Gas fire chimney cap too small

[Alex2017]

Hi, our builder recently fitted some clay drop-in chimney covers for our dripping chimneys and one of them was for a gas fire. The first two are regular 205mm ventilated caps but the spigot for the clay drop-in GC5 he used for the gas, in my option, is too small at just 170mm externally and 135mm internally. This is for a 5.3kW glass fronted natural gas fire in the living room and the chimneys are the original 1920's brick type that shares a breast with next doors chimney.



His response was "they didn't have any of the 205mm versions in stock and I'm not making two trips up the ladder".

The gas cap in question looks like this


Is this safe or is the smaller spigot size too restrictive? All of the existing chimney pots were the same size and the 205mm elephant foot caps look like they fit perfectly so that must be the size of all the pots.

Secondly he used silicone sealant to fill in the large gap left by the smaller spigot.

The builder feels as though the job is complete but the undersized spigot on the gas cap is really bothering me so any advice on this would be greatly appreciated!

 

[Chuck]

Hi, our builder recently fitted some clay drop-in chimney covers for our dripping chimneys and one of them was for a gas fire.

The builder shouldn't have touched the gas flue and probably shouldn't have been workng from a ladder either. Seems like you employed a cowboy...

Don't use the fire until a Gas Safe Registered plumber or heating engineer has inspected and remedied the work.

 

[Last Plumber]

If the fire is designed to work on a 5" flue (it more than likely is), it will be the correct terminal. However, after installing it he should have tested the fire for spillage. If he did not, you need a Gas Safe registered Engineer to test the flue for you as chuck says.
I personally, would carry out a smoke test, Gas rate and spillage test, along with a visual inspection of the fire and chimney.

 

[Alex2017]

Thanks for the advice everyone, I'll try and make sure nobody uses the fire.

I've drawn up a diagram in paint as a picture is worth a thousand words, turns out the internal diameter of the cap may be even less at 125mm!



There was no gas leakage test or any smoke bombs used when it was installed. The fire itself is a Flavel emberglow from the 80's and claims around 3.5kW of heat output at max, so efficiency from 5.28kW input is not that great.

 

[Last Plumber]

5" is 125mm. Near as damn it.

 

[Alex2017]

5" is 125mm. Near as damn it.

Yes but there are still massive voids around the sides, couldn't this hurt the flow of hot gases from the chimney flue as they cool and drop back down the flue? Also there is no flue lining so its just bare brick between the gases and the rest of the house.

 

[Last Plumber]

In some circumstances the height of a chimney or the mass of masonry it is constructed of and the temperature ( heat loss) of that masonry, can effect the performance of its flue.

However, in most cases the temperature of the gases in the chimney are higher than the outside air temperature and the room temperature of the property the appliance is situated in, so they should in theory be forced upward by cooler denser air descending under the influence of gravity and displacing them.

As long as the cross sectional area of the terminal is equivalent to or greater than the requirements of the appliance and the height of the chimney is enough to allow the ascension of flue gases to overcome the physical resistances of the chimney, it should work safely.

Having said that, theory is all well and good but you cannot beat practical testing.
Many chimneys work well when we do not expect them to and some don't work when in theory they should pull your socks off. There are many other things which can effect a flue, such as downdraught, wind, other chimneys in the same property, extraction fans, construction of the building and the position of the chimney etc. It is a long list.

That is why, they should always be taken on their own merits and be tested properly when any change occurs and in my opinion tested at least once a year during the service of the Gas appliance.