Discuss 15mm flow and return, plate exchanger blockage? in the Central Heating Forum area at PlumbersForums.net

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The combi boiler has 22mm flow and return outlets but they are immediately reduced to 15mm as they come out the boiler.

After the pipes come out of the boiler, they split off into 3 branches. Branch 1 is for a single radiator only. Branch 2 feeds about 9000BTU diownstairs and branch 3 feeds about 13000BTU upstairs.

The plate heat exchanger is accumulating debris and getting blocked or reduced flow. After it is cleaned out it is OK for a few months and then starts to have problems again.

Could the flow and return pipe size being only 15mm affect the way sludge and debris move through the system and cause the plate exchanger problem?

Is it worth upgrading the pipes to 22mm?
 
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Riley

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As above which boiler is it? Plate to plate heat exchanger will not affect your heating
 
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You need to tell us what unit you have and something about the set up. Standard things such as magnetic and non magnetic filters should be in place as shoukd a history of chemical stabilisation of the ch water. centralheatking
 
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It is a Baxi platinum combi 28he condesing boiler. It was originally fitted with a salamander SESI device which was replaced with a magnetic scale inhibitor a year ago. There has always been inhibitor fluid in the system which is about 12 years old.

The heating is working fine although the diverter valve needed replacing last year. There is boiler bang when hot water is turned on as the plate exchanger is overheating due to debris.

Given that the plate exchanger is heated by the same water as the heathing, I was wondering if having too small pipes could make it worse? They are mostly hep20 so are even smaller than copper.
 

Riley

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Get a Registered Gas Engineer in. 15mm is not ideal for the heating side but it’s effectiveness will be based on how many radiators you have in the property and the space you are trying to heat. Regarding the plate to plate heat exchanger though you need to be Gas Safe in order to work in the boiler.
 
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Of course. I am asking here only to be better informed. Due to other works, the pipes at the start of branch 3 will soon become permanently inaccessible so I want to get them changed from hep20 to copper for peace of mind. I want them done right and done to last.

The question is whether they sould be increased to 22mm while he is at it.

The heating has never had trouble getting hot enough quickly but I was wondering if having too small pipes might cause other issues?

The rest of the 15mm hep can be upgraded to 22mm later. Might there be an issue with part of the system just in one room being 22mm?

I would expect it not to make much difference as the bottle neck is the rest of the 15mm?
 

Harvest Fields

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Main flow and return should be at least 22mm dependant on size of system. 15mm to rads only.
 
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Agree that is a standard set up on 25 kw boilers and some bigger ones might be 22mm but 30kw and bigger like 28mm feeds and returns ...it certainly makes the system go well in my experience, why choke the the primary hot water at birth
chking
 
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Thanks for the answers.

Looks like I will have to ask the engineer to replace the pipes that will become inaccessible under the new floor with 22mm copper.

That leaves the rest of the system in 15mm hep2o which will have to be sorted another year.

That leads to another question. The original installer branched the flow and return off into separate runs for upstairs and downstairs. Was this because of the smaller pipes so that there were less radiators on each branch?

Is it better to have a single flow and return circuit through the whole house in 22mm?
 

Darren Jackson

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Better to have seperate circuits up and down so you can zone them independently with their own stats to save energy.
I would have 22mm copper F&R and 15mm branching off to the rads. And most manufactures don't allow plastic pipes within at least 1m to 2m of the boiler. Power flush to get rid of debris & Mag filter on the 22m return to the boiler. Good inhibitor added after flush. Should keep the plate hex free.
 
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The general principle is one rad per 15 mm branch. If they are small ones maybe 2. I once tried in my own gaff two monster 3 kw doubles on one 15 mm feed and return..no good. Rob Foster aka centralheatking
 
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Thanks for the replies. The system has been heating the house OK for years so it will be a pain to re-do the flow and return unless there is a good reason.

Back to my original question... Is there any theory why having too small pipes could make a difference to how debris / sludge, etc move through the system and how it might cause the boiler to suffer?
 
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The smaller the pipe the easier it is to block
or the smaller dia the pipe the faster the liquid passes thro so the scour will be better in my opinion. Rob Foster aka centralheatking
with pump cavitation when the ports are smaller the scour is much greater in the pump body and around the impellors, which is not good on a pump set but might be useful on a small bore ch system
 
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