Why is the chimney cleanout blocked off

How high is your chimney?

We're looking at installing a wood burner and had someone round to quote. We discussed the options and they were adamant that they would need scaffolding erecting to reach the top of the chimney. They said that manoeuvring the liner up the outside of the house would be nearly impossible without it.

Now our's is a three storey house so that's a lot of scaffolding and I expect that to be a major part of the cost. So the quote could well include that which you might not have accounted for.

This would make it, for me, something I'd be happy to pay someone to do. We fitted a lightweight pipe in another chimney where we had access to the top from inside the house and that was very easy, but a heavy weight liner for a fire/furnace would be a different matter altogether.

In your case - for a single storey house the cost of scaffolding (if any) will be a lot less.

What you should do is to ask for a quote that's broken down into parts. So you want to know (as a minimum):

• the cost of materials (not just the pipe)
• the cost of labour
• the cost of scaffolding (if any)
• the cost of removing waste - in the UK this can be significant

Given that the cost of the pipe you found was less than $1,000, $2,000 for the whole job doesn't seem too outrageous when you consider the other factors. As a rule of thumb I look at the cost of the parts and then double it to account for the labour charges.

The ultimate standard is going to be your local building department and inspector, but the International Plumbing Code only specifically prohibits plumbing in elevator shafts or in elevator equipment rooms (301.6). The only requirements specific to location of venting deal with size, distance, and type of connection to the system that they are venting. Everything else is deferred to the International Mechanical Code.

For chimneys that are used only for venting gas-fired appliances, the IMC defers to the International Fuel Gas Code. The relevant portion (501.15.4) of that code states only that there be airspace clearance to combustibles per the International Building Code. Note that there is a specific exemption for masonry chimneys with a low clearance liner.

This gets you back to what it appears that you already know from your question (R1003.18) "Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm)." However, again this is exempted if a low clearance flue liner is in place:

Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777 and installed in accordance with the manufacturer’s installation instructions are permitted to have combustible material in contact with their exterior surfaces.

PVC will ignite at 435°-557°, roughly the same temperature range wood will (although it will be a molten puddle at that point). So, unless your chimney liner is rated for contact with combustibles you would need galvanized or some other form of non-combustible pipe. If you don't know how the chimney is rated, I would take the presence of the existing air gap to indicate that it is not contact rated.

That said, if you can maintain the 2" minimum to a PVC pipe and the local code authority will sign off on it, I'd do it. Oversimplified, a 90% efficient gas furnace would basically dump only 10% of it's BTU output into the flue gas. I don't ever recall finding a brick chimney that was even warm to the touch on the outside, except right next to the inlet from the furnace.