Why is the Hive so popular?

[JonathanM]

There is a saving with a modulating boiler but it's just one of many little improvements in the thing that actually costs you money when operating, the boiler itself.

What is your view on whether a boiler actually operates more efficiently when the burner turns down? I don't mean because of increased condensing or less cycling but the actual act of burning gas and transferring heat. I've read several different takes on this. Most seem to say yes but that there is a point at which greater modulation stops being more efficient because of things like more excess air being needed to keep the flame stable. But at least one comment I read said the opposite and that it was less efficient when turned down. If it does operate more efficiently then what is the explanation? The main thing I've read trying to explain it was the quote below from Heat Geek, which seems sensible on the face of it, but is it correct?

When boilers components i.e. the heat exchanger and the combustion chamber, are designed, they are sized to effectively transfer the maximum amount of heat as efficiently as possible. Both of these components are more efficient, bigger.

A larger combustion chamber gives more room for the natural gas and oxygen to evenly mix and give a more complete combustion/flame efficiency. A larger heat exchanger gives more chance for the heat to transfer into the heating system water.

When boilers modulate down these components stay the same size, meaning they effectively become oversized. This increases the heat exchanger's relative surface area and 'heat transfer coefficient'. The larger combustion chamber gives lower NOX levels and fewer unburned gases.

 

[vulcancontinental]

What is your view on whether a boiler actually operates more efficiently when the burner turns down? I don't mean because of increased condensing or less cycling but the actual act of burning gas and transferring heat. I've read several different takes on this. Most seem to say yes but that there is a point at which greater modulation stops being more efficient because of things like more excess air being needed to keep the flame stable. But at least one comment I read said the opposite and that it was less efficient when turned down. If it does operate more efficiently then what is the explanation? The main thing I've read trying to explain it was the quote below from Heat Geek, which seems sensible on the face of it, but is it correct?

From your other posts I know you realise that there are many aspects to the combustion process so it is correct in one respect that the surface of heat exchanger can increase the possibility of improved transfer also increases, however not so sure about NoX as that's flame temperature not surface contact and unburnt gases are a function of the burner not heat exchanger.

The lower the flame flow the more unsteady the flame so to get over that add additional air which stabilises things however the air is not required for combustion but stability and is therefore excess air which is not good as it will serve no purpose other than to remove heat with it as it passes into the flue.

People are getting their knickers in a twist looking for product plus marketing points which do not tell the whole story. There are tales of very high modulating boilers cycling at really low temperatures theoretically maxing out efficiency then losing it because when the boiler refires it fires at a fan speed for safe ignition and produces too much heat for the low temp system, so flow thermistors shut it down again. The manufacturer says that's normal after marketing tit as a plus; which it is on a test bench. It can be overcome but with changes in system and running not mentioned in the marketing.

It's quite possible to have a boiler running at high temperature outperforming a boiler running at condensing temperatures if the heat exchanger is that much better. Before thinking about latent heat you have to remove the sensible heat.

It's not all b*ll*cks but marketing makes much of it so.

 

[JonathanM]

Thanks, great info and really interesting as always.

The lower the flame flow the more unsteady the flame so to get over that add additional air which stabilises things however the air is not required for combustion but stability and is therefore excess air which is not good as it will serve no purpose other than to remove heat with it as it passes into the flue.

I hadn't understood the reason before but that makes sense now.