My spreadsheet with 21C and a water temperature of 45C gives a DeltaT24 and with 40C a DeltaT19.

This morning my favoured contractor came back at me about my spreadsheet with a water temperature of 45C + an extra DeltaT10 that gave DeltaT19 so the same rad value as my water temperature of 40C. I have asked him what it’s about but while I await his answer has anyone got any idea what it’s about?

He is using CIBSE software and tells me that according to their tables the rad conversion at DeltaT19 would be 3.35 whereas Stelrad say 3.04 for DeltaT20 - I wonder who knows best.

Meanwhile Green Energy Group (whom I was just using as a check price) have told me this evening;

“With regards to ASHP installs, every install requires new rads and 15mm pipes to ensure the system will work correctly. The job would not get signed off if not as it hazards so many issues in the future.”

“This typically is a 5 days work”

I wonder if MCS know how their accreditation is being used to misrepresent? Either these people don’t want the work (then be honest) or they I button up the back.

Posted by: @alistairgbarron

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He is using CIBSE software and tells me that according to their tables the rad conversion at DeltaT19 would be 3.35 whereas Stelrad say 3.04 for DeltaT20 - I wonder who knows best.

It's a minefield. In fact the whole thing is a minefield. 

As I have said before I prefer using a correction factor, ie you down rate the rads output for lower delta T values. To me, it is more intuitive, others may have different views. Your CIBSE chap (and you) appear to be using the over size factor (because the numbers are > 1), ie how much bigger the rad needs to be at lower delta T values to match the delta T 50 output. Theoretically, one is the reciprocal of the other, eg a correction factor of 0.5 (half the output) means an over size factor of 2 (twice the size). One problem with the over size method is you now have to find a rad that is (at least) twice the size. With the correction factor method, you have the actual output values for your available rads, and can see how much over/under the needed output the actual output is.   

I used the Stelrad tables because they use a one degree increment, other tables tend to use 5 degree increments, but they all have very similar figures for a given delta T eg for delta T 20, the correction factor is 0.302 (Stelrad) or 0.304 (KRAD). The corresponding over size factors are 3.31 and 3.29.

My Stelrad correction table only goes down to delta T 20, but if we extrapolate (fair enough as it is pretty much a straight line) we get a output down rating correction factor at delta t of 19 of 0.282, and that gives us an over size factor of 3.546. The key point here is that even one degree change in the delta T does make a visible difference.

Is it possible you have mixed up the numbers? You quote a Stelrad figure of 3.04 for a delta T 20, and because this is > 1, it implies an over size factor, but at the same time the correction (down rating) factor is 0.302 or 0.304 (depending on source). Has the decimal place jumped the rails? It is easy to get mixed up because at delta t 20 the numbers look very similar: output down size factor is say 0.303, size upgrade factor is say 3.3.

The other thing to remember is that while thermodynamics may be an exact science, designing heating systems isn't. I use the good enough test, something definitely needs to be good enough, but it doesn't need to be perfect. 0.302? 0.304? Either will do, or if you want, use 0.303.
 

Posted by: @alistairgbarron

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“With regards to ASHP installs, every install requires new rads and 15mm pipes to ensure the system will work correctly. The job would not get signed off if not as it hazards so many issues in the future.”

I had a number of installers take similar positions. They are jobsworths, hiding behind regulations. Solution? Fire them. 

@cathoderay oops - yes I was using 0.304 and now I realise that he is multiplying up by 3.35 so does get back to close to the ∆50 output.

Fortunately at the lower ∆19 we are using (however we have both arrived at it) all but 6 of the rads have 10-20% excess output capacity to achieve heat loss. For the worst one, our bedroom, it is easy and indeed beneficial to add a second radiator. Buying a couple of extra rads and juggling the other 3 around which is easy with minimum exposed pipe and the job's a good 'un.

My ∆19 is based on water temp of 40C but his based on a water temp of 45C with his extra ∆10 thrown in. He is happy quoting 45C off the heat pump and we should allow for some heat loss in a buffer tank. However the buffer tank and the cylinder will both be on the ground floor in the core of the building so any loss from them will be a gain to the building. 

Now we are running the boiler very low that seems to give an off temperature of between 30 and 40. With an outside temperature below 0C last night and a daytime of 4C the house is staying warm. Set at 20C during the day, 21C in the evening and 17C overnight the boiler is not firing very much and the rads are generally running at between 30C to 36C. This morning it only took 45 minutes to raise from 17C to 20C. By Friday when the installer is coming to do his own heat loss calls we are looking at -4C overnight and 0C during the day so that will be a good test. All the indications are that it should be fine.

Posted by: @cathoderay

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I had a number of installers take similar positions. They are jobsworths, hiding behind regulations. Solution? Fire them. 

I spoke with them a couple of years ago and quickly realised that they were still selling double glazing. However they are "popular" and spend a lot on internet advertising so I thought I would use them as a check price. When I first spoke to them their guide price was £11K. Next I had them round in September to quote on a hybrid and the price was .... £11K all with no physical survey and a fancy proposal - oh and if you shell out £500 up front they will send the designer round. I can guess what happens next.

I called MCS after their text message to ask them if the rads and pipes would prevent anyone for "signing off" (signing off clearly being a function of meeting the design the MCS accredited installer has developed) and of course it wouldn't. They asked me to send them a copy of the message so I did😉

What really annoys me about this is that, using their accreditation as an excuse, they would rip an unsuspecting person off.

Rant over😁

Posted by: @derek-m

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The ^1.3 formula I found somewhere on the internet, but cannot remember where. It does appear to work.

In the meantime, I've tracked this down, it is the so called radiator exponent n equation, which is used by rad manufacturers to calculate their correction factor tables. Different rad brands/families have slightly exponents, but 1.3 is a typical value. Apparently Stelrad's is 1.29, but reverse engineering their tables, I find it is nearer 1.305: (20/50)^1.305 = 0.302474167 which rounds to 0.302. I have no idea whether the exponent is derived empirically, or calculated in theory, or whether it is a magic number, but as @derek-m says, it seems to work. The actual formula is (your delta t / standard/reference delta t of 50) ^ n. You can either use n = 1.3, which is likely good enough, of track down the rads particular exponent, and use that. Apparently the formula is a simplified version of yet another formula, but as it seems to work, and the complicated one involves using logs, I reckon the simplified version is indeed good enough.    

@cathoderay well done - good to know the provenance of things like that - and simple suits me fine 🤣

@alistairgbarron - could you please clarify the point about your 'favoured contractor', who crops up here regularly...

Can we assume that this is a local company who don't need to advertise, who live by their reputation and are able to pick up new customers based on recommendations?

Out of interest, what's the background of the proprietor? Electrician? Plumbing & heating? Engineering?

Please remember that others will read this topic in the months to come, whilst they're looking around for an installer. So please share how you found this company in Scotland and what criteria you were looking for. Thanks.

@transparent I have 2 installers I am considering both not so far away from me, they are both experienced, competent and well spoken for. However probably best wait until a) I have made a decision, which I am keen to do, and b) I have the finished installation, which will be some months yet. Perhaps then I will make a separate blog post / thread about who I eventually chose, how I arrived at that decision and how well (or not) they performed. I might even keep it going for a while to feedback on the performance of the system.

Well we had the contractor visit on Friday to do their heat loss and radiator calcs. They use a piece of software that is way more complicated than plain old Q=U*A*ΔT and they arrived at 7280w as opposed to my 5400w. Seeing how it does it and the factors it takes into account I won’t argue.

Anyhow after some discussion about which unit and how many new radiators they have come in at 20% more expensive than the competition. Needless to say they won’t be getting the work. The contractor we are going with, a local firm from Dundee, has offered a good package for all the elements, i.e. Heat Pump, Solar PV and Battery.

My funding request is now back in with Home Energy Scotland and all things being equal we are looking at a March install.