Need help with Radiators cold at Bottom - not sludge

[wedge1988]

Yes that calculation above is correct, I think the 12.86LPM is a good choice as well.
what is the norm for the UK in choosing the lowest ambient temp, do you size for say from -5C outside temperature or what??. is there a standard figure?.

We can get lower than -5 but i think generally where I live anyway (Midlands) it gets on cold months about -3. I think the default curve on the boiler is also set to -7 but would need to read up on that

Have they? I may have missed something in the original posts, wouldn't be the first time

heads back to the OP - DOH!!

(edit) I think this was the part that confused me - "I have set the Boiler to 100% for (3) - 23~24~kw and the rads are all summing up to about 22kw" - as this boiler can't be set to 100% - max is 85% on the 28/24

Yep, they're rather large.. Mostly T22 and5/6 T21.

Appears i can get them down to T20 though so i'm buzzing about that - longer warmup times granted but by any means not a problem.

Sorry i'll clarify that point, i rated the rads to 23.63kw in actuality. T50

When looking before renovating i converted to T30 - https://www.radiatorsdirect.co.uk/delta-t-conversion

So my theory was this.. Room needs 100BTU at T50 then find a radiator that outputs 100BTU at T30

Turns out i can reduce to T20 as the rads are a bit bigger :'-) LOL

Thanks chaps i will have a play with the calcs and the spreadsheet and will keep you updated with my progress. ANy more input is welcome

legends

 

[Johntheo5]

We can get lower than -5 but i think generally where I live anyway (Midlands) it gets on cold months about -3. I think the default curve on the boiler is also set to -7 but would need to read up on that

So presumably you have weather compensation, WC, can you post that default curve + any other available curves.

So my theory was this.. Room needs 100BTU at T50 then find a radiator that outputs 100BTU at T30

Correct

Turns out i can reduce to T20 as the rads are a bit bigger :'-) LOL

A bit bigger?, should, theoretically, be oversized by a factor of 100/51.48, 1.94, say 2. (if you want to run them as T30s)
What is the actual heat demand in kw (approx) at say -5C or whatever?.

Thanks chaps i will have a play with the calcs and the spreadsheet and will keep you updated with my progress. ANy more input is welcome

legends

 

[Johntheo5]

Just did a few numbers there using the factory set WC where a OT of -7C to 25C = Boiler Target temp of 80C to 25C.
Assuming your rads are T50 sized to 22kw then you will still get 16.25kw at a OT of 0C and 11.23kw at a OT of 7C (fairly typical OT temp around here) assuming the room temperature of 21C is achieved.
It states you can change the WC parameters in 10.3 but I read somewhere else that this is only possible on some Intergas models.

Edit: at -5C OT this curve will give 20kw at a target temp of 76.6C, might be enough??.

 

[wedge1988]

So presumably you have weather compensation, WC, can you post that default curve + any other available curves.

Correct

A bit bigger?, should, theoretically, be oversized by a factor of 100/51.48, 1.94, say 2. (if you want to run them as T30s)
What is the actual heat demand in kw (approx) at say -5C or whatever?.

nope, afaik no WC, although that would be great. The setting (6) is Min. outside temperature of the heat curve which is -7, suspect that this is then related to WC

A bit bigger?, should, theoretically, be oversized by a factor of 100/51.48, 1.94, say 2. (if you want to run them as T30s)
What is the actual heat demand in kw (approx) at say -5C or whatever?.

Yep, i added 10% BTU to them as well for wiggleroom at T50.. actual heat demand at -5, not sure ho to calculate this

I used chat GPT to help wuth the u loss /W and those are below based on 21 degrees inside and -5 outside.. (Avg U value is 0.0147 W/m²·K)

Regards

--------

recalculate the weighted average U-value and heat loss for your building.

Given the updated U-values and areas:

• Ufront (50mm phenolic insulation) = 0.018 W/m²·K
• Uback (125mm rockwool insulation) = 0.044 W/m²·K
• Area of the front half (Area_front) = 1463.89 sq ft ≈ 136 sq m
• Area of the back half (Area_back) = 645.84 sq ft ≈ 60 sq m
• Total Area (A) = 1,700 sq ft ≈ 158 sq m
• Temperature difference (ΔT) = 26°C

Calculate the updated weighted average U-value (U_average) as follows:

U_average = (Ufront * Area_front + Uback * Area_back) / Total AreaU_average = (0.018 W/m²·K * 136 sq m + 0.044 W/m²·K * 60 sq m) / 158 sq m

Now, calculate the U_average:

U_average = (2.448 W + 2.64 W) / 158 sq mU_average = 5.088 W / 158 sq mU_average ≈ 0.0321 W/m²·K

Now, you can calculate the heat loss (Qloss) using the updated U_average:

Qloss = U_average * A * ΔTQloss = 0.0321 W/m²·K * 158 sq m * 26°C

Calculate the updated heat loss:

Qloss = 134.1368 W

 

[Johntheo5]

OK but I'm still not sure as to the total heat demand at -5C.

Is it 22kw??, if so, and you wish to run the rads as T30s then the rads should have been sized to ~ 44kw at the T50 rating to give you 22kw at T30?.

Your calcs, above, might indicate a heat demand of , 158*134.14/1000, 21.2kw?.

 

[wedge1988]

I think 50 degrees on the boiler should be more than adequate, it's set to 50 at the moment and within an hour the house is WARM

Also means as temps drop it has a bit more umph, 45 might be a bit on the low end.

It can get to a stale 7 degrees ish here in the UK in the house if* we dont use the heating.. even if 0 or negative outside.. I'm hoping that the u value of the house is going to help a lot too

 

[wedge1988]

OK but I'm still not sure as to the total heat demand at -5C.

Is it 22kw??, if so, and you wish to run the rads as T30s then the rads should have been sized to ~ 44kw at the T50 rating to give you 22kw at T30?.

Your calcs, above, might indicate a heat demand of , 158*134.14/1000, 21.2kw?.

Ah i see

Well i based the heat demand on BTU. I sized the rads at T30 based on the BTU needed for each room, which converted to kw.. see table below

Name	KW dt50	KW dt30	dt50 BTU output	dt30 BTU output	Required BTU	Total BTU
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 600MM WHITE	1.025207​	0.52778​	3499​	1801.2852​	8920​	9007.456​
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 1200MM WHITE	2.050414​	1.05555​	6998​	3602.5704​
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 1200MM WHITE	2.051​	1.05585​	7000​	3603.6​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 700MM X 700MM WHITE	1.063883​	0.54769​	3631​	1869.2388​	1661​	1869.239​
Capri - Anthracite Towel Radiator 1420 x 500	0.610905​	0.31449​	2085​	1073.358​	1047​	1073.358​
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 500 X 1200MM WHITE	1.765618​	0.90894​	6026​	3102.1848​	2994​	3102.185​
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 1200MM WHITE	2.050414​	1.05555​	6998​	3602.5704​	2108​	3602.57​
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 500 X 1400MM WHITE	2.05979​	1.06038​	7030​	3619.044​	5387​	5428.566​
FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 500 X 700MM WHITE	1.029895​	0.53019​	3515​	1809.522​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500MM X 1000MM WHITE	1.14856​	0.59128​	3920​	2018.016​	1945​	2018.016​
Milano Arno - White Bar on Bar Heated Towel Rail - 600 X 1738	1.074724​	0.55327​	3668​	1888.2864​	1828​	1888.286​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500MM X 1400MM WHITE	1.608277​	0.82794​	5489​	2825.7372​	2456​	2825.737​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 600 X 400MM WHITE	0.534432​	0.27513​	1824​	938.9952​	779​	938.9952​
Milano Bow - Anthracite D Bar Central Connection Heated Towel Rail 1533mm x 500mm	1.193389​	0.61436​	4073​	2096.7804​	1947​	2096.78​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500MM X 1400MM WHITE	1.608277​	0.82794​	5489​	2825.7372​	2456​	2825.737​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500 X 1200MM WHITE	1.378565​	0.70969​	4705​	2422.134​	2159​	2422.134​
FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500 X 1200MM WHITE	1.378565​	0.70969​	4705​	2422.134​	2011​	2422.134​

 

[wedge1988]

DT30 KW requirements: 12.1657kw

 

[Johntheo5]

I won't labour the point but the above implies that the total heat requirement to heat your house to 21C from a OT of -5C requires 12.17kw?? even though your calculations show a requirement of, 158*134.14/1000, 21.2kw? so how do the rads running as T30s supply this 21.2kw??.

Its either one or the other, either the house requires 12.17kw or it requires 21.2kw to heat it to 21C rom a OT of -5C, which is it??.

 

[wedge1988]

Please remember i'm not a heating engineer so not all of this makes total sense at the moment

so 158sqm * the energy loss (134.14watt) based on chat gpt which isnt 100% acurate but will be aroundabout, /100 is what you said above

Confusion here is this; my calcs to get the heating requirement were based on the BTU of the rooms at T50.

Lets say that the lounge needs 100BTU. So i have sized the rads to output 100BTU at DT30

So based on that assumption, the house needs 12.17kw

Sorry if i am being a bit here and there i need to get this right in my head

 

[Johntheo5]

No problem, forget about T50 or T30 for the moment, you do/did a calculation on the heat required to heat your house from -5C OT, you came up with ~ 22kw, you could supply that heat with all electric heating or UFH or, in your case rads, it doesn,t matter because 22kw must be supplied. All rad manufacturers now use this T50 standard when sizing their rads and the rads you have installed have been based on this because the sum of their outputs is ~ 22kw, if you now decide to run the rads at reduced temperatures to become T30 rads then they will only output 51.48% of what they would at T50 conditions, 22X51.48%, 11.33kw. You have just converted the T50 outputs to T30s which is mathematically correct but doessn't change in any way the heating requirements, you will be able to heat the house to 21C if running with T30s once the OT rises but you don't have to rigidly stick to the T30, you just increase/decrease the boiler flowtemp to suit the conditions, this is what WC is doing in effect, its changing the boiler flow temo so the rad outputs as a T50 or T45 or Twhatever to give you your required room temperature.

 

[desktop987456]

once the OT rises but you don't have to rigidly stick to the T30, you just increase/decrease the boiler flowtemp to suit the conditions, this is what WC is doing in effect, its changing the boiler flow temo so the rad outputs as a T50 or T45 or Twhatever to give you your required room temperature.

So theoretically you are saying at 5degC the boiler flow shold be 59.4 as per the Excel table of OT above to heat the house effectively, correct?
I did something similar but I agree with John that boiler flow needs to be adjusted as per outside Temperature. I keep mixing OT open therm with outside temperature. Outdoor reset is a US term for WC.
PM sent to John

 

[Johntheo5]

Read your PM so I'll answer it here.
re table, above, yes, thats what the Intergas chart shows, there is a linear relationship between heat loss and heat demand (I'm a bit skeptical about this but this is what the gurus tell us). the heat loss may be designed for a 20C room temperature for a minimum ot of -5C, a 25C dT and you size your (T50) rads for this, now suppose the OT rises to 7.5C, then the heat demand is 50% but a T25 rad will not provide this because rad output has this 1.3 exponent so a "T29.3 rad (say T30) is required and that's why WC use (or should use) curves, the intergas one looks linear.

Your heat demand is 18kw so based on a -5C to 20C basis, your query re ~ 63% output, 11.34kw, (a T36 rad) should, theoretically mean that the OT has risen to 4.5C.

 

[desktop987456]

Read your PM so I'll answer it here.
Your heat demand is 18kw so based on a -5C to 20C basis, your query re ~ 63% output, 11.34kw, (a T36 rad) should, theoretically mean that the OT has risen to 4.5C.

Thanks John, Yes that's correct 11.34 is what I require based on my designed temperature is 5deg C outside temperature so my dT would be 15degC (assuming room temp of 20degC). or What flow temperature should I be running on in my scenario?

 

[Johntheo5]

Thanks John, Yes that's correct 11.34 is what I require based on my designed temperature is 5deg C outside temperature so my dT would be 15degC (assuming room temp of 20degC). or What flow temperature should I be running on in my scenario?

Don't know what the boiler output is but assuming 20kw then the flowrate to give a dT of 20C will be 14.33LPM, based on this then you will require a flowtemp of 63C.