Central Heating - Pipe Sizing Query?

[STEVEN BANKS]

A couple of years ago I had a new central heating system installed as part of a full modernisation. Unfortunately I’ve been having problems with other aspects of the renovation so for a while I’ve been running it without a control system or TRV’s (I have a Honeywell EvoHome set ready to go but didn’t want these damaged by other work going on). Another reason is that I don’t have downstairs doors fitted yet so wouldn’t get the benefit of that extra control..

Now that I’m nearing completion, I would just like a bit of a sanity check on the plumber’s workmanship before I start fitting the control system and carpets etc.. I’m a bit concerned that some radiators feel cold lower down, I’m more accustomed to them being cold at the top if they need bled. Here are the specs;


- BAXI 28kW Combi boiler feeding 7 radiators in 3 bed terrace house with 2 receptions.

- Boiler located in kitchen.

- 10mm O/D speedfit pipes to all radiators

- I believe these all branch off 12 or 15mm speedfit pipe from the boiler (would need to lift boards to check).


Basically, I’m a bit concerned that the 10mm O/D pipe is too small and causing the cold spots lower down the radiators? Although this seems to be more of a problem downstairs than up? And could it be that they just need to be balanced properly and all will be well once TRV’s fitted?

Appreciate input from any experienced plumbers/heating engineers out there, also are there any relevant British Standards for selection of pipe sizing for heating systems you could point me to?

Thanks

Steve

 

[oldbuffer]

1. 10 mm pipe will be OK for all but the very biggest radiators, generally good for between 2 and 3 kW.
2. 15 mm pipe is generally OK for 4 to 6 kW. If you have 7 radiators, at an average of 1.5 kW your system should be carrying 10.5 kW at peak load. In this case a 15 mm "spine" of flow and return pipes would not be enough. 12 mm would be even less adequate.
3. 22 mm pipe is good for around 12 to 14 kW, and is what is generally used for the spine in all but the largest domestic systems.
4. The above figures:
4a. Are based on using copper pipe, which has a lower wall thickness, and hence large internal diameter, than plastic, so has better heat carrying capacity.
4b. Are given as ranges because the heat carrying capacity depends on a number of factors, one of which is the difference between flow and return temperatures. The bigger the difference, the bigger the heat carrying capacity. With a modern system you are looking for a small difference (typically around 11 degrees) in order for the boiler to run in condensing mode more of the time and yielding efficiency savings.
5. You don't need to wait before balancing the system. It can be done using the lock shield valves alone, with all wheel head valves / TRVs fully open.

 

[Gasguru]

Pipework should be ok, a complete new system installed in the last 15 years should be based on a 20 degree drop across the flow and return pipework so the heat capacity has increased considerably from the old 11 degree drop.
Start with balancing the system paying attention to small radiators/towel rails that may only need the lockshields opening an 1/8 of a turn....use a screwdriver to the ear to detect when the valve is opening.
Don't waste your time with heat loss calcs at this stage...unknown air change factors render the calculations pretty futile.

 

[dal5band]

I am just a diy but I have never come accross a boiler with main flow and return pipes of 15mm. They are nearly always 22mm in domestic houses up to 4 bed detached.

 

[dilalio]

I am just a diy but I have never come accross a boiler with main flow and return pipes of 15mm. They are nearly always 22mm in domestic houses up to 4 bed detached.

As a diy-er, how many heating systems have you appraised then?

 

[Bodd]

If I fit a new system I will run 22mm flow and return
Tee off each rad 15mm or 10mm.
Would not use 10mm poly as this is effectively 6mm inside diameter with the inserts.

I may do the last two rads in 15mm

 

[Madrab]

I have to agree though, would never use 15mm for main flow and returns unless it's a really small setup of 3/4 small rads, would always use 22mm.

Unless it's a really tight and awkward under floor setup (old flats with balast as insulation/sound proofing etc) then it would always be 15mm to the rads too. I find it makes for quicker warm up times and easier servicing in the future.

Whilst I would always try to use the above, 10mm is used a lot these days in new builds where the insulation and window/door seals are good and heat loss is low so small type11/21 rads are used and rad capacity is low, it's also much quicker (and cheaper) to install .

Accurate balancing is certainly needed though if all your rads aren't heating up properly.

 

[DP]

With a modern system you are looking for a small difference (typically around 11 degrees) in order for the boiler to run in condensing mode more of the time and yielding efficiency savings.

Perhaps I am wrong on this one, 11 degree temp difference was for cast iron boilers. That temp difference adhered to for band C or D as condensation within the chamber often caused corrosion

Roll in steamers. 20 degrees quoted, oversized rads too, to keep return temperature low as possible (UF being a fine example) as return temperature, lower it is, more heat removed from the flue gases.

 

[Gasguru]

The 11 degree drop originates from the old Fahrenheit temperatures of 180 flow temp and 160 return temp, these convert to Celsius 82 and 71 and hence 11 degrees.
The original temperatures were a compromise and AFAIR were kept at an 11 degree delta to prevent thermal shock to traditional cast iron heat exchangers.

Roll on to modern condensing boilers and we have heat exchangers with highly restrictive waterways and a EU bent on saving every last gram of carbon so the industries gone towards a 20 degree drop.
The pumping losses with a 20 degree drop are halved (and without fitting a very high head commercial pump) at design temperatures you could never achieve the flowrate through a modern heat exchanger if we kept the 11 degree drop due to the excessive friction.

So although the pipework can in theory carry twice the heat load that must be countered by the increased head loss (friction) through the heat exchangers ie. safer to stick with 22mm off the boiler if calcs are not to be done.
And as you say rads should be increased in size a little due to the lower average temperature (with a 20 degree drop) and a shift towards lower running temps in general to help boiler condensing.