Sizing radiators for rooms

[Joshua Graham]

Hi, I had an installer fit an ASHP to my existing heating system. They advised i should upgrade my radiators at the time as it was about 30 years old and micro bore etc. I was a bit strapped for cash at the time of installation so I had no choice but to refuse and come back at a later date to fit new radiator system. As a result I have left my self unsure as to how to scale the radiators for the rooms and what BTU they need to be. I was hoping to run the ASHP as low as possible around 35-40 degrees to reduce long term strain on the system, so have no issue over sizing the radiators. How would I calculate the sizes? I was left with these documents and I am unsure if they are what I need to calculate this out. Also attached is a detailed floor plan of the area needing heated in case that helps. Thanks in advance for any replies, help and support offered.

Mike

 

[JohnMo]

You use the power in watts to size each room radiator, so room 1 needs 618W output from radiator.

 

Now you need to convert the radiator size from manufacturer data to your chosen flow temp. There is a calculation on here somewhere so do Google search on buildhub.

 

You need to look at the pipe sizes then as most of the micro bore may need replacing(?).

 

Did they install a buffer?

[Joshua Graham]

13 minutes ago, JohnMo said:

You use the power in watts to size each room radiator, so room 1 needs 618W output from radiator.

 

Now you need to convert the radiator size from manufacturer data to your chosen flow temp. There is a calculation on here somewhere so do Google search on buildhub.

 

You need to look at the pipe sizes then as most of the micro bore may need replacing(?).

 

Did they install a buffer?

That is great thanks. I have replaced the micro bore with a manifold system and 15mm pex pipe. There is a tank if that is what you mean by a buffer (sorry for my ignorance!) built into the valiant internal unit. Should i install an expansion vessel on the radiator loop also?

Thanks for your reply, I will lookup the calculation tool.

[Nickfromwales]

12 minutes ago, Joshua Graham said:

That is great thanks. I have replaced the micro bore with a manifold system and 15mm pex pipe. There is a tank if that is what you mean by a buffer (sorry for my ignorance!) built into the valiant internal unit. Should i install an expansion vessel on the radiator loop also?

Thanks for your reply, I will lookup the calculation tool.

The vessel inside the internal unit may just be an expansion vessel (EV). 
 

A buffer tank is a stand alone cylinder which is in addition to your EV’s and hot water cylinder.

 

Have a look at the manufacturer’s installation guidelines, it should tell you there what system volume the OEM (factory) EV can cope with, so only add more if the upsized rads increase the water volume in excess of its current capacity.

[Joshua Graham]

Gosh, I have to admit, I feel more confused then before. Thanks everyone for the input. I am probably going to have to have a serious sit-down and look into this, or if there are suggestions on who to hire to help with this?

[Joshua Graham]

I am very confused, some sources say 2.5 times the BTU (I would imagine watts would work also) for ASHP but this just sounds a bit loosey-goosey advice and could lead to cold rooms. Is this advice sound? It doesn't seem to take into account that I would like to run the system at lower temperatures.

[Joshua Graham]

Ok, I will have to take a look at the manual see what the case is.

Edited June 2 by Joshua Graham

[marshian]

On 02/06/2025 at 12:17, Joshua Graham said:

I am very confused, some sources say 2.5 times the BTU (I would imagine watts would work also) for ASHP but this just sounds a bit loosey-goosey advice and could lead to cold rooms. Is this advice sound? It doesn't seem to take into account that I would like to run the system at lower temperatures.

 

If you have a heat loss (w) required by the room then it really is simple

 

Lets take the example of the Room 1 - from the table you supplied heat requirement/loss is 612w

 

So you need a radiator that supplies 612 w at your target flow temp

 

Radiator outputs are normally stated at Delta 50 (Flow temp 75, Return Temp 65 so mean rad temp 70 Deg minus room temp 20 gives difference of 50 Deg.

 

Rad outputs by size and type can be found here

 

https://www.stelrad.ie/wp-content/uploads/2015/08/Compact-K3-Technical-Specification-Brochure.pdf

 

Taking a T22 Rad (double panel / double convector) which is 600 mm tall and 1400 mm wide it has an output of 2.425 kW (2425 w) at Delta 50

 

Correction factors

DT50 is 1

DT45 is 0.872

DT40 is 0.748

DT35 is 0.629

DT30 is 0.515

DT25 is 0.406

DT20 is 0.304

 

At Flow temp 45, Return Temp 35 so mean rad temp 40 minus room temp of 20 gives a difference of 20 Deg (or Delta 20) so the correction factor for that is multiply the original rad output by 0.304

 

2355 multiplied by 0.304 is 737 w so a little over what the heat loss required is so you could reduce the size a little but I'd go a little bigger everywhere and then tweak the flow temp back a smidge.

 

The key for me when up-sizing rads is to try and ensure all are up-sized by similar factors for all the rooms - I made that mistake myself in not thinking about whole house heat up being balanced.

 

With ASHP your rad flow temps and return temps are going to be a lot closer than the 45 / 35 but the important thing is the radiator mean temp so a rad is going to give the same output at 45/35 as it will at 42/38 or even 41/39

 

If you are looking at designer rads, vertical rads or towel rails up-size a little bit more because in my experience they are a little optimistic in terms of output and that impact gets worse as flow temps get lower

 

Highest Flow temp (central heating) that I ran my gas boiler at through Dec, Jan and Feb was 34 deg C - house is all rads (no UFH)