How much heat could I expect a heat transfer system to move from a log burner?

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davecla

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#272818 17-Jul-2020 17:20

I know there are a lot of variables for this, but hoping someone has done this before me and can share their experiences...

I'm in the early stages of getting a new log burner put into a smallish living room (5m x 4m), it previously it had an older Metro Aspire log burner which was rated at approximately 18-20kW max output. The old log burner did a great job of heating the room and some of the rest of the house, without being too over powering. This living room has just been relined, lots of wall and ceiling insulation and double glazing - so should be much more user friendly.

What I'm wanting to do, is take some of the heat from the new log burner and move it around the house. Directly behind the spot where the log burner will be is the return vent for a ducted heat pump, this return vent is mounted in the ceiling. My plan is to add opening near ceiling level behind the flue to allow hot air to flow directly into the heat pump return, and hopefully then just run the heat pump in circulation mode to move the air about the house. The return vent duct is 400mm in diameter, so there is an opportunity to move a fair bit of hot air about the place.

What I can't get any sort of feeling for is how much heat I can expect to move this way. One guy in the heating industry suggested 2kW at best which sounds like it would be hardly worth the effort.

If the heat transfer system could move, say 5kW of heat, then I’d look to buy one of the bigger log burners available, something around 24kW. But if the transfer system will only move 2kW of heat, then this will be far too big for the small room.

(the heat pump itself is an older one and is useless when it's cold outside < 4 degrees, 1950's house, poorly insulated, about 240m2 in total)

I'm mostly leaning to get another mid size log burner - say 20kW like the old one, but if this system could move a lot of heat, then bigger would be better.

What to do??

Dave

snnet

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#2524871 17-Jul-2020 17:39

I have done many of these installs -- use a good system with insulated ducting, install the inlet at the opposite end of the room so that room heats up first, if there's a thermostat place it at the opposite end of the room as well. Every time I've done this with a wood burner it's heated the whole home great (with outlets in each room)

I wouldn't try and hook it into an existing heat pump system as the flow of air will be restricted by the system's thermostat

Tracer

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#2524971 17-Jul-2020 20:18

snnet:

I have done many of these installs -- use a good system with insulated ducting, install the inlet at the opposite end of the room so that room heats up first, if there's a thermostat place it at the opposite end of the room as well. Every time I've done this with a wood burner it's heated the whole home great (with outlets in each room)

I wouldn't try and hook it into an existing heat pump system as the flow of air will be restricted by the system's thermostat

You'd also want the inlet to be away from the door going to the hallway/bedrooms otherwise you end up circulating cooler air through the hallway and not so much heat.

-e- s/outlet/inlet/

AklBen

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#2525050 18-Jul-2020 08:30

We had a quote for one the other week and the installer recommended not stretching to a 4 bedroom system and only going with 3. He said the heat transfer once you go past 3 rooms becomes "disappointing" for the money. Not sure if this is true, generally I've seen most systems are a max of 3 vents.

Since getting the quote we had our fireplace made compliant (from the previous owners) we are wondering whether or not the system is required as the heat is actually moving through the house quite well.

Have most people installed heat transfer systems because their fireplace doesn't tend to distribute heat that well? Because if your fireplace distributes heat, then are you mostly doing it for speed of transfer and consistency?

PANiCnz

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#2525055 18-Jul-2020 08:57

snnet:

I have done many of these installs -- use a good system with insulated ducting, install the inlet at the opposite end of the room so that room heats up first, if there's a thermostat place it at the opposite end of the room as well. Every time I've done this with a wood burner it's heated the whole home great (with outlets in each room)

Surely you want the inlet reasonably close to the fire? That way the temperature of the intake air is at its highest and helps to offset temperature lost in the ducting.

AklBen

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#2525057 18-Jul-2020 09:02

PANiCnz:

snnet:

I have done many of these installs -- use a good system with insulated ducting, install the inlet at the opposite end of the room so that room heats up first, if there's a thermostat place it at the opposite end of the room as well. Every time I've done this with a wood burner it's heated the whole home great (with outlets in each room)

Surely you want the inlet reasonably close to the fire? That way the temperature of the intake air is at its highest and helps to offset temperature lost in the ducting.

Reasonably but not too close as it can draw too much heat from the room that it's in.

davecla

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#2525079 18-Jul-2020 10:39

AklBen:

Reasonably but not too close as it can draw too much heat from the room that it's in.

I guess, this is the bit I wanted to know - if there is the potential to take too much heat - I'll buy a bigger log burner. My concern is I wont be able to take enough heat away.....

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SomeoneSomewhere

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#2525134 18-Jul-2020 11:21

Most ducted heat pumps have variable fan speeds. If it's moving too much heat, turn the fan speed down.

It will depend on what the system is, but if it's a really old one you might be able to do some clever things with the controls like run it in fan mode when the room is above a certain temperature. Unfortunately most of the newer ones are too integrated to have much flexibility.

SomeoneSomewhere

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#2525136 18-Jul-2020 11:30

A 400mm duct could take the better part of a cubic meter per second. This suggests that with a 10C split (e.g. 30C intake, 20C in the rooms), you could move as much as 12kW.

davecla

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#2525146 18-Jul-2020 12:09

I'm thinking perhaps the best approach is to hire something like this first and see what happens.

Bung

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#2525150 18-Jul-2020 12:20

"This heater uses radiating infrared heat which heats objects rather than air. The heat is evenly distributed in all directions while maintaining a great level of safety. The radiating heat is never scalding and the heater itself never becomes hot. "

How will you transfer that?

davecla

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#2525152 18-Jul-2020 12:30

Bung: "This heater uses radiating infrared heat which heats objects rather than air. The heat is evenly distributed in all directions while maintaining a great level of safety. The radiating heat is never scalding and the heater itself never becomes hot. "

How will you transfer that?

Most log burners advertise themselves as being radiant or convection types. Blurb here.

Most seem to be radiant.

My experience with radiant log burners (I have another one in another room) as the air still gets hot, they just don't construct them to move the air though like those made specifically for convection.

I'm no expert so hopefully someone can correct or confirm.

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snnet

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#2525328 18-Jul-2020 21:00

AklBen:

We had a quote for one the other week and the installer recommended not stretching to a 4 bedroom system and only going with 3. He said the heat transfer once you go past 3 rooms becomes "disappointing" for the money. Not sure if this is true, generally I've seen most systems are a max of 3 vents.

Since getting the quote we had our fireplace made compliant (from the previous owners) we are wondering whether or not the system is required as the heat is actually moving through the house quite well.

Have most people installed heat transfer systems because their fireplace doesn't tend to distribute heat that well? Because if your fireplace distributes heat, then are you mostly doing it for speed of transfer and consistency?

Yes, the ones I have done have wanted it because the heat from the wood burner doesn't penetrate thru the house. The last few I've done have been addons to a smartvent system where there are at least 4 outlets in rooms and they've been very happy with the results.

snnet

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#2525329 18-Jul-2020 21:01

PANiCnz:

snnet:

I have done many of these installs -- use a good system with insulated ducting, install the inlet at the opposite end of the room so that room heats up first, if there's a thermostat place it at the opposite end of the room as well. Every time I've done this with a wood burner it's heated the whole home great (with outlets in each room)

Surely you want the inlet reasonably close to the fire? That way the temperature of the intake air is at its highest and helps to offset temperature lost in the ducting.

4m away seems to be a good place