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New boiler - can i have a new boiler fitted if there is a a small gas leak?

Hi there,

Question – can I have a new boiler installed if there is a “small gas leak” but within the permissible drop? (Hope this makes sense as I am not quite sure I’ve got the terminology correct)

I was suppose to have a new boiler fitted last weekend but when the engineer tested the gas meter he said there was a 2 millibar drop. He said this is under the permissible drop of 4 millibar and I don’t need to have my gas switched off or call Transco. We were also not getting any gas smell so he said it is safe. However he said he cannot install the boiler as it is against regulation to have any pressure drop for a new boiler.

The engineer had checked both the internal pipes (one going to the kitchen for my cooker and the other going up to the loft for the boiler) and found that the small leak is in the pipe going into the kitchen). The engineer then recommended that I isolate the gas pipe going to the kitchen and do a new gas run or buy a new electric cooker.

I then called my internal gas pipe insurer and they sent someone round. After testing the gas meter the guy from the insurer said he could see that there is a drop of 1 millibar but this is totally safe as I am in an old properly and there is no smell of gas. The insurer is therefore not going to do anything and I cannot claim for reimbursement for a new gas run. The guy from the insurance company also said he can’t understand why the engineer couldn’t fit my boiler in as he would have done it without any issue as the drop was within permissible range. I also rang the Gas Safe Register and the impression they gave me was that it is fine to go ahead and install a new boiler because the drop is less than 4 millibar.

I am really confused as I do not understand why a new boiler cannot be fitted when the gas leak is within permissible drop and I can use all my current appliances safely? Is it true that a new boiler cannot be fitted in unless there is no drop?

Any advice or guidance will be very much appreciated as I do not want to putting in a new gas run if it is not necessary.

Thank you.


10 Answers from MyBuilder Gas Engineers

Best Answer

OK this is how it goes,and the only way it goes.You are not allowed any drop on the gas line at all if the engineer has isolated your gas appliances.There is a permissible allowance of up to 4mbr with appliances not isolated on the gas line,thats U6/G4 Meters E6 (Ultrasonic Meters) you are allowed up to 8mbr with old appliances connected,but if they are isolated from the gas line you are allowed no permissible drop.


Answered 21st Nov 2016

I would advise to replace the gas carcass when replacing the boiler, it seems pointless to leave a known drop on a carcass which with no smell is ok depending on which gas meter you have.
If it was my house or job I wouldn't be leaving a leak of any sort.


Answered 19th May 2019

Hi Michelle

This is a confusing regulation and can understand why it is mis-understood. You can have your new boiler fitted, the permissible drop is only allowed on existing pipework. Any new or altered pipework to the new boiler MUST be gas tight. This can be checked by leak detection fluid on all new joints and personally i would use a gas sniffer to be 100% happy.

Hope this helps.

Many thanks
Mower Development


Answered 18th Nov 2016

The permissible drop is only relevant with all the appliances connected. If the appliances have been isolated (Gas cock to appliance turned off) and a tightness test has been carried out then there is no permissible drop and you have confirmed the leak is on the gas pipework so it should be repaired or replaced. You cannot use gas pipework knowing that it is leaking.


Answered 9th Mar 2019

1mb drop per old gas appliance? I've never heard of that before.
In answer to your question, yes you can have a new boiler fitted onto a system which has less than 4mb drop providing there is NO smell of gas.

If there is currently a 2mb drop with not smell of gas, then after the new boiler is fitted there can only be the original 2mb drop.


Answered 26th Apr 2021

All others are wrong!!!! Let me explain clearly.

You are allowed zero pressure drop on all gas pipe work.

You are allowed zero pressure drop on new gas appliance installation.

However you are allowed upto 1 mb drop PER OLD gas appliance like boiler, hob, fire etc individually. So if you have 1 old gas appliance your allowed 1mb drop etc etc.

Also if any gas pipes are lead, they have to be removed if work is being carried out on the gas line EVEN if the lead isn't causing the the leak.

Leaks dont get better over time, they get worse.
You house is worth zero pounds on the market with a gas leak and a life is priceless so get it sorted.

Theirs no such thing as acceptable gas leaks on the pipe work, only old appliances.


Answered 14th Dec 2020

There is no such thing as a permissable drop on a gas CARCASS. In short yes you could of had a new boiler fitted. However now the leak has been proven to be on the gas carcuss by disconnecting appliances no permissable, drop regardless of size or smell is permitted. I would call you insurer back as the supply has been tested with appliances disconnected and proved to be on the the carcuss. The supply needs to be capped and classed as immediately dangerous. Irrelevant of any smell of gas. Also a riddor should be raised against whomever is leaving your gas connected. Some of these answers are shockingly worrying on here.


Answered 20th Sep 2021

Get the "drop" sourced & repaired/disconnected using a gas safe registered engineer.

I personally would not start any gas work with a "drop"....permissable or not, present.


Answered 2nd Jan 2021

I cant believe qualified gas engineers are saying that leaks are permissible....
Who in their right mind would want a leak anywhere,
Gas or Water!!!


Answered 1st Feb 2022

Christopher plumber, obviously not a Gas engineer. If you were you would understand PERMISSABLE pressure drop, there is a reason for it which I don't have time to explain. QgH boiler repair from Bolton is 100%


Answered 8th Jun 2022

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