Rads - your rules of thumb

[Diggory]

I’m looking to replace my rads; preferring where possible to use a type 22 convectors.

I’ve got my heat output requirements and am now looking at size and fit in rad locations. What is good practice to allow extra for pipes and TRVs. My rads are all drop feed on 15mm copper.

Thanks D

 

[Madrab]

What is good practice to allow extra for pipes and TRVs.

Are you talking about the rad outputs? If so, always go over spec, never under, is all I'd suggest

 

[Diggory]

Thanks reply @Madrab.
I was meaning extra mm.

One of my window cills is at about 600mm.
I was trying to work out if a 450mm H rad will be okay. I’m unsure how much I need to allow for pipe work and TRVs.

Thanks D

 

[Madrab]

Rad with standard tails compression tails and then out to the valve/pipe centres can be anywhere from 45mm to 70mm

 

[Diggory]

Thank you - sounds like 450mm should be okay under a 600mm cill.

D

 

[Madrab]

As far as height is concerned then as long as the rads output is suitable then the height won't make much of a difference and would be purely driven by aesthetics and how you want it to look.

Length of curtains can be a driving factor with that one.

 

[endecotp]

The radiator manufacturers often specify a minimum distance from the floor; for example, this Stelrad diagram shows 150mm:

Presumably the issue is that without enough space below there is reduced incoming airflow for the convection to work, so you don’t get the expected output.

If e.g. the window cill height makes this difficult, it has never been clear to me whether it would be better to use a smaller radiator with the required gap, or a larger radiator without the required gap. Thoughts anyone?

 

[Diggory]

Presumably the issue is that without enough space below there is reduced incoming airflow for the convection to work, so you don’t get the expected output

@endecotp - yes.
By coincidence, I’m looking at STELRAD too.
So that diagram very useful - thanks.

450+150=600 cill height, so I’m on limit.
I might have to put wooden batten spacers behind brackets as the cill might make access to hang on bracket tricky (as I won’t have a gap).

D

 

[Diggory]

Are you talking about the rad outputs? If so, always go over spec, never under, is all I'd suggest

I’ve since found that the 300mm high STELRADs I’m looking at, increase in length by big increments of 500mm.
So I’m either a tad under spec or significantly over spec output wise.
Noting earlier chat / advice - how much latitude do I have on being over spec.

Thanks D

 

[Diggory]

Having searched forum for ‘oversized radiator’ threads.
There seems to be advice that oversizing of the entire (combi boiler) system can be beneficial.
As I’m changing all my rads - I can do this / fit TRVs

If I go through my BTU numbers (on-line tools) and err on side of caution…….
- Assume solid walls where unsure about ‘cavity’
- Assume single glazed where old secondary glazing fitted
- Add 10% rooms North facing

What then would be sensible oversize factor to add ?

Thanks D

 

[Diggory]

For system context - brand new boiler; ecoFIT pure 825 combi; 25 kW / 85,000 BTU with two zones:

- Z1 17sqm UFH / 16mm pipe under LVT (installed)
- Z2 x9 rads all being replaced

D

 

[Madrab]

'Oversizing' the system rads can allow the system to be run cooler, whilst still satisfying the rooms heat loss requirements, that obviously reduces energy costs with lower deltas and more consistent condensing and reduces the strain on the boiler. Its how low temp heat pump systems are primarily designed alongside UFH.

The actual size of the rads required are again down to the deltas. Each radiator will have a specific output at a given delta and that coupled with the heat loss cals allows. So the rads aren't actually oversized, they are just sized to align with what temp that the system will be run at and what they room needs in the way of heat output

Have a look @ radiator deltas.

 

[gas112]

@endecotp - yes.
By coincidence, I’m looking at STELRAD too.


D

if your looking at stelrad then have a look at wickes radiators these are made by stelrad and far cheaper