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Nostrum

Plumber
Gas Engineer
Anyone else finding the heat emitter guide a pain to work with? I keep finding myself with the same problem.

I can size a radiator based on flow temp and heat loss requirements, but the calculated oversizing factor is in the lower star rating than the flow temp.

Example @ 40°C flow temp

Bedroom with a heat loss of 394w/hr @ 18°C

Radiator 600x0900 K2 output @ 18.5°C ΔT = 439 w/hr
Radiator 600x0900 K2 output @ 50°C ΔT = 1559 w/hr

1559 / 394 = Oversizing factor of 3.9

Star rating chart gives ASHP with 40°C flow temp a 5 star rating but with an oversizing factor of 3.9 only 4 star rating!

So which one is right!?!

Any ideas? I don't fancy oversizing radiators just to please the oversizing factor if it can be avoided.

Thanks in advance
 
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L

lame plumber

I love being a dinosaur who does AGAs n rayburns and gas/oil boilers, so dont know, however, I'd oversize to cover my ass with an ASHP install :)
 

Worcester

Plumber
Hi Nostrum,

Think you're confusing yourself

Slightly confused W/hr is energy W is power

Sizing the radiators we are looking at power (Watts)

Assume flow temp of (40°C) 45°C and a ΔT on flow of 5°C

Power output required = 394W
Heat emitter guide says for (40°C) 45°C flow you need multiplier of 3.1 on a radiator with a flow / room ΔT of 50°C

So you need a radiator with output of (at least) 3.1 x 394 = 1221.4

Taking the manufacturers figures (Stelrad)
Radiator 600 x 900 K2 output @ 50°C ΔT = 1559 w/hr
Would be OK as it is MORE than 3.1 times bigger

Simples; with the 600 x 900 you are just putting in a bigger radiator than required, or if existing, it is more than adequate. You'll still be 4 stars, though you might be able to reduce the flow temperature a couple of degrees and give the client abetter CoP / SPF , no impact on RHI as the star rating gives the SPF, lower running costs though.

If new, you could get away with a smaller K2 radiator 600 x 800 K2 = 1386W so still adequate (greater than 3.1 multiplier)

You MIGHT be confusing Oversizing factor with SPF used for calculating the renewable element!

Download the ORIGINAL Heat Emitter guide as opposed to the current MCS one; the original was 6 pages and simple to follow, the MCS one includes the 6 pages and was written so you have to do a training course to understand it!! Load of extra words not necessary if your a heating designer!
Original: http://www.microgenerationcertification.org/images/MIS_3005_Supplementary_Information_2_-_Heat_Emitter_Guide_v2.0.pdf
Print Version (It's what I use :) ) http://www.microgenerationcertification.org/images/MIS_3005_Supplementary_Information_2_-_Heat_Emitter_Guide_v2.0_Print_Version.pdf

With retrofit we tend to design for 45°C flow as that gives a good balance between radiator size and return through RHI; the extra radiator size and pipe size needed for a 40°C flow doesn't work add up to a benefical increase in RHI payemts or running costs when using radiators. To go that low, radiators will only work with a brand new build with great insulation and MVHR. On a retrofit to go that low, it would have to be underfloor heating designed for that flow and the mixing valves removed.

Make sense?
 
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OP
N

Nostrum

Plumber
Gas Engineer
Hi Worcester,

Thanks for the reply. Looking at the chart my understanding of it was that, for example, an oversize factor of between 4.3 & 6.8 would attract a 5 star rating, which also indicates a flow temp of 40 degrees.


HEG.png



Bedroom @ 18 degrees
40 degree flow temp
7 degree ΔT
MWT of 36.5
A-W ΔT of 18.5 degrees

Room heat loss 559.99 W/hr
Emitter output at 18.5 606.86 W
Emitter Equivalent output at 50 2152 W

Oversizing factor 3.84 (2152/559.99) = 4 stars!


What am I missing?! :disappointed:


Appreciate your help.
 
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Worcester

Plumber
Aha I see you're confusion! - my mistake

You're right in my example for a standard radiator my figures should be:
"Assume flow temp of 45°C"

I've corrected it above so as to give people a correct example

At 40°C flow you need a multiplier of (at least) 4.3

So to answer your original question
According to the emitter guide the answer is 4 stars as 40°C flow won't put enough heat into the room with an oversize factor of only 3.9, it needs to be 4.3

I don't know if MCS would take the manufacturers data for a ΔT of 18.5°C . - where did you get that data from? If you can prove the output is 439W, then I'd stand by it and fill the Compliance Certificate as a 5 star rating. bearing in mind if you;re going to do that you should also add in the CIBSE f2, f3 and f4 factors (Section 10.5) so that you can prove the case at any future audit by Ofgem, MCS, or your certifying body.
 
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OP
N

Nostrum

Plumber
Gas Engineer
Yes , i've used the CIBSE factor F1 to reduce emitter output, as it is the only approved, documented reference I can find. I have then used the emitter manufacturers info for the 50 ΔT data and then applied the F1 factor.
 

Worcester

Plumber
Using the f1 factor with an MW - A ΔT of 18.5 degrees, you get a multiplier of 1/Ave(0.265 +0.284) = 3.64, if you want to use that (along with f2, f3, and f4) and therefore wish to depart from the simplistic approach of the MCS guide requiring a generic / fixed factor irrespective of actual room temperature (anywhere between 18°C and 22°C for inhabited rooms) and actual average flow temperature, i.e fixed flow / return ΔT, and max flow temperature, then you're going to have to lock down the heat pump so that the client can't ever change the parameters, or change any of the lockshields (which would affect the radiator flow ΔT). (also of course assumes that the radiators actually perform to those factor at those flows).

You can do this of course - the MCS document allows you to, so long as you document everything, including the commissioning process.

Get ready for a long discussion at your next MCS assessment, - we had one NC last year that when we submitted the same evidence to the head office as we offered at the inspection was accepted and the NC removed - we had to refer them back to the Building Regulations rather that the Approved Document, which they insisted on using at the assessment :)

That's why the MCS design parameters and RHI use generic multipliers - 99% of people actually on site doing the installations don't even know what the design flow / return ΔT for the system that they are installing is... MCS is designed for the lowest common denominator, as under the MCS scheme none of the 'operatives' doing the actual install need to have any qualifications whatsoever.

Proof of the pudding - we spent 4 months last summer going round fixing so many faults on other parties installations, that we have seen almost everything that could be done wrong, from plumbing return flows that should go to the master going to the slave and vice versa (cascade system), return sensors wired up as flow sensors, undersized circulation pumps, radiator heating thermostats placed in an area heated by underfloor heating, the 'front' of the heat pump pointing straight at a vertical wall, so the airflow just goes straight round the back of the heatpump, with no fresh air circulation ... the list is endless.
 
OP
N

Nostrum

Plumber
Gas Engineer
I knew someone had been following me around. Sixth sense.

This is the reason for the original post. I will email MCS in the morning for their opinion.

The sticking point will be on the MCS compliance certificate which asks for both the flow temp and oversize factor if I remember. Last thing I want is for it to be refused 2 months down the line when someone takes a look.
 
OP
N

Nostrum

Plumber
Gas Engineer
Forgot to reply to this.

The reply I got from MCS was that the flow temperature is what governs the RHI. However they did state that this may result in a non compliance and that my MCS certification body may pick it up..

Very unlikely seeing as they were the ones who suggested I phoned MCS for guidance.
 
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