Recoheat

[ZKlongisland]

Hi everyone,

I just saw a YouTube video on something called “recoheat” and found it interesting enough to look it up on their website: https://www.recoheat.co.uk/flue-pipes-how-it-works

It’s a heat recovery unit, which looks like a coiled tube which is sits inside of the flue. ambient air from the room enters through one end of the tube to circulate through the coiled tube inside the flue and returns back to the room after capturing heat from inside the flue.

The idea behind this tool is to reclaim some of the heat that’s lost up through the flue. Anyone in the US have any experience with something like this?

 

[bholler]

Hi everyone,

I just saw a YouTube video on something called “recoheat” and found it interesting enough to look it up on their website: https://www.recoheat.co.uk/flue-pipes-how-it-works

It’s a heat recovery unit, which looks like a coiled tube which is sits inside of the flue. ambient air from the room enters through one end of the tube to circulate through the coiled tube inside the flue and returns back to the room after capturing heat from inside the flue.

The idea behind this tool is to reclaim some of the heat that’s lost up through the flue. Anyone in the US have any experience with something like this?

View attachment 299066

Just another version of an old (and very bad idea) if run properly there should not be heat left to spare in the flue.

 

[begreen]

Yes, another variation of stack robbers which are generally to be avoided.

 

[ZKlongisland]

Thank you all for the feedback. It was a novel idea to me, but I see it's been tried before.

 

[begreen]

They were popular back in the late 70s when unbaffled smoke dragons sent lots of heat (and smoke) up a single-wall stovepipe. They were often known as creosote generators, especially when someone was burning poorly seasoned firewood. Modern stoves connected to double-wall stove pipe are much more efficient when burning dry wood properly.

 

[Recoheat]

Your other respondents are quite right in that the risk of taking heat out of the flue gases is that it's done to the extent that the flue ceases to draw or causes a build up of creosote. That's very easy to detect and is a very fast process, as they'll know. We avoid it by using turbulent air to extract the heat very efficiently, using the turbulent boundary layer effect, from the small proportion of the total volume of flue gases that pass over our coil. The coil itself also causes turbulence in the flue gases that reduces the pressure on them and causes them to accelerate around the coil - exacerbated by the foiling effect of the curved surfaces. The result is that the theoretical heat extraction in a steady (and therefore non-realistic) flow is 5%. In reality, the external test engineers who conducted our safety certification could detect no variation in the flue gas temperatures or velocity. The consequence is that the device does actually work, and although it would offer additional surfaces for creosote to sit on in a badly functioning flue, in a normal healthy installation the coil will attract less soot than the flue sides, because the coil is in the hottest part of the flow. We've sold over 400 now and they achieve an increase in heat displacement from the stove that is far greater than anything else, to the extent many people use them with their stove in place of central heating - as attested on our TrustPilot reviews. So while the concept is perhaps old, and the theoretical objections are absolutely valid, our device does actually work really well!

 

[begreen]

This may be a partial solution for those with older inefficient stoves. At $350-400 it would be better for most to invest in an efficient stove and not have to deal with the extra hardware and noise of the pump.

 

[Recoheat]

This may be a partial solution for those with older inefficient stoves. At $350-400 it would be better for most to invest in an efficient stove and not have to deal with the extra hardware and noise of the pump.

The device addresses the efficiency of the stove by circulating the heat from combustion via the powered system rather than convection, so by definition it transforms that, given it uses 17w of electricity to distribute up to 1kw of heat before the stove is hot and for hours as it cools. But another major factor is the distance of the distribution rather than just the volume: stove efficiency does not equate to useful distributed warmth if the heat produced goes straight into a cavity or chimney breast, so driving the heat away from the stove and circulating it as the device does, brings an increase in the effective heat distribution that no stove can achieve by itself. It's certainly not something everyone will want, but in terms of performance, it achieves an enormous amount.

 

[begreen]

Yes, for a freestanding, radiant stove placed inside of a fireplace cavity I can see the potential benefit, if one is ok with the pump noise. Normally, we recommend a convective insert for this type of installation. There are many insert models available here in the states. UK stoves and installations are often different from here in the US and Canada and frequently less efficient.

 

[Recoheat]

Yes, for a freestanding, radiant stove placed inside of a fireplace cavity I can see the potential benefit, if one is ok with the pump noise. Normally, we recommend a convective insert for this type of installation. There are many insert models available here in the states. UK stoves and installations are often different from here in the US and Canada and frequently less efficient.

Yes, that's really interesting to see. Quite a lot of the issues the Recoheat addresses come from the fact that UK legislation has turned a chimney from a heat storage unit into an insulated vent, so that heat that would traditionally be harnessed to release warmth over time and higher up the house is just wasted. There are few systems as efficient as a Roman hypocaust, which contravene UK regulations. Great to see how things are addressed more intelligently in other traditions, technologies and regulatory frameworks. We have a small number of customers in the States and have seen it as a potentially wonderful fit for the product for many reasons - and the quality of the technological innovation in the market is one of them.

 

[begreen]

Yes, there are still many legacy, pre-EPA stoves in service here. Most are from the 1970s and early 1980s. They were often built like tanks and thus their long lifespan, but they are much less efficient with high flue temperatures unless the stove was choked down to smoldering. This led to the trend at the time of using a stack robber (magic heat and others) to recover some of that waste heat. That fad thankfully has died out with much more efficient stoves and double-walled stove pipe.

It's good to see that a more efficient solution has been developed. The old fan powered versions sold here were very noisy and often known as creosote machines. We have a Classics forum here for the legacy stoves.

Classic Wood Stove Forums (prior to approx. 1993)

This forum is for discussion of older stoves - built during the stove boom of the first oil shocks up until the EPA regs started to take hold. Brands include Fisher, All Nighter, Timberline and many models of current makers like Vermont Castings, Jotul, etc.

www.hearth.com

 

[Recoheat]

Yes, there are still many legacy, pre-EPA stoves in service here. Most are from the 1970s and early 1980s. They were often built like tanks and thus their long lifespan, but they are much less efficient with high flue temperatures unless the stove was choked down to smoldering. This led to the trend at the time of using a stack robber (magic heat and others) to recover some of that waste heat. We have a Classics forum here for these stoves.

There's certainly a place for them, and our device certainly adds more to them than more efficient models, but the majority of the units we're running on will be about 80% efficient. There is plenty of high quality stove designs and manufacturers in the UK - the issue is more in the constraints on ancillaries. Using a catalytic converter is not recommended by a leading industry body, for example - absolute nonsense.

 

[bholler]

Your other respondents are quite right in that the risk of taking heat out of the flue gases is that it's done to the extent that the flue ceases to draw or causes a build up of creosote. That's very easy to detect and is a very fast process, as they'll know. We avoid it by using turbulent air to extract the heat very efficiently, using the turbulent boundary layer effect, from the small proportion of the total volume of flue gases that pass over our coil. The coil itself also causes turbulence in the flue gases that reduces the pressure on them and causes them to accelerate around the coil - exacerbated by the foiling effect of the curved surfaces. The result is that the theoretical heat extraction in a steady (and therefore non-realistic) flow is 5%. In reality, the external test engineers who conducted our safety certification could detect no variation in the flue gas temperatures or velocity. The consequence is that the device does actually work, and although it would offer additional surfaces for creosote to sit on in a badly functioning flue, in a normal healthy installation the coil will attract less soot than the flue sides, because the coil is in the hottest part of the flow. We've sold over 400 now and they achieve an increase in heat displacement from the stove that is far greater than anything else, to the extent many people use them with their stove in place of central heating - as attested on our TrustPilot reviews. So while the concept is perhaps old, and the theoretical objections are absolutely valid, our device does actually work really well!

If you aren't reducing the flue gas temperatures where are the btus your device is putting out coming from?

 

[Recoheat]

If you aren't reducing the flue gas temperatures where are the btus your device is putting out coming from?

We are, by a calculated 5%. The total can only be relative to the surface area of the coil because the flue gases aren't constrained so we can only remove as much heat as is passed through that surface area by the gas that comes into contact with it - which is a small proportion of the total passing through the flue. That total surface area equates to a 10cm increase in flue length on a 5" flue, and a 6cm increase on a 6" flue. Any stove has to be able to deal with a variation in flue length of many metres, so this negligible difference can't effect the healthy function. The effect of the device doesn't actually derive from the volume of heated air it produced, which is very small in comparison to the area it heats. It works by creating an oscillating jet that entrains and stirs the heat from the flue and crucially from the stove so that it warms the whole area. The physics derives from the thermal qualities of turbulent air, so it's fairly bizarre, but entirely consistent, demonstrable and effective.

 

[bholler]

We are, by a calculated 5%. The total can only be relative to the surface area of the coil because the flue gases aren't constrained so we can only remove as much heat as is passed through that surface area by the gas that comes into contact with it - which is a small proportion of the total passing through the flue. That total surface area equates to a 10cm increase in flue length on a 5" flue, and a 6cm increase on a 6" flue. Any stove has to be able to deal with a variation in flue length of many metres, so this negligible difference can't effect the healthy function. The effect of the device doesn't actually derive from the volume of heated air it produced, which is very small in comparison to the area it heats. It works by creating an oscillating jet that entrains and stirs the heat from the flue and crucially from the stove so that it warms the whole area. The physics derives from the thermal qualities of turbulent air, so it's fairly bizarre, but entirely consistent, demonstrable and effective.

So your only taking 5% of the btus out of the exhaust? On a modern stove that is an extremely low number

 

[Recoheat]

So your only taking 5% of the btus out of the exhaust? On a modern stove that is an extremely low number

It's not actually. You're getting too caught up in the theoretical proportions: if you light a fire, it kicks out a lot of heat, which vents via the flue - it's just that the heat is carried in a lot of air. A kindling burn heats the air flow to 520C within a couple of minutes, and a steady burn produces a flow at around 420C at the nozzle. Modelling it does reflect that, but the empirical evidence is really easy to demonstrate, however counter-intuitive it is. The reason it's not been exploited in this context before is that it is so counter-intuitive but that the system works is beyond question - we've got too many customers who'll vouch for it, but it's as simple to demonstrate as lighting a fire and switching it on. It's a really satisfying product to work with because of the effect - it's not subtle! If you look at our TrustPilot reviews, including from the States, you'll see there's no question of how effective it is. How safe is a longer term question, but we've had units in place since 2014 and have been in the market properly since 2020, so as well as all the engineering testing, we have a solid customer base to attest that.

 

[bholler]

It's not actually. You're getting too caught up in the theoretical proportions: if you light a fire, it kicks out a lot of heat, which vents via the flue - it's just that the heat is carried in a lot of air. A kindling burn heats the air flow to 520C within a couple of minutes, and a steady burn produces a flow at around 420C at the nozzle. Modelling it does reflect that, but the empirical evidence is really easy to demonstrate, however counter-intuitive it is. The reason it's not been exploited in this context before is that it is so counter-intuitive but that the system works is beyond question - we've got too many customers who'll vouch for it, but it's as simple to demonstrate as lighting a fire and switching it on. It's a really satisfying product to work with because of the effect - it's not subtle! If you look at our TrustPilot reviews, including from the States, you'll see there's no question of how effective it is. How safe is a longer term question, but we've had units in place since 2014 and have been in the market properly since 2020, so as well as all the engineering testing, we have a solid customer base to attest that.

Again where are the btus coming from if you aren't reducing stack temps? Many modern stoves are already running really low exhaust temps and don't have much of any heat to spare.

 

[Recoheat]

Again where are the btus coming from if you aren't reducing stack temps? Many modern stoves are already running really low exhaust temps and don't have much of any heat to spare.

It's not that it isn't reducing the stack temperature, as stated: it's the fact that the heat extracted is so relatively insignificant and falls within the necessary redundancy to allow a stove to have a variable height stack. A stove does not have a balanced flue - and our system is not made for and does not comply with the requirements for a balanced flue. An unbalanced flue cannot be so sensitive it can't deal with a variation of at least 10 metres of stack height or it also wouldn't be able to function in a varying external temperature range of 10C. You'll know that a single skin flue passes 1kw of heat per metre - again, a stove has to be able to deal with that or it's not commercially viable. It is true that some manufacturers are throttling the output of their flues to the extent that the stoves suffer from smoke-back in order to achieve impractical efficiency ratings, and our units shouldn't be fitted in those circumstances. But in a healthy flue system, the device functions without causing issues.

 

[Recoheat]

It's not that it isn't reducing the stack temperature, as stated: it's the fact that the heat extracted is so relatively insignificant and falls within the necessary redundancy to allow a stove to have a variable height stack. A stove does not have a balanced flue - and our system is not made for and does not comply with the requirements for a balanced flue. An unbalanced flue cannot be so sensitive it can't deal with a variation of at least 10 metres of stack height or it also wouldn't be able to function in a varying external temperature range of 10C. You'll know that a single skin flue passes 1kw of heat per metre - again, a stove has to be able to deal with that or it's not commercially viable. It is true that some manufacturers are throttling the output of their flues to the extent that the stoves suffer from smoke-back in order to achieve impractical efficiency ratings, and our units shouldn't be fitted in those circumstances. But in a healthy flue system, the device functions without causing issues.

Here's a model of the thermal profile, conducted by external engineering consultants Cambridge Engineering Analysis and Design: The video has two models running in synch - one with the Recoheat functioning and one without.

 

[bholler]

Here's a model of the thermal profile, conducted by external engineering consultants Cambridge Engineering Analysis and Design: The video has two models running in synch - one with the Recoheat functioning and one without.

Again for older less efficient stoves I'm sure it will work fine. But new high efficiency stuff I wouldn't be comfortable using it

 

[begreen]

Again where are the btus coming from if you aren't reducing stack temps? Many modern stoves are already running really low exhaust temps and don't have much of any heat to spare.

And, if they are in single-story homes, not a lot of draft to spare.

It's not that it isn't reducing the stack temperature, as stated: it's the fact that the heat extracted is so relatively insignificant and falls within the necessary redundancy to allow a stove to have a variable height stack. A stove does not have a balanced flue - and our system is not made for and does not comply with the requirements for a balanced flue. An unbalanced flue cannot be so sensitive it can't deal with a variation of at least 10 metres of stack height or it also wouldn't be able to function in a varying external temperature range of 10C. You'll know that a single skin flue passes 1kw of heat per metre - again, a stove has to be able to deal with that or it's not commercially viable. It is true that some manufacturers are throttling the output of their flues to the extent that the stoves suffer from smoke-back in order to achieve impractical efficiency ratings, and our units shouldn't be fitted in those circumstances. But in a healthy flue system, the device functions without causing issues.

A ten meter stack is likely to be a problem anywhere. That doesn't sound like an average use case. I think the greater concern is in single-story homes that are more common here. They often have near marginal flue system height of around 4 meters. The average US flue system height is 4 to 8 meters here. More than that tends to overdraft and needs dampering.

 

[Rich L]

I'm convinced this item will work for me.I'll be the test dummy and report back next season as how it works out.If I understand correctly what Recoheat is saying I'll have more heat for a longer period of time.If true no needs for a CAT stove at 4-5 grand plus CAT purchases whenever needed.

 

[bholler]

I'm convinced this item will work for me.I'll be the test dummy and report back next season as how it works out.If I understand correctly what Recoheat is saying I'll have more heat for a longer period of time.If true no needs for a CAT stove at 4-5 grand plus CAT purchases whenever needed.

They are saying lots of stuff

 

[kborndale]

I'm convinced this item will work for me.I'll be the test dummy and report back next season as how it works out.If I understand correctly what Recoheat is saying I'll have more heat for a longer period of time.If true no needs for a CAT stove at 4-5 grand plus CAT purchases whenever needed.

If the goal is to create a lot of creosote and have a chimney fire and burn your house down then yes, it will work for you!

 

[Todd]

Keep an eye on those flue temps