HP firing up and running for 5mins and then turning off and then for another stat somewhere else to be calling for heat and the HP starting up again..... compressor starts need to be minimised as much as possible. Lots of compressor cycling will result in a dead HP in quick time!

    So, it "looks" at the house overall and looks at the outside temp and drives heat into the house as its needed. It does work but you need to get away from the idea of turning it on/off or setting timers like you would with an oil burner. HP's are very different.

    You have alot of stuff already in place here in terms of Solar, UFH, water stores etc so you really just need to get someone onsite to see how to integrate a HP with it, but it should be no problem really. Having UFH is the key, so should fit nicely with the HP.

    In terms of sizing - how is this computed? I assume 16KW, but have no clue.

    Based on the heat loss of the house. If you have well insulated and reasonably airtight you would have no need for a 16kW HP unless its a McMansion!

    As a reference I've a 12kW in a 350m² house and it works fine. Newer HP's are probably even better so you might be able to go smaller again but it does require an assessment to be done by the HP installer to see what your heat demand is and calculate it from there.

  • Registered Users Posts: 2,528 ✭✭✭bullit_dodger

    True - I'd missed that KCross (the bit about solar thermal verses solar PV), that does change the complexion a bit! And your right a DIY is the way to go there if you have the skills. Considering it myself.

    There are however off-the-shelf house batteries out there for non-DIY capable folks who don't have solar PV. The math, you'd have to work out for yourself as to the economic value, but considering the way energy prices are going....I'd be optimistic of a sub 15 year payback. Just highlighting that they are "out there" so that people can then investigate themselves.

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