@alistairgbarron

Hi Alistair,

I have attached a small spreadsheet that you may find useful with regard to your radiator sizes and heat loss.

Varying any of the highlighted cells will either calculate the expected heat output (left-hand table) and/or the required water temperature (right-hand table).

The lower the water temperature then the higher the efficiency of a ASHP. The limiting factor will be the room with the lowest radiator heat output, to the heat loss for that room, particularly if doors are kept closed which may limit the movement of heat energy from room to room.

@derek-m thx for that. I’ll apply those equations to my radiator table. Meanwhile just putting in my total radiator output and my heat loss gives me a water temperature of 40C which I think is good. I have anticipated a bit of radiator juggling now while it’s zero VAT.

As a matter of interest where does the /50 and ^1.3 in the equation come from?

@derek-m Perhaps it’s me, now I have had a play with your spreadsheet, but if I reduce the DeltaT water temp to 30C the output falls off a cliff to 215w. Whereas according to Stelrad reducing a 2000w rad from 50C to 30C reduces it to 1030w

Posted by: @alistairgbarron

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All in all it still leaves the problem of installers being anal about MCS Excel spreadsheet data

I had a few installer quotes based on the MCS software. They didn't get the job.

Good that you are double checking things, typos are easy to do.

@cathoderay

Problem at the moment IMHO is that there are precious few good installers and too many double glazing salesmen. And they must be MCS certified to access the funding - I can get £10K grant and £19.5K loan from Home Energy Scotland plus the Zero VAT toward the cost of HP/Solar/Bat project which is an absolute must given cost.

I have identified what appear to be 2 good installers and am just trying to keep on top of them. Then I need to get a final quote in to secure the funding and hopefully the price too. 

@alistairgbarron You are confusing water temperature (average temperature of water flowing through the radiator) with delta T which is the difference between room temperature and radiator flow temperature.

The Stelrad radiator specs are in delta T form, so when they say 50 degrees they mean an average radiator temperature of room temp plus 50 degrees I.e. 70 degrees water temperature.

@alistairgbarron - I had similar painful hoops to jump through, and had the additional constraints caused by a small listed building, the listing limiting options of itself, and the small interior walls and often low set window sills limiting available wall space. The least bad compromise was a high LWT (some installers couldn't handle the heat and conked out) and big ugly K3s but in practice they don't look too bad, because although ugly compared to a K2 on a bare wall, they are partly masked by the surrounding furniture which almost always projects further into the room than the K3. The only room that doesn't work from an aesthetic point of view is the living room, where a necessarily large K3 dominates one wall, making that part of the room look like a plumbing warehouse.

You are right that some installers are over-obsessed with MCS compliance. There is compliance, and then there is over-compliance, which is when focusing too much on compliance actually produces a worse outcome. The extreme example, which very nearly happened to me, is the installers fussing means you miss a grant deadline, and the installation never happens. In my case, that would have meant going back to oil fired heating - I had a working boiler, the only thing that needed replacing was the tank. Self-funded ASHP at £12,000+ a pop or new tank at £2,000 a pop? One of the reasons I liked my installer was that he understood all this, and bent over backwards to get the ASHP installation done in time. He also understood the wall space limitations in the house, because he visited the house and saw for himself what the constraints were. Other larger companies sent out goons to do the survey, and had someone else in the office design (a rather grand term for dialling in numbers) the system, who had no idea of the constraints involved. Even photos with tape measures showing dimensions failed to convince then that you can't fit a 900mm high rad under a 700mm off the floor window sill and expect it to look OK. They were too busy ensuring their 'design' was MCS compliant.

What makes for a good installer? It involves many things, but one of them, given the importance of grant funding to making things happen, is that they understand deadlines.  

Posted by: @cathoderay

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The 10mm tails might mean the circulating pipe has to work a bit harder, but if the main pipe runs are 22mm where they need to be there is a good chance you will be OK.

Assume that this 10mm (ext diameter) PEX pipe has the usual 2mm thick walls, and that we're comparing with the more common radiator feed pipe using 15mm copper (0,7mm wall thickness). The cross sectional area of the internal bore is one-fifth of what's really required. I'm ignoring the possibility of the rad-valve restricting the pex-pipe connection even further.

Yes, the circulating pump will have to work harder, and choosing a more powerful model would help.

But there will also be considerable time-lag before the rooms reach their set-temps.

Perhaps that's not too much of an issue. I have one section of solid concrete floor with ufh pipe buried within. That takes a good 12-hours to reach temp from 'cold'.

Two possible points to discuss with the proposed installer:

1. What about setting up the ASHP control system as if it really is feeding a house with ufh in 100mm thick concrete pads?
2. The system needs to be considered as one which is 'always on'. There's little point turning it down just because there will be no one at home overnight.

Posted by: @transparent

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But there will also be considerable time-lag before the rooms reach their set-temps.

Perhaps that's not too much of an issue. I have one section of solid concrete floor with ufh pipe buried within. That takes a good 12-hours to reach temp from 'cold'.

I wonder if this is something we don't give enough consideration to. I too have noticed my heating takes a very long time to reach temp, sometimes measured in days rather than hours, but when it gets there, it tends to hold it. I wonder if this is because it needs more heat energy the raise the temp of a structure than to keep it at the same temp? By the laws of common sense that does, err, make sense. More technically, perhaps I am getting at thermal mass, something that once warmed up holds heat. Or maybe it is latent heat. The proper terms escape me. It's all a bit like charging a battery, high charge rate to start with, trickle when fully charged. Plus a large battery will need more energy to charge it, but it will hold more energy when it is charged. 

If there is anything to this, then it is another probable argument inf favour of always on systems, with the regulation done by altering LWT ie using weather comp and/or possibly flow, but on a Midea the latter is internally set.    

@transparent I expect to run the system continuously with a slightly lower night time than day. However not with a single temp thermostat for the whole building because that simply won't work. With a buffer tank the HP will maintain the tank temperature and I will use my existing EasyControl system to regulate the rooms, it will both call the DHW pump and regulate the TRVs. At a constant night/day temperatures this will maintain the balance throughout the building. Meanwhile I am hoping for a wee bit of lee way to call for a bit extra in a room when desired.

So your time-setting to bring the temperature up to 'daytime' again will need to be an hour or two before you actually want to experience that temperature!

Some HP systems require a central thermostat to be present in order to cope with stuff like anti-freezing routines. Your installer will need to assure you that the presence of such a sensor won't interfere with the room-stats that you want to configure.

Posted by: @alistairgbarron

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I am hoping for a wee bit of lee way

I'm just imagining that being said with a Scottish accent 😉 

Posted by: @alistairgbarron

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@derek-m thx for that. I’ll apply those equations to my radiator table. Meanwhile just putting in my total radiator output and my heat loss gives me a water temperature of 40C which I think is good. I have anticipated a bit of radiator juggling now while it’s zero VAT.

As a matter of interest where does the /50 and ^1.3 in the equation come from?

The 50C is the standard Delta T used by radiator manufacturers to designate the heat output capacity. A radiator may be specified to produce 2014W with a water temperature of 71C and an indoor air temperature of 21C.

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