Wood burning stove -hearth fitting issue

7 Aug 2020
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United Kingdom
After any advice from any experienced wood burning stove fitters.
We recently had a wood burning stove and new hearth installed (by a HETAS fitter) in an existing chimney breast. The previous hearth and surround was taken out and a new limestone hearth single slab fitted. The hearth was fitted on the remnants of previous hearth base which seemed solid (not sitting on floorboards) however once the hearth and stove was fitted we realized the hearth had some movement and turned out the fitter hadn't cemented the hearth down at all. The movement seems to be coming from the hearth sitting on a slightly uneven previous cement base allowing it rock ever so slightly. The fitter tried to rectify with a couple of spacers on one end to prevent the movement which did improve it and subsequently finished the install. When we questioned the fact it wasn't cements he advised it was too late given the stove fitting was fully complete.

Problem is even after fitting spacers if i stand on one end of the hearth there is still a very slight movement from the other end of the hearth and a very small movement visible where the stove meets the pipe (its accompanied by a cracking sound from the join which sounds like fire cement on the stove/pipe joint moving or separating).
Is it acceptable to have a hearth not cemented down?
Would love to hear from a experienced fitter if this sounds safe or an acceptable install?
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All your fitter has done by wedging is created a solid perimeter with unsupported middle which could eventually crack the slab over time so your install as stated by foxhole is a bodge. I don't know why fire cement which sets solid is ever used to seal flue pipes when there are alternatives that allow thermal movement without cracking the seal.
If they have bodged that part I wouldn't be confident in the rest if the install being up to scratch
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he should have dug all of it out to a depth of at least 5 inches and poured a concrete mix on a damp proof membrane. then laid your limestone on a wet cement bed or suitable tile adhesive
Lapsed HETAS.
The fire cement doesn't like the kind of movement your description creates as far as I am imagining it. However absent the flue being blocked, the Co doesn't 'try' nor usually succeed to get out of tiny little gaps. Like electricity in a weird way it has a preferred route, the provision of which is pretty straightforward. So poisoning isn't afaik an issue.
Personally speaking, the rocking hearth would bother me hugely. If you withheld payment and asked for input from HETAS I would imagine he'd put it right. Pretty sure he would find himself trying to tell you it's 'sound' or 'stable' when it's clearly not.

There's no major reason ( again based on my imagination of the installation?!) that it can't be undone and redone in an afternoon.

As to whether he needs to just rebed the hearth or go wild and dig out 5 inches, depends on many things, but mainly whether the underside of the stove is rated to under or over 100c. Former can sit on 12mm glass onto a wood floor iirc.
. I don't know why fire cement which sets solid is ever used to seal flue pipes when there are alternatives that allow thermal movement without cracking the seal.

Which one are you thinking of? Trades have a tendency to use generic terms for sealants.

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