15mm or 22mm central heating pipe

[bowytt]

Much appreciated if anyone can help me with this problem.

I am currently having problems with my 4 year old Chaff and Maury (MX 2 30 FF) which i am thinking of replacing as it seems a dog of a boiler however one installer seems adament that i have a major issue with my central heating piping.

The pipe leading to the radiators (and return) from the boiler is 22mm but then reduces to a 15mm for the rest of the system (as far as i can tell as after 2 meters or so it goes into a stud wall)
He wants to replace up to 6 meters of pipe to the 22mm incorporating at least 2 rads. In the process destroying my newish kitchen, bathroom and small bedroom.
I have had other installers saying that the 15mm pipe while not ideal will not cause the new boiler problems, just lose some efficiency whereas the installer in question stays it will knacker the new boiler up and may have led to the downfall of my Chaff and Maury.

Some more info that may be relavant - looking for a 28w Vaillant, worecester or veissmanna. Have 9 rads in the system.

Thanks for reading my post

Bohdan

 

[Agile]

The rule of thumb is that you can only run THREE rads from a 15 mm pipe.

Its not clear from what you have said how many rads you really have only fed by 15 mm.

If they are all on 15 mm then it will not work properly and may damage the boiler as he says.

You are only concerned about your new kitchen and decorations so you had better stick with your old boiler. Please dont ask me to fix it as they should be replaced not repaired.

Tony

 

[gas4you]

4Kw on 15 mm pipe work.

What is the problem you are having that is causing so much concern?

Too small pipe work will usually just make the rads take a lot longer to heat up, whilst giving too greater a temp differential as well.

 

[bowytt]

Well i am concerned about the re-decorating as it would involve major time and finance to be put right. If it needs to be done then it needs to be done however like i say i have had mixed messages for 2 diff installers and dont fancy forking our for something that strictly speaking doesn't need to be done.
Have even phoned up vallient and bosch and they say it shouldn't be a problem for their boilers respectively however I always thinks its better to get as much info as poss.

Anyway all the rads (7, ''medium'' sized or smaller) are fed of the 15mm circ loop (from what i can see, i have no reason to doubt that it changes in size through the floor boards etc.) The old boiler when working was fine, didn't take long to heat the rads apart from one which was in the extension which is fed on micro pipes.

The Chaff LED board code (50) stated that there was a primary circ defect. Last time i had it repaired there was a crack in the expansion vessel which was replaced, upon advisement (from chaff technical, unhelpful mind). Problem still persisted. Rang again. Could be scale build up in the heat exchange (boiler 4 years old and in manchester so soft water) and rang up again and another guy said it could be the pump. Read plenty of posts stating that the boiler is crap and so thinking that it may become a long list of problems and need some other work doing on the system (less invasive, new rad put in and changing micro pipes to extension)

Not too certain what you mean by 4KW on 15mm

Thanks again for your help

Bo

 

[D_Hailsham]

Not too certain what you mean by 4KW on 15mm.

That's a rule-of-thumb figure for the maximum amount of heat which can be carried by 15mm pipe.

The amount of heat is determined by the temperature drop and the flow rate (litres/sec). Assuming temperature drop is constant, if you want to carry more heat you have to increase the flow. But increasing the flow increases the resistance in the pipe and, the smaller the bore of the pipe, the faster the water as to travel through the pipe. If the velocity is too high, then the water can be heard travelling through the pipe.

For an 11°C drop a 15mm pipe can carry 2kw at 0.3metres/second and 10kW at 1.5m/s
For a 20°C drop a 15mm pipe can carry 3.63kW at 0.3m/s and 18.10kW at 1.5m/s.

If the velocity drops below 0.3 metres/second, there is the risk of sludge build up; and, if the velocity is more than 1.5 metres per second, the noise of water flowing through the pipe could be unacceptable. If the velocity exceeds 2.5m/s, then the internal friction could cause erosion of the pipe. A velocity of 1m/s is a good compromise.

4kW is equivalent to a velocity of 0.6m/s in a 15mm pipe at an 11°C drop.

Ideally, every pipe would be sized for its heating load. But that would mean carrying a different pipe for each radiator. So compromises are made with 22mm or 28mm used for the main feed and 15mm for the feed to individual radiators.

 

[bowytt]

Thanks for the explanation, From what i gather then the amount of heat through the rads wouldn't be as much as if i had 22mm pipes.
Is 4kw bad compared to what i would expect ? As written above the old boiler when working was fine in terms of the rads heating in a given time period. Heard no water travelling through the pipes.

Phoned up Bosch (thinking of going for the worcester again) and went through it very slowly (for my beneifit) with then and they said it wouldn't be a problem for the boiler being fitted in 22mm then reducing to 15mm after 1-2 m of visible pipe. No problem in terms of warrenty or anything else.

Again if need be i will replace but have costed it up that it will cost me an additional £1K plus to do the remedial work and the length of time to complete over crimbo. (half a kitchen/ half a bathroom to re-insatll, new plaster, new flooring etc etc. Is it really worth it?

 

[namsag]

Simple if your rads are getting up to temp and everything is heating ok , having a system in 15mm will not cause the boiler to pack up .
Its dead easy for people to advise you to spend money when its not theirs, yes 22mm would be better but not having it will not knacker your boiler

 

[Agile]

If you do an in depth search on this forum you would find a posting from a customer who had a new Worcester boiler fitted to his microbore system.

The circulation was so poor that the boiler gave lots of problems and Worcester just said that their boiler should never have been fitted to such a system.

I expected that he ended up having to have his system repiped in 22.15 mm.

It would make a big difference if your system was fed in 22 mm to a SPLIT in two directions each in 15 mm. Thats not so bad.

If the 22 mm just reduced to a single 15 mm then I would never be fitting a new boiler for you!

Tony

 

[D_Hailsham]

Thanks for the explanation, From what i gather then the amount of heat through the rads wouldn't be as much as if i had 22mm pipes.

No that's not correct. You can, in theory, put any amount of heat through a pipe but if the pipe is too small the velocity will be so high that the noise would be unacceptable and the pipe would soon wear away from friction caused by the water. The "heat capacity" of a pipe is just a recommendation of what is a suitable maximum for the pipe to carry. A 2kw radiator will still produce 2kW, irrespective of what size pipe is used, provided the water can flow through the rad at the required rate to provide the necessary temperature drop across the radiator.

Looking at you list of suggested boilers, I assume you are thinking of a combi boiler as 9 radiators is unlikely to add up to 28kW. Furthermore, the boilers suggested all work best with a 20°C temperature drop. This means a lower flow rate for the same amount of heat, so your 15mm pipe will be able to carry 12-15kW.

This basic diagram may help explain how flows can vary along the pipework

Untitled

• D_Hailsham
• 18 Dec 2008
• 3

 

[namsag]

Agile save me doing a in depth search why not put any worcester MI instructions that say don t fit on microbore.
Why would it cause boiler to pack up

 

[bowytt]

The boiler i was looking at was a combi worcester junior 28i and have re counted my rad, doh i have 7 and a towl rail in the bathroom (is that counted). One of the Installers called up bosch with me again!! and was advised that there will be no problem with the set up as it is although the system will need to be balanced fully. They did point out that 22mm was advisible however they have had boilers been put on 8mm micro-bore and been fine. Obviously will different setup. Connecting to 15mm will not invalidate any warrenty or damage the boiler long term. Will have this in writing for what thats worth.

Noise issue, if the chaff if a 30 and didn't cause any noise problems would i be correct in assuming that the 28i would be no better or worse?
Would the flow rates be okay in terms of wearing away the pipes?

 

[Steady]

One of the Installers called up bosch with me again!! and was advised that there will be no problem with the set up as it is although the system will need to be balanced fully. They did point out that 22mm was advisible however they have had boilers been put on 8mm micro-bore and been fine. Obviously will different setup. Connecting to 15mm will not invalidate any warrenty or damage the boiler long term. Will have this in writing for what thats worth.

You would be advised not to employ the installer who had to ring Worcester to ask this.How many installations has he done in his short career??
There are many heating systems working perfectly happily with 15mm main circs and 10mm to the rads.
Unless your house is particularly large it shouldn't be a problem.
Probably heat up quicker too.

 

[bowytt]

Sorry that was on my bidding. He said it would be okay. Just being very pedantic. Hes done work for me before and is very relaiabel. Actually lives a street down form me and comes highly recommended. The initial guy had be going though. Though that would be a very messy crimbo.

House is smallish but extended 2 up 2 down end-terrace. (small box and bath)

 

[D_Hailsham]

have re counted my rad, doh i have 7 and a towel rail in the bathroom (is that counted).

1. Yes the towel rail does count
2. 8 rads (inc towel rail) will only be about 8-10kW, which is well within the capacity of 15mm pipe for a 20°C temperature drop at the boiler and across each rad.

The pipe leading to the radiators (and return) from the boiler is 22mm but then reduces to a 15mm for the rest of the system (as far as i can tell as after 2 meters or so it goes into a stud wall)

Can you see where it changes from 22mm to 15mm, i.e before the stud wall, or are you assuming that it is all 15mm because that is what you see coming out of the wall/floor for each radiator? If you can't see the change from 15 to 22, then you cannot make the assumption that all the hidden pipework is 15mm.

Noise issue, if the chaff if a 30 and didn't cause any noise problems would i be correct in assuming that the 28i would be no better or worse?

If you are not getting any noise with your old boiler, there is no reason why the new one will give any.

Would the flow rates be okay in terms of wearing away the pipes?

The flow rates will be well below that required to wear the pipes away.

 

[heatingman]

You will not kil the boiler and if set up and balanced very carefully it will work relatively ok, but (theres always one) the system heat up time may be longer. As the new boiler will be working with a reduced flow rate it will keep modulating down to a reduced rate. You wont notice it and will be ringing up your installer telling him it takes longer for the system to get up to full temp "my old boiler worked fine". Fit TRVs and a correctly adjusted auto by-pass to a well cleaned system and it'll be fine