Combustion air on a Powermax

[Backboiler]

I went to do a Gas Safety check on the top flat of a three story property yesterday. You could hear my jaw clang on the floor when I was told that the boiler was a Potterton Powermax and was in the bathroom airing cupboard. For those who don't know, these boilers have a thermal hot water store and have twin 60mm flue pipes. One option when installing the twin flue on a pitched roof is to take the combustion air directly from the loft; the criteria for doing this are, the air inlet pipe has to be more than 75% of the total length of the flue pipe and the loft is to have a minimum of 234cm² of permanent free air ventilation. I found no evidence of any free air ventilation in the loft so the boiler was AR'd. I was talking to a respected colleague who used to work for BG in technical roll and he told me that BG had issues with this boiler/loft/ air intake arrangement many years ago. The conclusion was that the developer had used a breathable membrane called Tyvek under the roof tiles which claimed to provide enough free air for complete combustion and was deemed acceptable. When googling Tyvek I downloaded their brochure which states that thee loft is airtight in cold weather.
How are we to know that products like this may be acceptable? we are taught nothing about this. How is the free air measured? has anyone else come across this?

 

[tamz]

Ha. Who said being a gas man was easy :lol:
Having a sound knowledge of construction methods comes in handy. New (ish) roofs have been built to be fully ventilated for around 30 years or so now. Look up at the soffits for either a continuous vent strip (usually plastic) or round vents fitted about 2ft apart. If there is a dry ridge system on it (these have a continuous vent built in) look at the apex of the roof from inside to check the felt has been cut and folded back at the ridge (it should be). Tyvek is breathable. Just look at it. It is full of microholes. The only time it could theoretically become airtight is if it became heavily frosted over.

Apart from all that "common knowledge" :smile: if you look in BS5440 it says this

6.2.2 An air vent shall not communicate with a ventilated roof space or underfloor space if that space communicates with other premises. Where an air vent draws air from a ventilated roof space or ventilated underfloor space, the total effective free area of the grilles or louvres into that space shall be at least equal to the total effective free area of any air vents drawing air from that space. The ventilated roof space shall be visually inspected to confirm that it conforms to the minimum unrestricted ventilation requirements in BS 5250.

COMMENTARY ON 6.2.2
To avoid condensation and associated problems that can result from low ambient seasonal temperatures in the roof space, adequate ventilation should be provided. Existing air vents, grilles or louvres that have previously been installed to meet the requirements for roof space ventilation and that communicate directly to the outside air may have apertures of less than 5 mm, and are deemed to be acceptable if the total free area is satisfactory and openings are checked to ensure that they are free from obstruction. In order to prevent condensation occurring within a roof space that can result from low ambient seasonal temperatures, ventilation is normally required and further guidance can be found in BS 5250:2002, 8.4.2.1.2 and 8.4.2.2.3.
A roof space constructed in accordance with the ventilation recommendations in BS 5250:2002, 8.4.2.1.2 and 8.4.2.2.3, may be considered to be outside air.
It is important that any ventilation provision is not blocked at the eaves and at the point where the ventilation passes through the ceiling insulation into the dwelling. This can be caused by poorly installed roof/cavity wall insulation, for example. Some modern construction methods employ the use of a “breathable” roofing felt designed to allow similar ventilation of the roof space. Where this method has been used, the roof space should not be considered to be outside air for the purpose of gas appliance air supply, due to its propensity for reduced performance of the membrane over time.
It is also important that the requirements of Approved Document B regarding fire safety [10] are taken into consideration and that any ventilation provision for a gas appliance provided through the roof space or loft does not compromise fire safety.
Further guidance on ventilation and fire safety can be found in BS 5250.
A roof space not constructed to conform to BS 5250 can only be regarded as another internal space and therefore should be treated as such when calculating air vent free-areas.

BS 5250 is about preventing condensation in roofs (ventilation). You may find this interesting.
http://www.glidevale.com/downloads/condensation_control_bs5250_white_paper.pdf

 

[SafeGasInstall]

I did have a job this monday , remove one of this power max boilers . It had a twin flue but was taking air from outside . I did strugle to get it out , it has got a drain of right under it and could not drain it as it was a flat in the 4th floor . They are good for scrap powermax !

 

[Hybrid]

I'll rip out a powermax but that's as far as i'll go, don't like these boilers at all. In fact I think BG refuse to touch them.

 

[Backboiler]

Some modern construction methods employ the use of a “breathable” roofing felt designed to allow similar ventilation of the roof space. Where this method has been used, the roof space should not be considered to be outside air for the purpose of gas appliance air supply, due to its propensity for reduced performance of the membrane over time.

So do I take it from the above paragraph that the Tyvek or any other brand name they use is NOT considered suitable for use as free air ventilation? It needs a permanent vent?
​

 

[diamondgas]

Purpose provided ventilation has to have slots/ holes no smaller than 5mm or larger than 10mm if my memory serves me right and has to be as said, "purpose provided" A breathable membrane will not pass IMHO

 

[Backboiler]

Purpose provided ventilation has to have slots/ holes no smaller than 5mm or larger than 10mm if my memory serves me right and has to be as said, "purpose provided" A breathable membrane will not pass IMHO

My thoughts exactly. This needs clarifying by the Gas Safe Register: Does anyone know if there's a TB concerning this? I'm seriously considering requesting Gas Safe to visit the property in question

 

[Billy Bob Bob]

Purpose provided ventilation has to have slots/ holes no smaller than 5mm or larger than 10mm if my memory serves me right and has to be as said, "purpose provided" A breathable membrane will not pass IMHO

:iagree::iagree::iagree::iagree::iagree:

 

[tamz]

Some modern construction methods employ the use of a “breathable” roofing felt designed to allow similar ventilation of the roof space. Where this method has been used, the roof space should not be considered to be outside air for the purpose of gas appliance air supply, due to its propensity for reduced performance of the membrane over time.

So do I take it from the above paragraph that the Tyvek or any other brand name they use is NOT considered suitable for use as free air ventilation? It needs a permanent vent?
​

Because it can potentially become airtight (alledgedly) in winter you should ignore it. That is why soffit and ridge vents are also required on roofs.

With regards the hole sizes of vents in roofs it expains in the bit i pasted that holes less than 5mm are acceptable in this situation.

Existing air vents, grilles or louvres that have previously been installed to meet the requirements for roof space ventilation and that communicate directly to the outside air may have apertures of less than 5 mm, and are deemed to be acceptable if the total free area is satisfactory and openings are checked to ensure that they are free from obstruction.

Existing air vents, grilles or louvres that have previously been installed to meet the requirements for roof space ventilation and that communicate directly to the outside air may have apertures of less than 5 mm, and are deemed to be acceptable if the total free area is satisfactory and openings are checked to ensure that they are free from obstruction.

At the end of the day it is your rse so you classify it as you see. You are not a building design engineer so you can only go with what you see and know.

​

 

[stevetheplumber]

My thoughts exactly. This needs clarifying by the Gas Safe Register: Does anyone know if there's a TB concerning this? I'm seriously considering requesting Gas Safe to visit the property in question

your only chance of getting gas safe out to give a definate answer is throw a dead body in the loft

 

[diamondgas]

your only chance of getting gas safe out to give a definate answer is throw a dead body in the loft

That would deffo get the police too and maybe the TV News network depending on what has happened elsewhere in the world on the day :lol:

 

[GrahamM]

I'll rip out a powermax but that's as far as i'll go, don't like these boilers at all. In fact I think BG refuse to touch them.

Unfortunately BG do still touch them and I have had the pleasure of visiting a few for them in past. Hate them!!

This is a bit of a grey area and is no definitive, 100% clear guide on it as far as I am aware. It is really down to your expert opinion to a degree. Some knowledge of house builds and building regs is handy.
Generally, if the building is built from 1985 onwards it can be said that the roof-space should have enough ventilation for the appliance. Older than that then it is your call.
Just make sure any insulation isn't pushed right into the eaves where there is a gap to allow for ventilation. If you can get right over into the eaves and feel free air then all the better.

Ideally there would be purpose provided ventilation at least 300mm above the insulation level, but as we know in this job it is not always that easy.

I have been to quite a few of these and have, just noted NCS and purpose provided vents advised.

If doubt cover your own.

 

[Backboiler]

UPDATE: I met with the developer of the property today (Wimpy Homes) and was told that the ridge tiles are ventilated. The ridge tiles were definitely not set on a bed of mortar. He had a gas engineer with him who cut the felt at the ridge. Underneath the felt was a very thin rubbery membrane which I closely examined. the developer told me that the membrane was attached to the ridge tile. I could certainly feel a draft under the ridge but there was no direct path to the outside. I told them there is no indication in BS5440 that ridge tile ventilation was acceptable but they both insisted it is! I'm waiting to see their report before taking it further. Oh nearly forgot, when I arrived the developer was in the process of fitting a CO alarm in the boiler cupboard. Is that worrying?

 

[AWheating]

well if the developer is happy it complies and meets the bs needed then ask them to put it into writing that the roof space has been inspected by a competent person and conforms to the said requirement and ask for it to be signed by the competent person. Then copy it, attach a copy in the appliance instructions and attach a copy to the landlord cert, keep a copy to cover yourself.

 

[Billy Bob Bob]

well if the developer is happy it complies and meets the bs needed then ask them to put it into writing that the roof space has been inspected by a competent person and conforms to the said requirement and ask for it to be signed by the competent person. Then copy it, attach a copy in the appliance instructions and attach a copy to the landlord cert, keep a copy to cover yourself.

we have to have it in writing its at this point said developer will become difficult/ignore the question

 

[AWheating]

we have to have it in writing its at this point said developer will become difficult/ignore the question

and forget his own signature LOL

 

[stevetheplumber]

i had the same with gas hobs in a new block of flats where the kitchens were internal the developer insisted that the 1/4 inch gap at top and bottom of the door and the cooker hod was all it needed and that it had been passed by the architect strangely the architect never did get back to me in writing we actually didnt do the job in the end
coincidently the boiler here were promax with flues in the ceilings i bet theres been some fun with those recently job was in victoria