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7 comments
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Join Date: Mar 2008
Posts: 6

Heat Recovery From Chimney of Domestic Boiler

08/28/2009 1:27 AM

Hi all, I'm looking for a heat recovery jacket that can be fitted to the chimney / flue pipe of a domestic boiler. I'm looking to preheat the domestic water supply prior to inlet of the boiler. Does anyone know of a supplier or manufacture who has this type of jacket available?

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Guru
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#1

Re: heat recovery from chimney of domestic boiler

08/28/2009 3:16 AM

A word of caution if considering doing this:

Do not let the stack temperature fall below the dewpoint of sulphuric acid as a result of operating the equipment, otherwise the corrosive effects within both it and the boiler will be worth rather more than the value of the combustion energy recuperated.

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#2
In reply to #1

Re: heat recovery from chimney of domestic boiler

08/28/2009 3:58 AM

Thanks.

I have considered the issue of gases condesating and as you so rightly pointed out the consequences of this happening will far out weigh the value if this happens.

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Guru

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#6
In reply to #2

Re: heat recovery from chimney of domestic boiler

08/29/2009 2:36 AM

The heat transfer coefficient on the flue gas side will be only about 2% of that on the water side. This implies that the heat exchanger may have to be designed with extended surface (fins) on the gas side. Fins tend to get clogged easily and pose cleaning problems.

In a countercurrent design, the dew point issue may not be severe as the residence time will be very short.

Bioramani

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#3
In reply to #1

Re: heat recovery from chimney of domestic boiler

08/28/2009 10:36 PM

Great answer.

milo

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Anonymous Poster
#4

Re: Heat Recovery From Chimney of Domestic Boiler

08/28/2009 10:47 PM

If things are close enough to be done cost effectively, you could install a coil over the outside of the exhaust at the top and preheat some water that way. or at the top maybe stick in a gas water heater to act as a preheat tank. just dont connect the gas line and use the boilers exhaust heat to warm the tank at the top. you could use a drip lip and drain to catch and divert any condensation from the tank. just watch for too high a temp, and keep all the tanks safetys in place

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Power-User

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#5

Re: Heat Recovery From Chimney of Domestic Boiler

08/29/2009 1:41 AM

Jacket can be fabricated by most fabricators. What can be the fluid in the jacket should be the question. The boiler inlet air (normally by FD fans) can be put in the jacket before going to the boiler to reduce fuel consumption

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Power-User

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#7

Re: Heat Recovery From Chimney of Domestic Boiler

08/30/2009 5:40 AM

We once lived in a caravan/trailer home where the domestic hot water came from a galvanised tank with a short section of chimney pipe through the middle. The pipe was single-wall and very hot up to a couple of feet above the stove. It worked well until the flue pipe corroded and the leaking water put the stove out! We lost not only the hot water but the heating stove too - pretty serious in Winter in the far North of Scotland with a new baby to look after.

To use this method of heat transfer the tank/coil needs to be close to the heat source and is acceptable on short chimneys. You will need to replace the normal double, insulated wall pipe with a single wall section or fit a pipe to stop the corrosive flue gases from eating the inside of the brickwork if it's an old chimney.

Be very careful if you have a long chimney as the flue gas depends on it's heat to rise up the pipe, if you take too much heat out the gas flow will stall and you could have a dangerous build up in the room below especially on calm days when there is no natural draught to pull the gases away or if your room is sealed to stop draughts. Remember carbon monoxide is undetectable until it kills you!

As long as you understand the risks and monitor the effects you should be ok.

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andyd53 (1); Anonymous Poster (1); b v rao (1); bioramani (1); Milo (1); PWSlack (1); tom (1)

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