New radiators - totally baffled!

A1ps

Hello All,

I'm going to be buying 6 new radiators and have gone in to several sites to do the BTU calculations. Thing is, each and every single website delivered very different calculations. 

An example - my bedroom in metres: 

Length: 5
Width: 4
Height: 2.3
Window area: 2.34 m2 double glazed
Heated rooms above and below
2 outside walls
Insulated timber frame walls

The results I got for BTU were: 4881, 7789, 2054, 4408, 2570 and...... 56,611

Why could this be happening (and yes, I did put the same figures in each time and in metres ) 

My current radiator in the bedroom is 600 x 900, but it definitely does not provide enough heat for the size of room. Unfortunately, I don't know the BTU figure of this rad. 

I'm going to be buying from Stelrad as these rads have a good reputation and currently have a sale on . 

The Stelrad BTU calculator is giving me an ideal BTU figure of 2824 (827w). But the rads that give this output are by far and way smaller in size than my current radiator.

To get the correct Stelrad BTU radiator I'd need to go for either a 600 x 700 OR a 450 x900 (Double Panel, Single Convector). Width wise, these would be way too small, and would look a  it odd.

If I went for my current sizing 600 x 900, the BTU output would be 4137 (1212w). BTU calculator says max of 2824.

https://www.stelrad.com/radiators/standard-steel-radiators/classic-compact/?attribute_pa_height-mm=600&attribute_pa_length-mm=1100&attribute_pa_model=p&attribute_pa_colour=ral-9016-traffic-white&attribute_pa_installation=hot-water

Would going for a higher BTU be super expensive to run? I have a Tado heating system so can control when the radiator switch off, if that makes a difference.

The loft has 2 radiators as the room is 8m x 5m, window area 2m2, and that's an even bigger confusion! And if fitting larger rads with higher BTU will affect energy use, then I'd most likely be skint 

These sites give a breakdown of the BTU and Watts

https://www.stelrad.com/radiators/standard-steel-radiators/classic-compact/

Stelrad Softline Compact P+ Double Panel Plus Radiator | 16 Sizes

Buy the Stelrad Softline Compact P+ Double Panel Plus Radiator today. Choose from 16 different size options. Free delivery to most UK addresses in approx 2 working days.

 www.heatandplumb.com

Thank you 

FreeBear

I'll often recommend the Stelrad heat loss calculator - It often suggests a higher rated radiator than many of the other sites you might come across. I had a recommendation for a ~900W radiator for my bedroom(s), but I fitted some rated at ~1350W at a flow temperature of 70°C (Delta T 50°C). This means I can run at a lower flow temperature and still heat the room effectively. This is important with a condensing boiler, as the lower the return temperature, the higher the efficiency. And having a low flow temperature means less work involved when a heat pump is fitted.

Suggestion - If the recommendation is for a 600x900 T21, go for a 600x1000 T21 - No real increase in price, but you get a little bit more heat out (just 150W or so). As long as you fit TRVs and balance the system properly, you won't increase the gas consumption. If anything, you'll be able to turn down the flow temperature and gain a little on efficiency. Even better if you can oversize all the radiators by 30-50%. But to get the best from low flow temperatures, you need decent size pipes. 22mm or even 28mm for flow & return, with short runs of 15mm to feed each radiator (micro-bore doesn't hack it).

Albermarle

As above - Go Large !

They will work quicker when the weather is really cold, and otherwise just turn the TRV down. Plus running with a lower average boiler flow temperature will reduce overall gas use.

ThisIsWeird

A1ps said:

I'm going to be buying 6 new radiators and have gone in to several sites to do the BTU calculations. Thing is, each and every single website delivered very different calculations.

my bedroom in metres:
Length: 5 Width: 4 Height: 2.3

My current radiator in the bedroom is 600 x 900, but it definitely does not provide enough heat for the size of room. Unfortunately, I don't know the BTU figure of this rad.

https://www.stelrad.com/radiators/standard-steel-radiators/classic-compact/?attribute_pa_height-mm=600&attribute_pa_length-mm=1100&attribute_pa_model=p&attribute_pa_colour=ral-9016-traffic-white&attribute_pa_installation=hot-water

Would going for a higher BTU be super expensive to run? I have a Tado heating system so can control when the radiator switch off, if that makes a difference.
The loft has 2 radiators as the room is 8m x 5m, window area 2m2, and that's an even bigger confusion! And if fitting larger rads with higher BTU will affect energy use, then I'd most likely be skint

Your bedroom. Could you post a photo of the current rad, and also tell us whether it heats up well - does it become 'hot' when required? We can probably give you a good idea of its effective output from that info.

Then you say it's definitely not enough output for that room, so we can probably give you a rough ballpark for just how much more powerful it needs to be to heat that room, depending on how 'close' it's currently getting to be 'ok'. 'Twice', 'thrice'?

And then we can compare that figure against calculators :-)

And then there's the advice given above to 'oversize' the rads so that they'll provide the same heat output as 'smaller' rads, but when running on cooler water. Say a rad gives out 1kW at 70oC water flow (hot), then a physically larger rad will also give out the required 1kW but with a water flow temp of, say, 60oC, or even less. Why does this matter? Because a condensing gas boiler relies on a cool return flow temp for it to 'condense', and extract most of the heat from the burner. And, should you go 'HeatPump' in the future, they also work at their most efficient if they deliver 'cooler' water. So, oversizing is good - it saves energy.

Will an oversized rad be too powerful, or use more gas? No, because you are hopefully going to run it on cooler water - so the same amount of heat will be emitted - and you can easily control the 'size' of a rad in any case by tweaking its lockshield.

This Tado - do you have Smart TRVs?

A1ps

FreeBear said:

I'll often recommend the Stelrad heat loss calculator - It often suggests a higher rated radiator than many of the other sites you might come across. I had a recommendation for a ~900W radiator for my bedroom(s), but I fitted some rated at ~1350W at a flow temperature of 70°C (Delta T 50°C). This means I can run at a lower flow temperature and still heat the room effectively. This is important with a condensing boiler, as the lower the return temperature, the higher the efficiency. And having a low flow temperature means less work involved when a heat pump is fitted.

Suggestion - If the recommendation is for a 600x900 T21, go for a 600x1000 T21 - No real increase in price, but you get a little bit more heat out (just 150W or so). As long as you fit TRVs and balance the system properly, you won't increase the gas consumption. If anything, you'll be able to turn down the flow temperature and gain a little on efficiency. Even better if you can oversize all the radiators by 30-50%. But to get the best from low flow temperatures, you need decent size pipes. 22mm or even 28mm for flow & return, with short runs of 15mm to feed each radiator (micro-bore doesn't hack it).

Hi all, thank you so much for the very helpful replies.

I forgot to mention in my first post that I'm running a mega-flow for my water, and a system boiler for the heat.

@F@FreeBear , Stalrad calculations are stating I need a 2824 BTU (827w) radiator. Would going for a 4597 BTU (1347w) radiator be within the oversized suggestion you mentioned above? Or is that over kill?

The 600 x 900 radiator comes in 4137 (1212w). I'm wondering whether a larger rad by the extra 100 would make a difference?

I currently have Tado TRVs, which are great as they're in all the rooms and hallway, and are set to individual temperatures, and switch off when each radiator hits the set temperature. But I have a feeling I will need larger pipes. Would these need to go all the way back to the boiler? Or is it just a case of somehow make the existing pipes going to the radiator larger?

Albermarle said:

As above - Go Large !

They will work quicker when the weather is really cold, and otherwise just turn the TRV down. Plus running with a lower average boiler flow temperature will reduce overall gas use.

@Albermarle as in my reply above, I have Tado TRVs, so if I set them at 21 degrees in winter, would the larger  rads use the same or possibly less gas? My thinking is, they'll reach temperature quicker, and therefore switch off quicker? Sorry if this is not right and silly. I really don't have much knowledge of this and off stuff.

ThisIsWeird said:

A1ps said:

I'm going to be buying 6 new radiators and have gone in to several sites to do the BTU calculations. Thing is, each and every single website delivered very different calculations.

my bedroom in metres:
Length: 5 Width: 4 Height: 2.3

My current radiator in the bedroom is 600 x 900, but it definitely does not provide enough heat for the size of room. Unfortunately, I don't know the BTU figure of this rad.

https://www.stelrad.com/radiators/standard-steel-radiators/classic-compact/?attribute_pa_height-mm=600&attribute_pa_length-mm=1100&attribute_pa_model=p&attribute_pa_colour=ral-9016-traffic-white&attribute_pa_installation=hot-water

Would going for a higher BTU be super expensive to run? I have a Tado heating system so can control when the radiator switch off, if that makes a difference.
The loft has 2 radiators as the room is 8m x 5m, window area 2m2, and that's an even bigger confusion! And if fitting larger rads with higher BTU will affect energy use, then I'd most likely be skint

Your bedroom. Could you post a photo of the current rad, and also tell us whether it heats up well - does it become 'hot' when required? We can probably give you a good idea of its effective output from that info.

Then you say it's definitely not enough output for that room, so we can probably give you a rough ballpark for just how much more powerful it needs to be to heat that room, depending on how 'close' it's currently getting to be 'ok'. 'Twice', 'thrice'?

And then we can compare that figure against calculators :-)

And then there's the advice given above to 'oversize' the rads so that they'll provide the same heat output as 'smaller' rads, but when running on cooler water. Say a rad gives out 1kW at 70oC water flow (hot), then a physically larger rad will also give out the required 1kW but with a water flow temp of, say, 60oC, or even less. Why does this matter? Because a condensing gas boiler relies on a cool return flow temp for it to 'condense', and extract most of the heat from the burner. And, should you go 'HeatPump' in the future, they also work at their most efficient if they deliver 'cooler' water. So, oversizing is good - it saves energy.

Will an oversized rad be too powerful, or use more gas? No, because you are hopefully going to run it on cooler water - so the same amount of heat will be emitted - and you can easily control the 'size' of a rad in any case by tweaking its lockshield.

This Tado - do you have Smart TRVs?

@ThisIsWeird thank you for the useful information.

My current radiator is OK but not great. My bedroom tends to be cold due to poor isulation and being north facing. I've posted a picture below. Please forgive the messy looking windows. A very very very long a,d infuriating story  

I have a Valiant 837 EcoTec Plus boiler. Is this a condensing gas boiler? Again, sorry if I sound silly. I'm on a severe learning curve   

And I have Tado TRVs in each room, which switch off when that particular rad reaches the desired temperature.

BarelySentientAI

A1ps said:

 I have Tado TRVs, so if I set them at 21 degrees in winter, would the larger  rads use the same or possibly less gas? My thinking is, they'll reach temperature quicker, and therefore switch off quicker? Sorry if this is not right and silly. I really don't have much knowledge of this and off stuff.

The same amount of heat in total goes into your room.  The same amount of gas is needed to burn to make that heat.

Close enough for government work anyway.

What matters, as TIW mentioned, is the low return temperature that allows the boiler to condense (and yours is condensing).  That's easier with bigger radiators.

Can always pretend the radiator is smaller by changing the lockshield valve setting.  Can't suddenly make them seem bigger very easily.

A1ps

BarelySentientAI said:

A1ps said:

 I have Tado TRVs, so if I set them at 21 degrees in winter, would the larger  rads use the same or possibly less gas? My thinking is, they'll reach temperature quicker, and therefore switch off quicker? Sorry if this is not right and silly. I really don't have much knowledge of this and off stuff.

The same amount of heat in total goes into your room.  The same amount of gas is needed to burn to make that heat.

Close enough for government work anyway.

What matters, as TIW mentioned, is the low return temperature that allows the boiler to condense (and yours is condensing).  That's easier with bigger radiators.

Can always pretend the radiator is smaller by changing the lockshield valve setting.  Can't suddenly make them seem bigger very easily.

Thanks BarelySentientAI.

So, a 600 x 1000 would be the ideal size as apposed to 600 x 1100?

If memory serves me well, the low return temp is something that's controlled on the boiler. If so, I think mine is currently set at 55.

Can you explain the lock shield valve scenario please?

And "Close enough for government work anyway."???

FreeBear

A1ps said:

FreeBear said:

I'll often recommend the Stelrad heat loss calculator - It often suggests a higher rated radiator than many of the other sites you might come across. I had a recommendation for a ~900W radiator for my bedroom(s), but I fitted some rated at ~1350W at a flow temperature of 70°C (Delta T 50°C). This means I can run at a lower flow temperature and still heat the room effectively. This is important with a condensing boiler, as the lower the return temperature, the higher the efficiency. And having a low flow temperature means less work involved when a heat pump is fitted.

Suggestion - If the recommendation is for a 600x900 T21, go for a 600x1000 T21 - No real increase in price, but you get a little bit more heat out (just 150W or so). As long as you fit TRVs and balance the system properly, you won't increase the gas consumption. If anything, you'll be able to turn down the flow temperature and gain a little on efficiency. Even better if you can oversize all the radiators by 30-50%. But to get the best from low flow temperatures, you need decent size pipes. 22mm or even 28mm for flow & return, with short runs of 15mm to feed each radiator (micro-bore doesn't hack it).

I forgot to mention in my first post that I'm running a mega-flow for my water, and a system boiler for the heat.

@F@FreeBear , Stalrad calculations are stating I need a 2824 BTU (827w) radiator. Would going for a 4597 BTU (1347w) radiator be within the oversized suggestion you mentioned above? Or is that over kill?

The 600 x 900 radiator comes in 4137 (1212w). I'm wondering whether a larger rad by the extra 100 would make a difference?

How you generate hot water has no effect on central heating - The zone valves effectively make it two separate systems sharing the same heat source.

Going from an 827W  to a 1347W radiator is some 60% oversized. A bit over the top depending on flow temperature - A 600x700 T21 would be sufficient with a flow temp of 70°C.. With a flow temperature of 55°C, you need to be increasing the size of the radiator to get 827W out by about 63% (unless you had run the Stelrad calculator with a Delta T of 35°C).

ThisIsWeird

A1ps said:
I forgot to mention in my first post that I'm running a mega-flow for my water, and a system boiler for the heat.
I currently have Tado TRVs, which are great as they're in all the rooms and hallway, and are set to individual temperatures, and switch off when each radiator hits the set temperature. But I have a feeling I will need larger pipes. Would these need to go all the way back to the boiler? Or is it just a case of somehow make the existing pipes going to the radiator larger?

@ThisIsWeird thank you for the useful information.

My current radiator is OK but not great. My bedroom tends to be cold due to poor isulation and being north facing. I've posted a picture below. Please forgive the messy looking windows. A very very very long a,d infuriating story  

I have a Valiant 837 EcoTec Plus boiler. Is this a condensing gas boiler? Again, sorry if I sound silly. I'm on a severe learning curve   

And I have Tado TRVs in each room, which switch off when that particular rad reaches the desired temperature.

That's a double panelled radiator. Looking down inside it, does it have 'fins'? If so, one or two sets?
How wide is it?

BarelySentientAI

A1ps said:

And "Close enough for government work anyway."???

There's a very slight difference accounting for the relative heat loss through the walls depending on the temperature gradient at different times, heat used to heat the pipes as they pass through the floor/walls of other rooms etc.  It just isn't enough to be worth thinking about in your case.

A1ps said:

Can you explain the lock shield valve scenario please?

There's a valve on the other end of the radiator from your TRVs, probably hidden under a plastic cap.  That's called a lockshield valve and what it does is control how much water can flow through your radiator - but not 'live', so it doesn't turn on and off like the TRV.  If you turn down the flow a little bit, so there's less water going through, then it's like having a slightly smaller radiator on full flow.  You also use these valves for balancing the system (usually for making sure that each radiator heats up at the same speed).

A1ps said:

If memory serves me well, the low return temp is something that's controlled on the boiler. If so, I think mine is currently set at 55.

It's normally flow (water to the radiators) temperature that's set on a boiler, not return (water coming back).  It's possible to measure return though, so perhaps yours does.  I haven't checked the manual.

A1ps

FreeBear said:

A1ps said:

FreeBear said:

I'll often recommend the Stelrad heat loss calculator - It often suggests a higher rated radiator than many of the other sites you might come across. I had a recommendation for a ~900W radiator for my bedroom(s), but I fitted some rated at ~1350W at a flow temperature of 70°C (Delta T 50°C). This means I can run at a lower flow temperature and still heat the room effectively. This is important with a condensing boiler, as the lower the return temperature, the higher the efficiency. And having a low flow temperature means less work involved when a heat pump is fitted.

Suggestion - If the recommendation is for a 600x900 T21, go for a 600x1000 T21 - No real increase in price, but you get a little bit more heat out (just 150W or so). As long as you fit TRVs and balance the system properly, you won't increase the gas consumption. If anything, you'll be able to turn down the flow temperature and gain a little on efficiency. Even better if you can oversize all the radiators by 30-50%. But to get the best from low flow temperatures, you need decent size pipes. 22mm or even 28mm for flow & return, with short runs of 15mm to feed each radiator (micro-bore doesn't hack it).

I forgot to mention in my first post that I'm running a mega-flow for my water, and a system boiler for the heat.

@F@FreeBear , Stalrad calculations are stating I need a 2824 BTU (827w) radiator. Would going for a 4597 BTU (1347w) radiator be within the oversized suggestion you mentioned above? Or is that over kill?

The 600 x 900 radiator comes in 4137 (1212w). I'm wondering whether a larger rad by the extra 100 would make a difference?

How you generate hot water has no effect on central heating - The zone valves effectively make it two separate systems sharing the same heat source.

Going from an 827W  to a 1347W radiator is some 60% oversized. A bit over the top depending on flow temperature - A 600x700 T21 would be sufficient with a flow temp of 70°C.. With a flow temperature of 55°C, you need to be increasing the size of the radiator to get 827W out by about 63% (unless you had run the Stelrad calculator with a Delta T of 35°C).

Thank you. Would I then be better of sticking to a 600 x 900 which is 1212 watts and bTU 4137 (no need to move piping also)?

Likewise, stick to 600 x 900 for my loft room, which is 8m x 5m and has 2 rads either side of the room?