Radiators delta T30 and room BTU requirements

Scr55

Hi All.

Some of my convector rads require replacing because of wear and tear. I have a combi boiler (gas) for rads and hot water for now, however, I think that I will have to instal a heat pump in a few years. Therefore, I will be looking for rads that are fit for both combi boiler and heat pump. I know that BTU/Watts requirements are very important from efficiency point of view.

BTU/ Watt requirements for my office is 430 Watts (14 sqm). As far as I know the delta T30 wattage is used for assessing heating output for rads when heat pump is installed. My question is: can I buy a bit bigger convector rad (700 Watts, 600mm x 700mm type 22) for that room. I believe that room will heat up a bit quicker and because of the size of the rad the efficiency of combi boiler and heat pump should be higher beacuse returning hot water temperature will be higher?

Thanks.

10-10-20

https://www.boards.ie/discussion/2058378149/radiators-delta-t30-and-room-btu-requirements

Yes, that's it in a nutshell. That has actually been my project this weekend - replacing 600mm x 600mm rads with spare 600mm x 800mm ones, meaning that at delta-T 50 I get a jump from around 1050W to 1350W on the larger, etc.

Stelrad have a calculator which allows for a delta-T (between the Mean Water Temperature of your system and the ambient temperature of the room) to be input. Do some calcs with that.

https://www.stelrad.com/basic-heat-loss-calculator/

Other items to note at that the oversized rads should improve the efficiency of the gas boiler too as the return temps should be lower.

Calahonda52

OP,

I trust you meant this as peened by 10-10-20 Other items to note at that the oversized rads should improve the efficiency of the gas boiler too as the return temps should be lower.

John.G

A T30 rad (51.5% of a T50 rad) will result in a very poor Heat Pump COP (coefficient of performance) because even with a flow/return dT of only 5C will mean flow/return temps of 52.5C/47.5C, and a oversizing factor of 1.94.

If you aim for a "T17.21" rad (25.0% of a T50 rad) the flow/return temps required are 39.7C/34.7C which will give a very respectable COP but, a oversizing factor of 4.0.

This is why rads, generally, are a poor choice with Heat Pumps.

T30 Factor.xlsx

10-10-20

https://www.boards.ie/discussion/comment/122771708#Comment_122771708

Try that again? 😀

Scr55

https://www.boards.ie/discussion/comment/122771805#Comment_122771805

Thanks for calculations, they are very impressive!

Now I am not sure if I understood deltaT correctly. I run some tests today with my boiler: I set boiler to 55 deg. C, temperature on my panel rads and my convector rads went up to around 55 deg. C but after a few minutes went down and remained at approx. 45 deg. C. Does it mean that my delta T is 25 (45 deg.C - room temp 20 st. C = delta T 25)?

Scr55

Thanks for that link! I was using calculator from bestheating.ie and the calculations are totally different 🤔

According to bestheating.ie room requirement is 430 Watts but on Stelrad website it states 970 Watts 🤨

My current rad is 70cm x 40cm (type 22) and it is not coping with warming the room efficiently so I believe that stelrad calculations are better.

John.G

https://www.boards.ie/discussion/comment/122772877#Comment_122772877

No, the "degree" rad is the mean rad temperature minus the (required) room temp, normally taken as 20C, the mean rad temperature is the (inlet temperature+the outlet temperature)/2, so a T50 rad might have a inlet temp of 75C so would have to have a outet temp of 65C to be T50, (75+65)/2 minus 20 which is 70 - 20 or (T) 50.

Your rad above with inlet temp of 45C and assuming a outlet temp of 35C will be a (45+35)/2 minus 20, a T20 rad with a output of (20/50) to the power of 1.3, written as (20/50)^1.3, 0.3038, or 30.38% of the T50 output.

If you increased the flowrate through the rad to give a dT of 5C then the rad becomes a (45+40)/2 minus 20, a T22.5 rad with a output of (22.5/50)^1.3 x 100, 35.41% of the T50 output.

Scr55

https://www.boards.ie/discussion/comment/122772997#Comment_122772997

Thanks for explanation. It appears that with my original boiler settings of 65C inlet temp I am not even getting deltaT 50. No wonder I was struggling with heating my rooms. Would you recommend going back to 65 or higher?

I have another (probably stupid) question. How should I use my boiler for heating rooms when it is around 0-5C outside? I presume that boilers are not designed to work 24/7 and I should turn them on and off just when I need heating. One day I forgot to turn on my heating schedule and individual TRVs were calling for heating when needed. It turned out that boiler was working for 8 hours. Is it very harmful to the system?

John.G

https://www.boards.ie/discussion/comment/122773074#Comment_122773074

It dependson whether your rads were oversized in the first place which they tend to be as they may be designed for a outside temp of -5C which you rarely see here.You will still get 75% output (T40) running at 65C/55C and 63% output (T35) at 65C/45C.

I run my system for 16 hours per day, no problems, oil fired boiler.

John.G

Regarding the rads having been designed for a minimum outside air temperature (OAT) of -5C and because (we are told) that heat loss is directly proportional to the dT between the required room temp and the OAT then, the above rads would be running at 100% output (T50) at a dT of, 20- - 5, 25C so output required per (one) deg dT is, 100/25, 4%, if the OAT is say 7C then the rad output required is, (20-7)*4%, 52%, (almost exactly T30).

Your rad above, if running with flow/return,45C/40C,output,35.41% will theoretically only satisfy a dT of 35.41/4, 8.9C so will not maintain 20C room temperature at anything below a OAT of 20-8.9, 11.1C.

Calahonda52

https://www.boards.ie/discussion/comment/122771805#Comment_122771805

I fail to see how the performance of the heat emitter can impact the COP of the heat pump.

In fact the COP of the HP is improved by minimising the temperature lift between the input (source) temperature: eg air or water

and the required output temperature, which is why the performance of a HP can decline somewhat in very cold weather as the source temp is lower.

So watt am I missing here?

The math in the excel is all fine and dandy but to make sense of it the hard coded factors such as 860 60 etc. need to be explained.

https://www.grundfos.com/ie/learn/research-and-insights/coefficient-of-system-performance

micks_address

https://www.boards.ie/discussion/comment/122774196#Comment_122774196

it kinda seems that if you have a room temp sensor that says i need to be 20 degrees and you have an undersized rad the heat pump will have to run a lot more to try and achieve the room target temp and might never actually achieve it?

Calahonda52

https://www.boards.ie/discussion/comment/122774222#Comment_122774222

That is true but this statement is my beef here

A T30 rad (51.5% of a T50 rad) will result in a very poor Heat Pump COP (coefficient of performance)

The math is simple enough, you have a heat requirement and this can be delivered by small rads at say 80 degrees average temperature or big rads at 30 degrees or medium at 50 because its the surface area by the heat emitter surface temperature that determines the energy delivered

Scr55

https://www.boards.ie/discussion/comment/122773961#Comment_122773961

Thanks for that! You have just fixed my issue remotely :D

Can I ask you for a piece of advice? I need to order a new rad for my office which now is W4.3m x L3.0m x H2.4m, window area 5.4m2 (I miscalculated that last time), 1 external wall, heated rooms above and under, timber frame.

Stelrad says heat loss is 1290 Watts at dT50.

Would you recommend buying Type 33 60cm x 80cm rad of output 1868 Watts at dT50? Type 22 60cm x 100cm 1580 Watts at dT50 is probably too weak to do the job? Type 33 60 x 100cm gives 2335 Watts is also an option. I am going to keep my combi boiler at 65 deg. C. Please let me know your thoughts.

I want to be sure that my room will stay warm even when OAT is slightly below zero.

Thanks for your help!

John.G

The sums are simple enough, if you want to run the rads at a average temp of 30C, say flow/return of ~ 33C/28C, (32.95C/27.87C), room temp 20C, then you will need a mighty big rad(s) as it will only emit 13% (a T10.41 rad) of a T50 and a oversizing factor of X 7.69.

Calahonda52

https://www.boards.ie/discussion/comment/122774346#Comment_122774346

Do you still stand by this

A T30 rad (51.5% of a T50 rad) will result in a very poor Heat Pump COP (coefficient of performance)

John.G

I don't have to stand over anything, a HP running with flow/return temps of 52.5C/47.5C will certainly have a far lower COP than when running with flow/return temps of 33C/28C.

10-10-20

https://www.boards.ie/discussion/comment/122774388#Comment_122774388

https://www.boards.ie/discussion/comment/122774196#Comment_122774196

I think it's the flow rates that are the problem when you have insufficient emitter surface-area, you end up pulling the water away from the rad quicker than it has time to cool, so the return ends up much higher than your objective and COP comes right down.

10-10-20

And on that note, I was mentally toying with a situation where if a HP was fitted to a two-zone system (upstairs and downstairs) - if instead of normally having the zones separated, if the flow from the HP was looped around the downstairs, then forced back upstairs before returning to the HP, that would lengthen the path considerably. The rads would need to be opened more-so than with a standard boiler to balance the pressures better, but I wonder if that would result in a better overall COP.

Then as an adjunct thought to that, if the system was fitted with 2-way valves you could swap the flows around to prioritise upstairs over downstairs… That concept would work in my house as I'm only ever heating either one zone or the other - very rarely we heat both at the same time.

Thoughts?

10-10-20

Oh and by the way @John.G - thanks for the sums above - It's not always easy to see them worked out like that.

Calahonda52

https://www.boards.ie/discussion/comment/122774427#Comment_122774427

When you state this

A T30 rad (51.5% of a T50 rad) will result in a very poor Heat Pump COP (coefficient of performance)

I think you do because you are peddling falsehoods here that may put people off a perfect low temp/high output solution IFF properly designed.

Calahonda52

https://www.boards.ie/discussion/comment/122774437#Comment_122774437

A flow rates fcukup is poor design and does not support the generalised falsehoods posted in

A T30 rad (51.5% of a T50 rad) will result in a very poor Heat Pump COP (coefficient of performance) because even with a flow/return dT of only 5C will mean flow/return temps of 52.5C/47.5C, and a oversizing factor of 1.94.

If you aim for a "T17.21" rad (25.0% of a T50 rad) the flow/return temps required are 39.7C/34.7C which will give a very respectable COP but, a oversizing factor of 4.0.

This is why rads, generally, are a poor choice with Heat Pumps.

Calahonda52

Ps dont bother replying, have put yeah all in ignore

MOD NOTE

I've issued a minimal warning to the above poster for their progressively uncivil posts.

John.G

https://www.boards.ie/discussion/comment/122774328#Comment_122774328

The 1868W rad will emit 1290W with flow/return temps of 65C/50.2C, the 2335W rad will emit 1290W with flow/return temps of 65C/38.4C, the better choice IMO since the lower return temperature will increase the boiler efficiency.

Scr55

@John.G Thanks for your help. Issue resolved.