BTU CALCULATIONS - IS THIS A CAVITY

Hi there - not sure if this is a heating or building forum question.

I’m looking to replace some old rads (circa 50yrs) and I’ve been using a couple of on-line BTU calculators that provide broadly similar results.
My question is about how I describe my outside walls in those calculators. My bungalow has been altered over the years - one wall (the oldest part) has an overall thickness of 240mm and another wall (added 1957) has an overall thickness is 270.
I’ve seen inside the 240mm wall and there was a very basic cavity (more of a gap really) of maybe 25mm.
I’ve never seen inside the 270mm wall but I’m guessing there’s maybe 50mm or so.

My question is; for the purpose of BTU calculators, should I describe these walls as ‘BRICK CAVITY’ or a ‘SOILD WALL 200mm’ ?

Thanks D

Since it breaks the continuity of the brickwork (assuming no snots or headers) and hence stops heat conducting directly across the wall from the inside to the outside I would consider it to be an air gap and apply a standard 0.18 R value

Since it breaks the continuity of the brickwork (assuming no snots or headers) and hence stops heat conducting directly across the wall from the inside to the outside I would consider it to be an air gap and apply a standard 0.18 R value

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Thanks reply.
As regards breaching the gap - on the narrower wall; in places, there was a fair bit of debris in the gap.
Would it be sensible to model the narrower 240mm wall as a ‘SOILD WALL 200mm’
And the wider 270mm wall as a ‘BRICK CAVITY’.
Best D

Well it will be detrimental to the calc so best play safe and assume it is a solid wall or you could do calc for both types of wall and take a middish value depending on how much of the gap is filled with debris

Thank you again - a midpoint seems a good idea / compromise.

Where I can’t hit the actual BTU - I was thinking I’d go for the nearest but always on the plus side.
Am I right to assume a TRVs to each rad, gives me some accuracy latitude.

D

Yes, go bigger and let the TRV do the controlling, main thing is confirm your boiler out put capable of providing necessary btus and everything else falls into place ( after you have balanced the radiators)

Thanks - it’s a brand new combi boiler fitted / sized by my plumber so should be fine.

It’s currently only running a small UFH zone in new extension.
The main house flow/return zone for conventional rads not yet connected.
I’m sourcing the new / replacement rads before power flush full commissioning.

Best D

I started to size up my radiators and it leads me to wonder if my previous rads were generously oversized.
For example, in the lounge I had two type 22 double rads that are each 300mm x 1,450mm.
I’m guessing their combined output would be well in excess of the 4,819 BTU I’m getting from the estimating tools.

The old boiler (now replaced) was a conventional / non condensing type (Potterton 60E 17.58kw).
Was this type of boiler more tolerant of oversized radiators ?

Diggory said:

The old boiler (now replaced) was a conventional / non condensing type (Potterton 60E 17.58kw).
Was this type of boiler more tolerant of oversized radiators

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Tolerant in what way? If it was oversized for the required output it would still heat the rads up until your room thermostat indicated the necessary temp had been reached and then switch off, possibly do the job faster and work less efficiently (but was only 75% in the first place)

Sorry; slightly vague question.

My limited understanding is that I need a certain temperature differential between flow and return for new boiler to ‘condense’. My old boiler wasn’t a condensing type, so did anyone care too much.

Would you agree that two type 22 double panel rads at 300mm x 1,450mm would seem generous for the ‘living room’. The estimator tools suggest 4,800BTU whereas the fitted rads must be near double that ?

Best D

Diggory said:

Would you agree that two type 22 double panel rads at 300mm x 1,450mm would seem generous for the ‘living room’. The estimator tools suggest 4,800BTU whereas the fitted rads must be near double that ?

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Taking the standard Delta T50 fig and working with Kudox radiators in theory you have around 10000BTU output, perhaps they were sized on a T30 Delta which would give you 5000BTU?

Thanks for confirming.
Maybe it was T30 - it was installed circa 40yrs ago.

I’ll crack on and buy my new replacement rads based on the (standard Delta T50 I assume) estimator tools.
I’m mindful this will lead me to replacing with smaller rads.
Anything I need to watch out for here to save dropping an expensive clanger.

Thanks D

If you are happy with your calcs go for it, I used 3 different sources for my calcs and they all differed ( its not that exact a science) so based everything on the larger fig.

Thank you very much for your advice.
Much appreciated.
D

I have questioned many times what decides the radiator size? I am an electrician, not a plumber. But a modern boiler is often rated at around 6 kW - 28 kW output, for example, so once the output is less than 6 kW it will start to use a mark/space ratio (switching off and on) we want to avoid this if possible, so I look at my house, 14 heated areas, and I will likely be using 2 or 3 at any one time, and the programmable TRV heads will be turning off rooms I am not using.

So if for example living room, hall, toilet and kitchen are being used, then I want to sink 6 kW between those 5 radiators, in fact more as they will not all be fully open.

Same overnight, again the hall as it also heats the landing, and 2 of the 5 bedrooms.

Every time the boiler turns off and on again, energy is wasted, if the boiler turns itself on and off less is wasted as if a wall thermostat turns it on and off, so we should aim as much as possible to have analogue control, a TRV opens and closes a bit at a time, I hear the motors every so often adjusting the radiators output. I have three thermostats which can turn my boiler on/off, one in the hall, one in the living room and one in the wife's bedroom, the hope is as a result the boiler will run for a reasonable fire each time it turns on. And the TRV heads will control each room.

Central heating would cost a fortune if we had all the controls we need, so it works on near enough engineering, and this time of year my boiler may fire up on the odd cold day, and no real way to stop is cycling on/off. But as we get into winter, then the bigger the radiators output the better, as then it will cycle less.

Unless in a two up, two down house, we rely on the TRV heads to keep rooms at the temperature programmed, in my case both time and temperature, but one just has to let the TRV do its job.