Using Radiators with an Air Source Heat Pump

theboss.com

Hi Folks,

Looking at putting in an Air Source Heat Pump to replace my ageing oil burner.

I dont want to put in underfloor heating as I have put new floors in recently.

The house is compact and a terrace, would I get away with just updating my radiators to the most modern efficent ones on the market?

Sleeper12

Usually you need to double the size of the rads when going from oil to heat pump as the heat pump doesn't get the water in the rads up to the same temperature. Air to water is a disaster in poorly insulated homes so you need a pretty high spec insulation to make it work

GreeBo

You'll need to do your BTU sums, a HP operates at a much lower temperature than oil or gas boiler, so your rads will be cooler and on for longer, but it all comes down to what BTU you need for each room and can a rad deliver that with the water temps you will get from the HP. You may need larger radiators.

dubbrin

How well is the house instulated? Type of construction? Cavities and attic? It would be a good idea to upgrade now as well if possible. See the available grants on SEAI.

GreeBo is right, try and figure out how much heat each room needs and choose rads based on the heat pump flow temperature and delta T (Flow minus return).

Your local builders providers might be able to help with rad sizing if you have an indication of current wall make up incl insulation. Try approaching them, they should be helpful if it means a few rads bought by you. Sometimes existing rads will still do some rooms and others need an upgrade when you switch from oil to a HP. "x2" across the board is too broad a statement...

cruizer101

"x2" across the board is too broad a statement...

Is there a better way.

Lets say you calculate the BTU or kW required for a room, there are various online calculators that will do it.

Then if you look at rads they will have a BTU rating, presumably that is based on a rad at 70 and room at 20 or something similar, so 50 degree difference.

With a heat pump the rad temp might be 45 so temperature difference is halved, which I would have thought would mean you need double the btu as heat transfer would be proportional to the temperature difference.

Is it possible to get btu rating for rads at lower temperatures?

Sleeper12

Is it possible to get btu rating for rads at lower temperatures?

Can't say for sure but I'd imagine with people converting to Air to Water that there must be a guide. I'd imagine a good plumbing trade counter will have the calculator or can help OP with sizes.

dudley72

They have hybrid boilers now, so a A2W with either a oil/electric top up to get the temp of the water up. I was looking at the Hybrid Oil to replace my old oil. So the A2W will bring the temp of the water up, then if not enough the oil kicks in to finish it off. So less oil

I also seen before High Temp A2W. I did ring a few years back about them but never got the full details. They run the water at a higher temp than a standard so should work better with rads.

If you go to a company expect them to tell you to swap out all the rads for aluminium and quote crazy money.

https://www.firebird.ie/images/2018/Enviroair_Hybrid_Heat_Pump_Oil_Jul18.pdf

High temp

https://www.daikin.co.uk/en_gb/product-group/air-to-water-heat-pump-high-temperature.html

Wearb

https://www.boards.ie/discussion/comment/118040235#Comment_118040235

The change of output for a given deltaT isn't linear. i.e. output is less than halved if deltaT is halved.

Off top of my head, the rule of thumb for rads needed for heatpump is almost 2.5 times bigger (Maybe not so much for aluminium rads) . Otherwise heat pump would need to be run at its upper limit. Another thing is that because of the slow heat up times, they need to be left on most of the time and controlled by thermostat. The last sentence is the one that hear most plumbers complain about. Not so much with new home owners, but retrofits, where owners have a difficult time leaving heating on.

cruizer101

The change of output for a given deltaT isn't linear. i.e. output is less than halved if deltaT is halved.

Off top of my head, the rule of thumb for rads needed for heatpump is almost 2.5 times bigger 

Yeah, I kinda knew that but wasn't sure what it would be, I know with just direct conduction it would be linear but obviously a radiator in a room is far more complex, conduction to air which then creates convection currents and even a minimal bit of actual radiation.

Bit more googling found me this https://www.buildingservicesindex.co.uk/entry/136540/AEL-Heating-Solutions-Ltd/How-to-calculate-the-delta-T-for-a-radiator/ which gives a correction factor based on deltaT so 50 degrees is 1, 25 degrees which might be typical for a Heat Pump with rads is 0.4, which means your 2.5 would be right.

Problem I see with the High Temp ones is you won't get the same COP so your payback will be far longer, no COP listed on link and have to put in details to get brochure.

Even the combined I'm not so sure on, I'd nearly prefer go for something like this ac/a2a heat pump split unit and keep old boiler.

dudley72

https://www.boards.ie/discussion/comment/118040577#Comment_118040577

With both the HT and the Hybrid you would need some references to be sure. I think I talked to Dalkin years ago, they mentioned about sitting in the heat pump alongside the existing boiler and then longer term swap out for a 7kW electric unit to top up the water. That would burn electricity so not really viable.

Other companies do the oil hybrid, so some of the rest might have the information

As you mentioned, the other option I thought about was A2A and then use existing system to heat up if required. Never priced

John.G

Heat Pump with Rads.xlsx

Heat Pump with Rads.xlsx

You may just get away with your own rads but ONLY if heating left on 24/7, roughly speaking my house with 20kw of installed rads averages around 5/6 kw heat demand with outside temp of 7C. Heat pumps, as pointed out above require to run with low flow temps to get a reasonable COP, say 35C to 40C. I ran my Firebird SE boiler at 40C last January (pipestat in series with control stat) and was able to maintain 20C throughout the house but it would certainly be prudent to oversize the rads by a factor of 2/2.5 if installing new, as pointed out above.

The attached screenshot gives a good feel for rad outputs at various flow temps.

Actual calcs included, as well. (twice, for some reason) Calcs are based on the fact that the "deg Rad" is the mean rad temperature minus the room temperature and the rad output, based on a "50 deg" rad is the (actual deg rad)/50)^1.3. (^ = to the power of)

A hybrid system can be used to get the house up to temperature rapidly (if heating has been off overnight) and then c/o to HP.

Heat Pump With Rads Screenshot.docx

GreeBo

https://www.boards.ie/discussion/comment/118040797#Comment_118040797

Even with them on 24x7 if your insulation is poor or worse, your house is drafty then forget about it, too many air changes means the low and slow rads will never catch up.

theboss.com

Thanks folks for all the feedback, FYI my house was built in 99, the grant work done on the house so far is triple glazed windows/doors, pumped cavity insulation, roof insulation and heating controls. Based of this, I think I am safe to go with larger sized radiators and install an ASHP, (after I do the above calculations).

sydthebeat

You need to do two things.

Confirm your HLI is 2 or less by a BER assessment and then get SR 50 calculations carried out based on the exact make and model of rads you want to install.

These SR 50 calculations will confirm the size required for each room, based on the BER information