We're about to have an ASHP and rads fitted, and I'm concerned that the rad sizes recommended are excessive. Obviously it's hard to give definite advice without seeing the property, but if anyone has any thoughts I'd be grateful...
The house is a 3-storey mid-victorian mid-terrace in the North Pennines, floor area in the order of 150m². Main rooms around 4m x 4.2m, 2.8m high ceilings. Stone built, we've added 100mm Kingspan internal insulation to all external walls and to the roof spaces, in addition to the usual roof insulation; as well as insulating under the ground floor. New triple glazing throughout.
We're going for underfloor heating for the ground floor, new rads for the first and second floors.
Installer has recommended a 8.5Kw Mitsubishi ASHP and a 210l DHW cylinder, which I presume will be adequate?
But the suggested rad sizes (calculated for a 45℃ flow temperature) look very big to me:
Could anyone comment on these rad sizes please? And also advise, if we install oversized rads will that have adverse consequences (apart from losing precious wall space!)?
Well you should have heat loss calculations for the rooms really, but your insulation looks healthy. One advantage though of bigger rads is you can use a lower flow temperature and hence a get higher efficiency (COP) and lower bills.
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there is plenty of online data about radiator outputs per size at different flow temps, you can easily check what wattage a size AxB K3 will do at temp T. there are also plenty of publicly available full house heat loss calc that you can do for yourself.
most important thing I can suggest is that you fully cross check the maths of your proposed installer, all rooms heat demand and radiator outputs, using several of the public tools.
its not been unknown for an installer to be wrong (not fully considering insulation for example), or for different installers to get wildly differing figures for the heat loss (as well as of course different costs) .
personally I detest K3's - I assume you are familiar with how far into the room a K3 projects?. for me they cross over the border into unacceptable room intrusion - but its up to you of course. If size is becoming an issue but you need the output, you can look at Fan driven units (some times called FCU's) of which again there are many on the market - fan driven means you get higher output for same physical size. In fact I think Mitsubishi have their own range of these.
physical space and purchase cost of the rad itself are the biggest downsides of bigger rads. bigger rads will always enable more efficient running of the ASHP - by enabling a lower flow temp - but only if you have this system wide (which to be fair it seems that you do have).
I should have said - we like to keep the house at about 18℃
Looking at your numbers quickly I'd agree; about 50% oversized. These rads are enormous. The sizes you quote aren't quite standard but a 2m wide K3 is huge, and expensive.
Is the 'watts required' column your calculated heat loss? The total heat loss is in the 13-14kW region. Even without the oversized rads I suspect something is wrong with that figure because that's a lot for a 150m2 terraced house with the insulation you describe. Most older but upgraded houses seem to be in the 50-60W per m2 region.
An 8.5kW Ecodan will not heat these rads to 45 degrees at your likely design temp (normally around -3C). Even if you allow for oversizing it's still not going to be nearly enough. OTOH an 8.5kW Ecodan could be fine for a well insulated/upgraded 150m2 terraced house with ufh. As I said, something's not right.
Oversizing rads is OK and will normally mean lower running costs, but not 50% oversized. And 210l is quite a small tank. For 2 people who have showers and a dishwasher it'll be fine. With kids/teenagers/baths maybe not.
also worth saying that on radiator purchase cost there is are significant non-linearities in cost and a reduction in suppliers you can pick from as you go larger (either K2 to K3, or over about 1400 wide)
some typical standard sizes, of which 600x1400K2 is pretty much the best "bang for buck"
600x1200 K2 = £80 , 600x1400K2 = £100 , 600x1600K2 = £180-200 (screwfix , toolstation, or any local plumbers merchant or online discount plumbing store)
do you have a itemised prices for all the radiators in your installers proposed bill of materials? by my guesstimate you've got 7 radiators at about 3-4 grand? in my larger (210m2) house I did a lot more rads for a lot less money.
It might be the installer is using the MCS spreadsheet you can find on their website where I think there is an error in their formula. It proposed nearly all K3 for my house (pessimistic MCS heat loss of 7kW at -3 C for 110m2) whereas what was actually needed was all K2 (and a K3 in the kitchen under the worktop) to run at 35C flow temperature.
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We're about to have an ASHP and rads fitted, and I'm concerned that the rad sizes recommended are excessive. Obviously it's hard to give definite advice without seeing the property, but if anyone has any thoughts I'd be grateful...
The house is a 3-storey mid-victorian mid-terrace in the North Pennines, floor area in the order of 150m². Main rooms around 4m x 4.2m, 2.8m high ceilings. Stone built, we've added 100mm Kingspan internal insulation to all external walls and to the roof spaces, in addition to the usual roof insulation; as well as insulating under the ground floor. New triple glazing throughout.
We're going for underfloor heating for the ground floor, new rads for the first and second floors.
Installer has recommended a 8.5Kw Mitsubishi ASHP and a 210l DHW cylinder, which I presume will be adequate?
But the suggested rad sizes (calculated for a 45℃ flow temperature) look very big to me:
Could anyone comment on these rad sizes please? And also advise, if we install oversized rads will that have adverse consequences (apart from losing precious wall space!)?
Many thanks
I fully agree with Kev's assessment that there is something wrong with your installers calculations, since according to the supplied table the standard heat loss is 13.6kW, but the proposed heat pump is only 8.5kW. I would suggest that you ask your installer to clarify how they arrived at the heat loss calculations with details of any assumptions that may have been made.
As you are no doubt aware, heat loss varies from room to room dependent upon the physical size, the level of insulation, but also its location within the property. A detached house will have 4 outside walls, plus roof and ground floor, so most rooms are likely to have at least one outside wall. A semi-detached or end terrace will only have 3 outside walls, and an inner terrace will have only 2 outside walls. These factors need to be taken into account when performing a heat loss calculation. Also note that the standard criteria for heat loss calculations is for most rooms to be at an indoor temperature of 21C and outside temperature normally in the -2C to -3C range.
The specified heating capacity of radiators is at a temperature difference (DT) of 50C, so if the indoor temperature is 21C, the average water temperature would need to be at 71C for a radiator to achieve its specified heating capacity. At lower water temperatures the heating capacity is reduced, but not in a linear manner. Taking the example of a Type 22 double panel double convector radiator of 600mm x 1200mm, the specified heating capacity is 2051 Watts.
If this radiator was installed in a room with a standard heat loss of 1000 W at -2C, to maintain an indoor temperature of 21C would require an average water temperature of 49.8C
If instead, a similar radiator of 600mm x 1000mm was installed with a specified heating capacity of 1709 W, this would require a water temperature of 54.1C to match the heat loss.
The correct sizing of heat emitters, radiators or UFH, is therefore important to not only achieved the calculated heat loss, but also to achieve maximum overall efficiency. To correctly size both the required heat pump, but also the heat emitters is dependent upon heat loss calculations being as accurate as possible.
Do you have any data for your present/past heating system that would indicate your normal energy consumption?
I have attached a spreadsheet that can be used to calculate the required water temperature or expected thermal energy output for radiators under different operating conditions. The values in the cells highlighted in yellow can be varied to see the effect.
The heating capacity of different size radiators can be obtained from the Screwfix website.
If you have any further questions please feel free to ask.
I fully agree with Kev's assessment that there is something wrong with your installers calculations, since according to the supplied table the standard heat loss is 13.6kW, but the proposed heat pump is only 8.5kW. I would suggest that you ask your installer to clarify how they arrived at the heat loss calculations with details of any assumptions that may have been made.
Could it be that the column labelled “Watts Required” is the value of the room heat loss estimate already modified by the radiator output factor at the lower delta-t (to the standard 50C) and possibly with a 20% overhead on top? This might account for the difference between the total heat loss given and the HP rating. That is, the actual room heat loss is more like half of that given in the table, which would seem more plausible given the insulation levels stated.
My goodness - what a wealth of expertise and wise advice: thank you all!
@iancalderbank thanks for the tip about FCUs: if we actually need the size of rads recommended, I'll certainly look into these. I don't have itemised prices at this point: for long and complicated reasons, I'm getting a local plumber to install the rads to the specs supplied by the ASHP installer; but I'm not going to pass on the rad specs until I'm satisfied they're right.
@kev-m good to know you agree and my (basic) understanding isn't completely wrong! There will be 3 of us with short showers and a dishwasher - no kids or teenagers!
@dougmlancs that's interesting. I'll try to find out what he's used (but annoyingly he's away on holiday for 3 weeks, which delays everything)
@derek-m thank you - this confirms what I thought I had understood, good to have it laid out so clearly. I hadn't picked up on the discrepancy between the heat loss and heat pump - thanks for highlighting it. Sadly no useful data from the previous system, as a) the insulation is new; b) my mother in law (who lives in the house) mostly used the solid fuel stove. But even before the insulation was installed, the oil boiler, rads and stove were overkill - the TVRs were all turned down low, or even off, most of the time.
This all confirms that I need to go back to the installer and get much more detailed info before any work starts. Thank you all - I'll keep you posted with any progress!
I fully agree with Kev's assessment that there is something wrong with your installers calculations, since according to the supplied table the standard heat loss is 13.6kW, but the proposed heat pump is only 8.5kW. I would suggest that you ask your installer to clarify how they arrived at the heat loss calculations with details of any assumptions that may have been made.
Could it be that the column labelled “Watts Required” is the value of the room heat loss estimate already modified by the radiator output factor at the lower delta-t (to the standard 50C) and possibly with a 20% overhead on top? This might account for the difference between the total heat loss given and the HP rating. That is, the actual room heat loss is more like half of that given in the table, which would seem more plausible given the insulation levels stated.
I'm afraid that I do not follow your logic, since heat loss is heat loss and is not dictated by the radiator size or type. Different size or type of radiators will merely require a higher or lower water temperature so that the thermal energy output matches the present heat loss.
I have attached the radiator heat output spreadsheet with an added sheet 3. This shows a comparison of the required water temperature for both K3 and K2 radiators.
