Are you using PHPP? I believe it is a requirement for both EnerPHit and AECB. If so, it will tell you how much heating you will need. If not, what model are you using? How do you account for thermal bridges etc? What are the difficulties that prevent you aiming for EnerPHit airtightness?

thankyou both..

<blockquote><cite>Posted By: djh</cite>Are you using PHPP? I believe it is a requirement for both EnerPHit and AECB. If so, it will tell you how much heating you will need. If not, what model are you using? How do you account for thermal bridges etc? What are the difficulties that prevent you aiming for EnerPHit airtightness?</blockquote>

Yes we are using PHPP, which I thought only provided heating demand for the whole house, not room by room. Thermal bridges are accounted for. Our builder is not a specialist so it was suggested that 1.5ach @50 was a good realistic target to aim for. We are keen to have a well performing home but not aiming for EnerPhit certification.

<blockquote><cite>Posted By: Nick Parsons</cite>Welcome to the Forum! Grab all the benefits of the collective consciousness while it's still here! ;-)

''Given that our house will have relatively low heating demands is combining an ASHP with radiators a viable option?

If we do go down the ASHP/rad route, how can I calculate radiator sizing for individual rooms?

And how can I calculate an appropriately sized ASHP?''

Pay circa £300 to a MCS-certified person to do a full room-by-room heat-loss calc (as required if you are going for the grant anyway). Ask/tell them to do it based on your realistic target U values and air-tightness target and ask them to calculate for your target flow temp and for 5 degrees either side. Whenever I see a quote based on 55 degrees I ask them to re-calc based on 40 and 45.

Thanks Nick, I had not heard of MCS but sounds like this is exactly what I'm after. What grant are you referring to?

What insulants are you using, BTW? Are you doing floating floors over the exg concrete? I have done floating floors with XPS and (for better U value) with PIR using only the self-weight of glued T&G 18mm OSB to hold it down and I am extremely happy with it. (Also got one floating floor with 200 wood-fibre, but that would make your doorways a bit short!! How are you coping at the stairs (unless it's a bungalow)?</blockquote>

I believe the roof will be 300mm warmcel within I-beams, EWI 200mm not yet specified (although I believe PIR type due to budget) and floor will be 50mm PIR.
Regarding floors, our current ceilings are 2550mm, so my plan was: 50mm PIR---glued 25mm T&G chipboard--- 22mm engineered wood. However, I am terrified of doing this as in the past I have always battened in between the insulation, and then screwed the chipboard down. Can you please reassure me it works?! And can I get away with 18mm OSB > 25mm chipboard? And did you batten the OSB in high traffic areas (like doorways) or high weight areas (like underneath kitchen cabinets/fridges etc)?
It is currently a bungalow so we are installing new stairs, new doors, so the only limiting factor in height is the ceilings.

Thanks again - really useful advice and any more opinions on ASHP and rads would be much appreciated

I have used the floating floor system on 3 occasions, each with click fit flooring over XPS. The first with oak on pine click fit laid on 50mm XPS about 10 years ago and the subsequent floors were 8mm thick composite click fit flooring laid on 20mm XPS. None of the floors had any fixings other than that provided by the click fit system and none have shown any problems. (All in rental properties)

I haven't, but I will when I have finished my insulation programme, and I know several who have. And if you want to find oodles more look in Wales for those benefiting from the ECO4 grants. Their ASHP installations are all with rads - hundreds of them (installations, not rads...)

What you will often find is that an installer will offer you a system designed to retain as many of your exg rads as possible. I appreciate that may save you money, but it is usually at the cost of a higher flow temp and lower CoP. In 2 cases while helping a friend of mine organise their installations the initial quote and schedule of rad replacement required was based on a flow temp of 50 degrees (and perhaps 55 in one case - I cannot remember perfectly). As soon as I see that I ask for the quote to be recalculated at 40 and 45 degrees. Depending on the levels of insulation (in both cases it's been pretty basic) trying to get down to 40 degrees sometimes gets you rads bigger than the space available!

They'll still perform better at a lower temperature. TANSTAAFL!

I've been involved with installing the Vaillant arotherm on a couple of projects. R290 refrigerant (propane) gives up to 75oC output. Do your own research on the noxiousness or otherwise of R290. No need to use direct elect immersion to do the legionella cycle (should you feel that is necessary in your private home). The maufacturers either have the SCoP data on their websites, or will provide if asked, for varying temperature differentials.

No idea where you're located geographically, but here I need to consider cold winter temps (-15oC), and need to look at the drop in power output as ext temp drops. ie. a 10kW unit may only output 7kW at -15oC, for example.

There's no issue connecting rads, have fitted UFH grnd flr and rads 1st flr. Sizing rads, I do the room calcs myself for heatloss, though last job needed an MCS calc, and the results were similar to mine.

The chosen design deltaT for the rads can vary, but also consider what external temp the heatloss calcs were run at. If it's quite low, then most of the time, the ASHP will not need to run at the high flow temp, as the ASHP control (heat curve) will drive the flow temp down. ie. just cause it's "sized" at 50oC water, does not mean it will always/often run at 50oC.

What is the carbon footprint of chucking out and replumbing new larger rads versus running a little hotter for a number of hours per year. If the rads are knackered/choked/way under sized, then probably necessary to replace. Also consider that the entire supply/install will be zero VAT rated, so now is the time to replace if you ever would.

Steel versus aluminium rads - ali are of course much lighter, which the plumbers like, and can be useful if you're worried about wall fixings. The ali do give a better heat output for the same size, like 20% more, but just read the charts for the rads you prefer the look of, for the approprite power output. Ali rads would be a bit smaller for the same output.

There is not a one size fits all answer. Problem is, lots of installers don't have the ability to arrive at the appropriate answer, or are risk averse in our litigious society, so just do the same every time. Probably more cost effective due to repeatability.

V gd evidence - thanks

maxsm1, I have a sort if similar problem with my system going from a wood burning boiler to a HP. Mine is a two fold problem 1) how much energy do I use (no meter on the wood) and 2) will the rads be big enough.
For 1) I put a heat meter on the CH to see the actual energy used, in your case the gas bill should give you a good idea
And for 2) I reduced the flow temp to the expected HP flow temp and monitored the effect.
I collected the daily data for January (our coldest month)
Answer = some rads need size upgrade but I can reduce the cost by moving larger ones to replace smaller ones and get bigger ones to replace the moved larger ones.
Will you have the luxury of time to do a similar test in January??

(My move to a HP will be a couple of years away (delayed from initial plan) but at least I have the data to make a decision)

You might still be able to use PiH's idea. You can supplement the heating just for this coming winter using cheap electric radiators or fan heaters if necessary.

thanks again, very useful

Rads or CH system are NOT of themselves zero rated, as they are not an energy saving device. However, with a HP, then the HP is the principle element, so rads and CH get bundled with it. See the cut/paste below for the HMRC link above...

Example 3
If you install a central heating system as described in Example 2, but with an air, ground or water source heat pump instead of a conventional boiler, this would be a single zero-rated supply because the principal elements are zero-rated.

Posted By: maxsm1Hi all,

After a long period of planning we are (hopefully!) a few months away from building work beginning on our eco retrofit project. There will be 300mm insulation in the roof, 200mm on the walls and 50mm overlaid on top of the existing concrete floors. This is expected to achieve around 30kwh/pa heating demand and we are aiming for airtightness to be somewhere in between enerphit and aecb.

Given that our house will have relatively low heating demands is combining an ASHP with radiators a viable option?

Short answer: Yes

It won't be quite as efficient as UFH but it'll be fine. Size your radiators and pipes right, and avoid zoning and buffers and it should be easy to achieve SCOP>4

Easiest way to do the sums is use 'heatpunk' online calculator: https://heatpunk.co.uk/
You have to create an account but it's free to use, and actually rather good. That will size your heatpump and do various checks. It does assume 'normal' construction but should work well enough (you may need to do a custom wall assembly).

The MCS spreadsheet also works fine.

In fact there are hordes of 'by-room' heat calculators out there, and making your own spreadsheet isn't that hard. It's just areas and U-values and inside/outside/floor/adjoining wall temperatures.

There are a few really expert heat pump practitioners putting content onto youtube
heatgeek
urbanplumber
both worth exploring.


ASHP run most efficiently (== best COP) at the lowest FLOW temp consistent with the required output being delivered. That output in turn depends on the radiator sizes and the bore of the CH piping serving them. In general microbore does not find favour with installers - but there are installations to microbore-piped houses documented on youtube that the householders are happy with.

CIBSE's publications are are useful deep dive. Lewis Litherland's 'An introduction to low temperature heating design' a better place to start