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Heatpump ATW system with radiators

  • 17-02-2022 11:54pm
    Registered Users Posts: 1,637 ✭✭✭ podge3

    I have searched here on Boards but can't find much info on this subject other than an occasional mention, apologies if it has been discussed at length before.

    Basically, I wondering if its really economically feasible to run an ATW system with radiators. My house is about 20 years old and the oil boiler needs replacing. I always thought that you needed UFH to have an ATW system but it seems that replacement aluminium radiators would make it economically feasible.

    My understanding now is that UFH is the best method but that the correct radiators will also work. I'm getting insulation work done on the house also.

    I've been quoted about €8k (after grants) for a complete Mitsubishi ATW system with replacement aluminium radiators for my 2000 sq ft house. Does this seem OK?



  • Registered Users Posts: 275 ✭✭ spose

    Was looking in to same recently. Boiler had to be changed out within the last 2 years but was wondering when it does next time what will my options be. found this good…

    I figured I’d test it out rather than wait for the time to come and be trying to rely on calculations. House is pretty well,insulated, good temperature control using a Hive and thermostatic valves on all rads. I dropped the boiler setpoint from about 70 to 55 and it’s been running at that for about 2 weeks without any real noticeable difference. Maybe a little slower to get up to temperature from cold but that’s really going by the temperature trends rather than actually noticing anything. Will probably drop it another bit just to see soon

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,611 Mod ✭✭✭✭ Jonathan

    Yay, another low temperature heating convert. 🙂

    Running mine around 55 as well and no ill effects noticed. Only downside is that the toddler isn't used to piping hot radiators in other people's houses and keeps burning themselves.

  • Registered Users Posts: 5,629 ✭✭✭ Ubbquittious

    Will a normal ashoop heat the rads to 55 degrees easily without undue strain?

  • Registered Users Posts: 3,201 ✭✭✭ cruizer101

    If you are only heating to 55 do you use an immersion on your hot water cylinder to heat it up more?

    The article references gas boilers which are far more common in uk, gas boilers can modulate i.e. change the flame output which allows them to vary the output temperature easier. Oil boilers can't really do that they are either on or off, I wonder would setting a low temperature result in a lot of cycling on and off for the boiler. Does the same principle apply to oil boilers I wonder?

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,611 Mod ✭✭✭✭ Jonathan

    I've a condensing gas boiler. The aim is to keep the return flow temperature low so that the boiler is always in condensing mode, and hence more efficient.

    I don't see why it wouldn't work if you had a condensing oil burner, but appreciate they are rare in Ireland.

    55C is more that enough for our hot water needs. Also have an Eddi, so the water hits 60C once a week for legionaries.

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  • Registered Users Posts: 1,637 ✭✭✭ podge3

    OK, so no real opinions yet...

    At least nobody has said I'm barking mad😁

    I'm not expecting huge savings over oil with the A2W system. I'm currently averaging 2 to 3 fills of oil per year, say an average of €2,500 per year. A saving of €1k per year would see the system paid off in 8 years - 5 years if you factor in the new oil boiler I need anyway.

  • Registered Users Posts: 3,561 ✭✭✭ graememk

    As the boiler needs replacing anyway helps the equation.

    Last time I got oil it was 76c and I just looked up the price now.. 90ish? Say you got 2000litres in a year.

    There's about 10 kwh of energy (heat) in a litre of kerosene, but a boiler isn't 100% efficient. And it's old, say best case 80%, that's 8kwh.

    So you need 16000kwh of heat in a year.

    If the heat pump averages an efficiency of 3, that's 5333kwh needed for the same heat (or 4000 if it can reach a cop of 4)

    Say your on a 24 hr tariff at 20c/kwh for 5333kwh that would cost €1066 in electric.

    So yeah your pretty on point with a 8 yr payback.

    I think my rough math checks out.

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,611 Mod ✭✭✭✭ Jonathan

    @podge3 I assume your house qualifies for SEAI grants? Particularly the HLI < 2, and the u-value figures for walls?

  • Registered Users Posts: 1,637 ✭✭✭ podge3

    I worked it out on current expenditure on oil and got more or less the same figures. I would probably lower the average COP a little in your calculations and increase the oil quantity but the end result is the same.

  • Registered Users Posts: 1,637 ✭✭✭ podge3

    Yes, getting €10.5k grant for the heat pump. It wouldn't be economically feasible for me if I wasn't.

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  • Registered Users Posts: 173 ✭✭ Baoithin66

    Hi I'm in a similar position. Did you get a technical assessment completed? I'm told it is needed in order to get the grant for Air to Water. How much were you quoted for the heat pump on its own??

  • Registered Users Posts: 1,637 ✭✭✭ podge3

    I'm using a "one stop shop" firm for the whole retrofit so they are taking care of the whole lot.

    I think the cost of the heatpump was €18,600, which included new rads. Less the grant of course?

  • Registered Users Posts: 3,100 ✭✭✭ John.G

    Maybe a few numbers might make it easier to see what rad output will be sufficient to heat/keep your house warm.

    First thing to bear in mind is that the rad output is based on a 50 deg basis, ie, it's the mean rad temperature minus the required room temp, 20C is generally used.

    Secondly: A heat pump will provide its greatest efficiency, COP, at the lowest discharge temperature, the lower the rad flow temperature the the greater the COP.

    The numbers below are based on a flow/return (dT) temp of 8C, not sure but I think a HP needs a minimum of 5C between flow/return to run without tripping out.

    If you run with flow/return temps of 46/38C then you will have a very good COP of 3 or more but the rad output will "only" be a third (34.4%) of its rated output, if your room/house is up to temperature then in most cases this should be adequate, my own oil fired boiler burner fires for 25 to 30% and keeps the house fine and warm even at a ambient of 5C or so.

    The above temperatures will also give hot water at 38/40C, this can be boosted to 60C once/week for legionella protection, a electric immersion is probably the best way and won't cost a fortune as HPs don't like running at 65C and also its COP will probably fall to < 2.

    You might also wonder why a "22 deg" rad doesn't output 44% of a 50 deg rad, the reason is that the calculation is (22/50)^1.3)x100, 34.4%.and ditto for any deg rad.

  • Registered Users Posts: 1,637 ✭✭✭ podge3

  • Registered Users Posts: 12,546 ✭✭✭✭ Dtp1979

    Jesus Christ these one stop shops. Do they supply the lube too?

  • Registered Users Posts: 1,637 ✭✭✭ podge3

    I thought €8k for a heat pump and rads was OK?

  • Registered Users Posts: 3,100 ✭✭✭ John.G

    You can do your own calcs here like changing the flow temp, the flow/return dT and the required room temp, all the rest are linked.

  • Registered Users Posts: 3,100 ✭✭✭ John.G

    And here are some theoretical Heat Pump COPs, Irish weather with high humidity at say typically 5 to 8c can result in frequent defrost cycles which has a big effect on the seasonal COP so these numbers do not obviously take defrosting into account.

  • Registered Users Posts: 12,546 ✭✭✭✭ Dtp1979

  • Registered Users Posts: 1,637 ✭✭✭ podge3

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  • Registered Users Posts: 1,637 ✭✭✭ podge3

    Some more great info there, John. I'll need to get my slide rule out but I think I can make sense of it.

  • Registered Users Posts: 2,707 ✭✭✭ MicktheMan

    I've added a rudimentary but highly effective boiler output control system to my conventional oil boiler.

    My dhw is on a separate zone to the ch circuits. I fitted a pipe stat to the return ch pipework. This pipe stat is connected in series with the burner circuitry and when the return temperature hit a predefined temperature the pipe stat opens the circuit thus killing power to the burner but not the circulation pump. The boiler thermostat is set to 65ºC for the hot water. I added a relay to disable the pipe stat when dhw zone is energised.

    The result is that I am running my ch at 40ºC and know that when the time comes I can replace with a hp without any change to the distribution system.

  • Registered Users Posts: 12,546 ✭✭✭✭ Dtp1979

    Yea but that ‘one stop shop’ are getting paid 18,600 euro! Extortion. These crowds are notorious for jacking their prices up once the Seai grant comes in to play.

  • Registered Users Posts: 12,546 ✭✭✭✭ Dtp1979

    Why not fit 2 boiler stats? One for heating, one for hw

  • Registered Users Posts: 2,707 ✭✭✭ MicktheMan

    Good question!

    1. I wasn't aware you could
    2. I guess my thinking was I wanted the control to be more demand led so used the return temperature to control. As I said, rudimentary but effective for the purpose of running a low temperature system through the original radiator distribution system.

  • Registered Users Posts: 3,100 ✭✭✭ John.G

    What make/type of boiler do you have?, HE boilers like Grant don't like a return temp below either 40C or 45C. If your stat is set to cut out at 40C then it won't cut back in until 32/35ish so long term hope no problems with corrosion, its high time that oil fired boilers don't have all stainless secondary heat exchangers although I think I read somewhere that its the primary Hx that causes corrosion problems which seems a bit strange as the flue gases leaving the primary Hx should be the same as a SE boiler, my SE firebird has a flue gas temperature of ~ 230/250C. Some gas boilers can control the firing based on looking at the return water temperature, no reason IMO why Oil fired can't do the same?. Grant also limit the minimum boiler flow temperature to 65C, if one wants a return of 45C then you have to be messing about throttling most of the rad lock shield valves, you can get the return quite low with A rated circ pumps but the pump head is then so low that some rads will scarcely heat at all. IMO they should allow a setting down to 55C, a dT of 10C will then give 45C return temp but far easier to get all rads balanced as the pump head is higher, also a dT of 8C/10C is what most rads run at, normally, unthrottled.

    I ran my boiler similar to yours simply to see how my rads would operate at very low flow temps, I had the stat on the flow pipe set to cut out at 45C, no problem in maintaining my house temp BUT if if one shut down overnight like I normally do then I would nearly be back to bed that night before the house would be back up to its required temperature, (took at least 5/6 hours with a outside temp of 3C/5C) so running 24/7 no problem really, a bit like UFH.

    Should also have said that I ran my A rated pump at 5M head to achieve a dT of 5C, 45C/40C, flow/return, which resulted in the rads outputting 35% of their full rating, so if/when I do install a HP I will upsize my rads by a factor of at least X 2.5, no big deal as all my upstairs rads are >40 years old and downstairs >20/25 years old.

  • Registered Users Posts: 3,100 ✭✭✭ John.G

    You mentioned getting aluminium rads, while they (the rads) might heat up faster and maybe contain less water I don't think they emit more heat per sq. meter so ensure that they are outputting your requirements if you have any info please post it and remember that they should be upsized by a factor of X 2.5 based on their 50C rating.

  • Registered Users Posts: 1,637 ✭✭✭ podge3

    Very little information yet, John, I'm waiting on the contract.

  • Registered Users Posts: 12,546 ✭✭✭✭ Dtp1979

    Or a mixing valve for the rads and take the cylinder circuit off before it mixes

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  • Registered Users Posts: 3,100 ✭✭✭ John.G

    Wouldn't you need another pump for that like a UFH manifold?.