I want to talk about Energy Performance Certificates (EPCs). Specifically, I want to talk about the bit that affects us chimney people more than most — and how the official system for assessing your home's energy efficiency handles chimneys. Spoiler: not very well.

You'll probably know that an EPC is legally required before you can sell or let a home. The energy efficiency certificate rates your property from A (excellent) to G (grim), and that little coloured graph influences everything from your sale price to the mortgage your buyer can get. The government also uses EPC data to track the country's progress toward net-zero targets. So it matters. It really matters.

Here's the thing though. I've been reading the official government guidance and the full methodology behind how EPCs are actually generated — the Reduced data Standard Assessment Procedure, known as RdSAP, which underpins 99% of all domestic EPCs in the UK. And what I found should give every homeowner pause for thought.

Half an hour and a good look around

A Domestic Energy Assessor visits your home for roughly 30 to 60 minutes. They measure rooms, note the heating system, check what type of windows you have, look at your loft insulation if the hatch is safe to open, and so on. All of this is entirely visual and non-invasive — the RdSAP specification says so explicitly. No drilling, no lifting floorboards, no opening up the fabric of the building. If something can't be seen, the software applies age-based assumptions instead. The government's own guidance is refreshingly blunt about this: where features can't be verified, they simply aren't included.

For most of the house, you can argue this is a reasonable compromise — nobody wants their skirting boards pulled off every ten years. But when it comes to chimneys, the limitations of this approach become genuinely problematic.

The chimney problem

Here's why chimneys matter so much to the calculation. If you've read our post about the stack effect, you'll know that an open chimney during winter pulls roughly 80 cubic metres of warm air out of your home every hour. That's an enormous amount of expensive heat disappearing up the flue — significant chimney heat loss that directly impacts your energy bills. Unsurprisingly, the RdSAP algorithm treats open chimneys as major sources of heat loss and uses the number of open and blocked chimneys to calculate your home's ventilation losses. Get the chimney count wrong and the whole EPC rating shifts.

So how does the assessor establish whether your chimney is open or blocked?

They look at it.

That's it. They look at the fireplace, perhaps glance up at the hearth to see if it appears sealed, check from outside whether the chimney pot is capped, and make a judgement call. The RdSAP documentation is quite clear: the number of blocked chimneys is recorded based on visual observation alone. No smoke test, no borescope, no removal of any register plate, no documentary evidence required.

If your chimney is blocked higher up the flue but looks open from the fireplace — it's recorded as open. If it looks blocked from the living room but there's actually a gap behind that board — the assessor records it as blocked and moves on. The software treats the assessor's visual impression as fact, when in reality it's an educated guess made from across the room.

If there's a piece of furniture over it, it gets overlooked.

One industry professional described the whole process as "a visual guess fed into a 20-year-old algorithm." Having read the methodology, I'd say that's a fair summary.

Does your chimney draught excluder count? Almost certainly not.

This brings me to a question we're often asked, and the answer is important if you're preparing to sell your home or simply want your EPC to reflect reality.

The RdSAP system records chimney status in one of three ways: open, blocked, or fitted with a ventilator. A "blocked chimney" in RdSAP terms means physically sealed — a register plate, a capped pot, or similar permanent-looking measure. A chimney draught excluder like a Chimney Sheep sits inside the flue and is, by design, removable. It is not a permanent seal, and that distinction matters enormously to the software.

This creates two problems.

First, a visual inspection will very often miss it entirely. An assessor looking at a fireplace with a Chimney Sheep fitted above the throat of the flue — out of sight behind the chimney breast — has no obvious way of knowing it's there unless you specifically point it out. Even then, without documentary evidence, many assessors will not record it as a blocked chimney, because the RdSAP guidance requires the blocking to be a structural or permanent feature to qualify for the credit.

Second, even if the assessor does notice it, a removable draught excluder almost certainly doesn't meet the RdSAP definition of "blocked." The software was written with fixed, permanent interventions in mind — a cemented register plate, a professionally installed cap, a ventilated closure system. A wool plug, however effective in practice, sits in a grey area at best.

The cruel irony is that a well-fitted Chimney Sheep genuinely does reduce infiltration and chimney heat loss — arguably more reliably day-to-day than a warped or ill-fitting damper that's been there since 1887. But the EPC system rewards what it can see and categorise, not what is actually performing.

So if you have a Chimney Sheep installed, don't expect your EPC to reflect it. Your real-world energy performance will be better than your certificate suggests — which is worth knowing, but cold comfort if you're trying to improve your EPC rating to secure a better mortgage rate or a higher sale price.

The real-world consequences

This isn't just a theoretical problem. Consumer watchdog Which? sent assessors to 12 real homes in 2024. Eight out of 11 EPCs contained significant errors — solar panels omitted, wood-burning stoves ignored, underfloor insulation wrongly recorded as absent despite the homeowner pointing to it. One home's EPC rating jumped two full bands, from D to B, after corrections were forced through. Recommended improvement measures were sometimes so out of touch with reality that one suggestion carried a payback period of 29 years.

For chimney owners specifically, the risks run in both directions. If you've gone to the trouble of blocking your flue — whether with a register plate, a Balloon, or a Chimney Sheep — but the assessor doesn't spot it or doesn't credit it, your EPC will underestimate your energy efficiency. You'll look worse on paper than you actually are, which could affect your sale price or mortgage terms. Conversely, if you have an open chimney tucked behind a modern fireplace surround that the assessor misses altogether, the EPC will overestimate your energy performance — and whoever buys your home may have a nasty surprise when their heating bills arrive.

What this means for the bigger picture

I care about this not just as someone who sells chimney draught excluders, but as someone who cares about whether the UK actually gets to where it needs to be on energy efficiency. EPCs are the data foundation on which national retrofit policy is built. Mortgage lenders are increasingly tying green finance products to EPC ratings. The government uses EPC statistics to measure progress toward its 2035 targets.

If the certificates are systematically inaccurate — and the evidence strongly suggests they are — then we're navigating by a faulty map.

Which? has called for urgent reform: better training for assessors, proper auditing, and recognition that a purely visual inspection has inherent limits. There's a compelling case for requiring documentary evidence of chimney blocking measures — just as you might provide receipts for cavity wall insulation — so that homeowners who've made genuine improvements get proper credit for them. That should logically extend to removable draught excluders too. A product that demonstrably reduces infiltration, can be photographed in situ, and comes with a purchase receipt is a perfectly evidencable intervention. The fact that the methodology doesn't currently accommodate it says more about the age of the system than about the effectiveness of the measure.

What you can do right now

If you're selling your home and you've taken steps to block unused chimneys — permanently or otherwise — gather your evidence before the assessor arrives. A photograph of the product in place, a note of what it is and when it was installed, and a purchase receipt will all help make the case. You may not win the argument with every assessor, but you're far more likely to get proper credit with evidence in hand than without it.

If your EPC has already been issued and you suspect the chimney status was recorded incorrectly, it is possible to challenge the assessment and request a correction — though the process isn't exactly painless.

And if you haven't yet addressed your open chimney? Well — you know where we are.

An unused, unblocked chimney is, as we've said many times, like leaving a window open day and night all winter. The stack effect doesn't care whether you've got the fire lit or not. Blocking it is one of the simplest, lowest-cost, highest-impact things you can do for your energy bills and your carbon footprint — regardless of what your EPC says about it.