Could someone double check my radiator sizing calcs? (conversion to ASHP)

[Crispy75]

I'll be insulating my house, as well as the new extension (one storey up, on the whole footprint). All new windows too. Aiming for EnerPHit.

 

The house is curently heated by a Vaillant Ecofit 825 gas boiler and mostly some rather fancy-looking cast-aluminium radiators; modular KAL models from Italy. The modern version is this: https://www.kazuki.co/en/product/radiators/aluminum-radiators/aluminum-radiator-kal-c  which looks almost identical. The boiler has a 9kW capacity and I calculate all the radiators to have a total output of 6kW at dt50.

 

After the works, the house will be 50% larger, but also much better insulated (U values: Walls 0.1, Roof 0.09 W/m²K). PHPP says my heating load will be 1.7kW at -10°C (or 2.2kW before taking solar gains and indirect heat in account, which is closer to the figure you'd get from a vanilla heat loss calc). This is a reduction of 75%, on a W/m² of floor area basis.

 

I want to replace the gas boiler with an ASHP (and also get some fan coil cooling on very hot days, but that's for another thread, another time). I think I can get away with using the existing radiators at dt20 or 30, simply due to the much lower heating load.

 

Now, the KAL radiators don't have data for dt20 or dt30, so I had to extrapolate from their 60/50/40 figures. There's also one Type 11 radiator which is easy to calculate. With the addition of small Type 11 radiators in the new bedrooms, I get a total output of 2.6kW at dt20, of which 2kW is the existing radiators. This roughly matches my load of 2.2kW, with a little headroom for error.

 

Does that sound about right? Going down from 6kW@dt50 to 2kW@dt20 with the same rads? Is my roughly linear extrapolation correct? Will the flow rates be so wildly different that the pipe diameter will matter?

 

I've attached the spreadsheet I used to do these sums and would greatly appreciate any advice. Thanks

 

^ KAL radiator. They're quite attractive so I'd like to keep them.

Radiator Sizing.xlsx

[Dave Jones]

dont forget fancoils are very good for heating as well, small footprint large output.

[Crispy75]

35 minutes ago, Dave Jones said:

dont forget fancoils are very good for heating as well, small footprint large output.

Yeah that was my 1st thought for the new rooms, but if I'm going to use FCUs for cooling also in rooms heated by radiators, it all starts to get a bit complicated in terms of control zones and plumbing. I'd want to send hot water to FCUs and rads, but cold water to FCUs only. Lots of valves! Also, I'm not really a fan (hah!) of having warm air blown at me. Cold, yes

[JohnMo]

Radiator heat output calculator, yellow cells is what to alter, use a dT between flow and return of 5.

Radiator Heat Output Calculator (1).xlsx

[Crispy75]

1 hour ago, JohnMo said:

Radiator heat output calculator, yellow cells is what to alter, use a dT between flow and return of 5.

Radiator Heat Output Calculator (1).xlsx 21.71 kB · 2 downloads

Cheers, very useful, and checks out pretty close when compared against manufacture data. Also comes out less than my extrapolation, so I'd be running at dt25 or so to keep some headroom. Still well within sensible limits.

 

If I try and get the boiler upgrade grant for the ASHP, will the installers accept PHPP's heat load calculations, or will they be forced to use cruder methods? (and therefore come up with larger rad requirements)

[Dave Jones]

7 hours ago, Crispy75 said:

Yeah that was my 1st thought for the new rooms, but if I'm going to use FCUs for cooling also in rooms heated by radiators, it all starts to get a bit complicated in terms of control zones and plumbing. I'd want to send hot water to FCUs and rads, but cold water to FCUs only. Lots of valves! Also, I'm not really a fan (hah!) of having warm air blown at me. Cold, yes

 

err, simplify. zero zones and open loop. nice and easy

[JohnMo]

7 hours ago, Crispy75 said:

Also, I'm not really a fan (hah!) of having warm air blown at me.

Run a low temp system, then there is no hot air being blown, just air a few degrees warmer than the room.

 

25 minutes ago, Dave Jones said:

 

err, simplify. zero zones and open loop. nice and easy

That gets my vote also. It's simple it works, others will come along and say you can't get it to work, but it does. The biggest issue you will have is a strong likelihood of the boiler short cycling if you do zone. This will increase gas consumption dramatically. Ours used double the expected amount of gas, when I had zones. Do the system so everything is a similar design temperature and get rid of any mixers or additional pumps.

 

17 hours ago, Crispy75 said:

Vaillant Ecofit 825

Big output boiler, low demand of 2.2kW, even lower at 7 degs. I assume the output min should be closer to 5 or 6kW. If that's the case you need a minimum of around 50L engaged water volume at all times to keep the boiler happy and not short cycle. That should give you about 8 to 10 min run time for the boiler.

[Crispy75]

24 minutes ago, JohnMo said:

Run a low temp system, then there is no hot air being blown, just air a few degrees warmer than the room.

 

That gets my vote also. It's simple it works, others will come along and say you can't get it to work, but it does. The biggest issue you will have is a strong likelihood of the boiler short cycling if you do zone. This will increase gas consumption dramatically. Ours used double the expected amount of gas, when I had zones. Do the system so everything is a similar design temperature and get rid of any mixers or additional pumps.

 

Big output boiler, low demand of 2.2kW, even lower at 7 degs. I assume the output min should be closer to 5 or 6kW. If that's the case you need a minimum of around 50L engaged water volume at all times to keep the boiler happy and not short cycle. That should give you about 8 to 10 min run time for the boiler.

You missed the bit where I'm replacing the gas boiler with ASHP!

[JohnMo]

4 minutes ago, Crispy75 said:

You missed the bit where I'm replacing the gas boiler with ASHP!

I did indeed. But then even more reason not to zone. Heat pump you really have too options no zones and super simple, or a big buffer (never a small one), additional pumps to buy and run and zones with controls. I am down the simple route and now only have a switch to change over from heating and cooling and a diverter valve for the cylinder heating to CH change over.

[John Carroll]

On 17/09/2024 at 10:59, Crispy75 said:

Cheers, very useful, and checks out pretty close when compared against manufacture data. Also comes out less than my extrapolation, so I'd be running at dt25 or so to keep some headroom. Still well within sensible limits.

 

If I try and get the boiler upgrade grant for the ASHP, will the installers accept PHPP's heat load calculations, or will they be forced to use cruder methods? (and therefore come up with larger rad requirements)

With a ASHP or weather compensation one should try and keep the dT as low as possible to achieve the lowest possible flow temperatures at any required output, I generally use a dT of 5C, but  I just use my attached calcs to give any combination required.

Radiator Output crispy.xlsx