Balanced Flue Gas Fire Question

[GeddyMortgage]

is that an opening velux window if so that flue terminal looks far too close the usual distance is 2metres away when below velux

It’s not, it’s a fixed unopenable skylight.

 

[JonathanM]

So ran it for 3hrs tonight, checked the meter on the hour, every hour, the fire if nothing else is consistent. But the figure has thrown another layer of confusion.

Did the room get really hot? I can't remember what the weather was like. But I'd have thought 7kW for three hours would have been pretty intense.

 

[polesapart]

Drying plaster will make the room feel cooler.

 

[GeddyMortgage]

Did the room get really hot? I can't remember what the weather was like. But I'd have thought 7kW for three hours would have been pretty intense.

Not massively. It was warm but not as warm compared to if I’d had my CH on in there for 3hrs. (2 x type 22 rads)

 

[MNW67]

We are looking to get a new gas fire and I remembered this thread.

I got some more info today about how glass fronted fires work. I was told that the glass traps most of the heat inside the firebox. Cold air passes through a heat exchanger behind the firebox and then rises by convection through a vent at the top of the fire and into the room . So, with glass fronted fires, most of the heat comes through convection rather than directly from the flame and coal bed etc.

 

[GeddyMortgage]

Yep, exactly right - I never did close the loop on this thread, but there is another post on this forum where I found all the roof and walls in this extension were incredibly shoddily insulated.... This has since been rectified

Since then, this fire has proved to be more than adequate. it's actually arguably too powerful, I could've gone with a 5kw honestly and it'd have been ample.

Shows the different of heat retention when the building work isnt done to standard!

 

[MNW67]

Now that we're looking for a gas fire ourselves, I've had to think about this a bit more, and I reckon I finally understand it.

Say a gas fire has an input of 3.7kW and an output of 2.7kW. A simple calculation shows that is 73% efficient. That is called gross efficiency. But 11% of the energy from a gas fire is lost up the chimney in the form of water vapour. So, they adjust for this to get to a net efficiency figure. In this case that would be about 82%. And that is the figure which is usually quoted. Which is a bit naughty, IMHO. They do the same on the continent with boilers, where it is common to see efficiencies of 108% quoted. Here in the UK, we seem to quote gross efficiency for boilers and net efficiency for gas fires.

I’m wondering if the difference between the input and output only partly influences the efficiency.

it definitely looks like it’s part of the calculation. Like on the image below the lower efficiency conventional flue types have a bigger difference between in and output. But still don’t add up. So there must be another factor that affects the calculation?

To be fair I don’t think you’re far off in real terms. They’re all like 75% based on in and output

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