Is 4 rads (18500BTU) on a 15mm branch ok?

[Action Joe]

Hi,

I was hoping someone qualified could please assist me?

I've recently had a large extension added to my house and I'm concerned the pipe work for the central heating is not appropriate.

My existing central heating system has a 22mm trunk for flow and return. Off the trunk, each radiator is branched using 15mm (one rad per branch). My concern is that the central heating guys who have put the pipes in for the extension have not followed the notion of one radiator per branch. They have put in 4 rads (totalling 18500btu) on a single 15mm branch off the trunk. This has effectively reduced the trunk to 15mm for those 4 rads They have respected the flow and return.

I had anticipated they would have followed the layout that had already been establish. I'm concerned that such a large load on the 15mm branch will mean the heating won't be effective.

FYI,I now have 14 rads in the house.

I questioned they guys who put it in, but they effectively told me to go away as there was nothing wrong with it.

Could anyone please advise on if I am right to insist on maintaining the one rad per branch model? Thanks in advance.

 

[sxturbo]

Will be fine, just needs to be correctly balanced.

 

[muggles]

As sx says, there nothing wrong with this, provided it's balanced properly. And nobody works in btu any more it's been kW since the 1970s

 

[oilhead]

As sx says, there nothing wrong with this, provided it's balanced properly. And nobody works in btu any more it's been kW since the 1970s

Plenty of boilers and radiators were still being sold primarily with Bthu ratings into the current century, and calculations are much more straightforward dealing in whole numbers rather than decimal places. Gradually, metrication is increasing dominance, but as an old oil service engineer, I still think in gallons for nozzles, and size pumps in gpm/Bthu's much more easily and to my mind understandably. (I do at least work in centigrade temperatures).

 

[Action Joe]

Thanks for your comments. Sounds like running a house off 15mm is acceptable.

Perhaps a follow on question is, why are there many people in the internet who say a 22mm trunk and 15mm branches to each rad is what should be used? Was that in the days before Combi boilers or for some other reason?

Thanks

 

[muggles]

Combi boilers have been around since the 1960s. Much like BTUs, it's a relic of a bygone era which still pops up from time to time. As you've asked, strap in for the science lesson...

There is a direct mathematical relationship between power (kW), volume of pipework (litres), velocity of water running through the pipework (metres per second), flow rate (litres per second), and system dT (the difference between flow & return temperatures). Flow rate = Velocity × Volume, and Flow rate = kW ÷ (dT × 4.2). We aim to keep the velocity between 0.5 and 1.5 m/s (ideally 0.9 although many people use 1.0 because it makes the maths easier). Below 0.5, system debris held in suspension in the system water starts to be deposited, eventually causing a build-up and possible blockage. Above 1.5m/s, the system can become unacceptably noisy.

In days of yore, systems were designed to run at an 11°C dT. This required a reasonably high pump speed, which could cause noise in the system if the velocity through the pipework exceeded 1.5m/s. A simple way to combat noise is to increase pipe size (IE volume), which reduces the velocity for the same flow rate. Pipe size may also be increased to reduce flow resistance over a long distance.

Modern condensing systems run at 20°C dT, which requires a much lower flow rate. Remember F=Ve × Vo, so to maintain the same Ve with a reduced F we can reduce Vo. In other words, you can get quite happily use 15mm where 22mm might previously have been used on an old dT11 system.

Incidentally this is one reason why modern boilers on old systems seem to suffer more with sludge. If we fit a new boiler to an old system and don't clean the system properly, when we set up to achieve our target dT of 20°C, we can end up reducing Ve to a point where debris falls out of suspension and becomes a deposit which then builds up. We've reduced F to achieve our larger dT, but a reduced F with the same Vo results in a reduced Ve.

Thus endeth the lesson. Hope you're glad you asked

 

[Mottie]

 

[Action Joe]

Thanks @muggles, that was quite thorough and I did follow it. Thanks for taking the time.

Cheers

 

[gas112]

muggles did you copy and paste that from your notes from kim
Btus are not an archaic term from pre 70`s they were still being used in boiler names up to the 2010`s aka baxi 105 along with many others .
Majority of guys i know even a fair few younger ones still talk in btu`s regards boiler and rad sizes required

 

[muggles]

muggles did you copy and paste that from your notes from kim
Btus are not an archaic term from pre 70`s they were still being used in boiler names up to the 2010`s aka baxi 105 along with many others .
Majority of guys i know even a fair few younger ones still talk in btu`s regards boiler and rad sizes required

Not quite although I've been on his 2 day course, very informative.

 

[winston1]

muggles that was very informative, thanks.

I looked into DIY central heating back in the '70s and put a system into my first house. Then flow temperatures were 180˚F and return 160˚F giving a dT of 20˚F or 11˚C as you said. Now if on a modern system dT is 20˚C and assuming the flow temperature cannot be increased for safety it must mean the mean temperature of the radiators is reduced and thus the output is reduced. I assume therefore if upgrading to a modern boiler the radiators should also be upgraded or is it possible to run a modern boiler with a dT of 20˚C?