Keeping a boiler in condensing mode

Cardew

Most of us are aware that to achieve the very high efficiencies of around 90% promised by modern condensing boilers, they must be kept in 'condensing mode'.

Well just how is 'condensing mode' achieved by home owners? I have seen all sorts of conflicting 'theories' advanced by posters on MSE and elsewhere. A good illustration is a thread currently running where advice is sought on how to efficiently run a CH system.

https://forums.moneysavingexpert.com/discussion/comment/66730715#Comment_66730715

Without(I hope) sounding patronising, two helpful and informed posters have posted the following conflicting statements:

Generally, if condensing then the boiler likes to be up high,

A condensing combi boiler will run more efficient the lower you have it set (it uses the exhaust from the boiler to pre heat the water)

In another current thread an informed poster is apparently confident that it is necessary to run in 'low power mode' and posts

The aim should be to run in condensing mode as much as possible (i.e. low radiator water temperature which mean low radiator output, at 60C diff (80C-20C) rad = 100%, at 40C diff (60C-20C) rad = 60%).

A quote I don’t understand!

Jeff Howell writes in The Daily Telegraph -

http://www.telegraph.co.uk/property/propertyadvice/jeffhowell/7243393/Boiler-scrappage-scheme-Should-I-replace-my-ageing-Potterton-boiler.html

In my opinion, the alleged efficiency savings of new condensing boilers are largely fictitious. They are based on the manufacturers’ own data, applied to idealised theoretical conditions, and do not correspond with actual results when these new boilers are connected to the existing pipework and radiators in actual homes.

When sizing a boiler for a property, the maximum output for a Combi boiler is determined by the Domestic Hot Water(DHW) demand. For instance a 30kW combi boiler might be specified, but the Central Heating(CH) demand might only be, say, 12kW.

With modern(non-Combi) boilers it is the CH demand that is the determinate, as a DHW tank can be heated with a few kW. I have read several articles that state the output of these boilers should be matched to the ‘heating demand’ for maximum efficiency. No longer should boilers be over-specified.

However what is the ‘heating demand’ for our properties? At the moment many of us will be just heating one room; at other times we might be heating every room in the house. Whilst modern boilers can modulate their output, even the minimum output is in excess of the heating demand much of the time.

For 6 months of the year we might only have the boiler(combi and non-combi) on for DHW. What steps do we take to get maximum efficiency from a condensing boiler

As said in previous posts I have tried to get some answers from manufacturers and got nowhere. I have no idea how to keep boilers in condensing mode. Have you?

Discuss!!

lstar337

Cardew wrote: »

Most of us are aware that to achieve the very high efficiencies of around 90% promised by modern condensing boilers, they must be kept in 'condensing mode'.

Well just how is 'condensing mode' achieved by home owners? I have seen all sorts of conflicting 'theories' advanced by posters on MSE and elsewhere. A good illustration is a thread currently running where advice is sought on how to efficiently run a CH system.

https://forums.moneysavingexpert.com/discussion/comment/66730715#Comment_66730715

Without(I hope) sounding patronising, two helpful and informed posters have posted the following conflicting statements:





In another current thread an informed poster is apparently confident that it is necessary to run in 'low power mode' and posts



A quote I don’t understand!

Jeff Howell writes in The Daily Telegraph -

http://www.telegraph.co.uk/property/propertyadvice/jeffhowell/7243393/Boiler-scrappage-scheme-Should-I-replace-my-ageing-Potterton-boiler.html



When sizing a boiler for a property, the maximum output for a Combi boiler is determined by the Domestic Hot Water(DHW) demand. For instance a 30kW combi boiler might be specified, but the Central Heating(CH) demand might only be, say, 12kW.

With modern(non-Combi) boilers it is the CH demand that is the determinate, as a DHW tank can be heated with a few kW. I have read several articles that state the output of these boilers should be matched to the ‘heating demand’ for maximum efficiency. No longer should boilers be over-specified.

However what is the ‘heating demand’ for our properties? At the moment many of us will be just heating one room; at other times we might be heating every room in the house. Whilst modern boilers can modulate their output, even the minimum output is in excess of the heating demand much of the time.

For 6 months of the year we might only have the boiler(combi and non-combi) on for DHW. What steps do we take to get maximum efficiency from a condensing boiler

As said in previous posts I have tried to get some answers from manufacturers and got nowhere. I have no idea how to keep boilers in condensing mode. Have you?

Discuss!!

Well to be honest, the statement 'Generally the boiler likes to be up high' at its best doesn't mean anything, and at its worst is completely wrong.

I'm trying to think of the reasoning for posting it, but I just don't know what I was talking about at that moment in time. I would blame it on a senior moment, but I am too far from senior for that to fly. I guess I was having an off moment.

I should have left it at the point where I said to check the manual for an indication of an efficiency sweet spot!

I'm not an expert, but condensing is about return feeds. So for one thing it only works for heating and not for DHW. I believe a lower return feed from the radiators starts condensing mode. 50c and lower at the return will start condensing and the lower the return the greater the efficiency. I believe figures quoted are between 9% at the best (low return temp, like 20c) and 3% at worst (borderline return temp, like 50c).

This obviously is going to be different for each install (number of and size of rads will all affect this), but the boiler itself (especially with use of an outside thermometer for weather compensation) is meant to be able to regulate this so that it can stay in condensing mode for a certain percentage of its time, something like 95% of the time the heating is in use.

A lot of these claims are from boiler manufacturers and I have no data myself.

Most of my understanding of it is picked up through reading and I cant give any guarantees for any of it as I have filled in blanks myself and forgotten which bits came from people in the know and which bits are my own.

Now, to try to redeem a little of what I said in that earlier post:

The combi will likely have a temperature at which it uses best use of the gas (where it is most efficient), on mine it is marked with an 'e' on the dial but you may have to consult your handbook.

[STRIKE]Generally[/STRIKE], if condensing then the boiler likes to be up high.

On my boiler the 'e' position is over 3/4 of the way around the dial, which to me is 'high' at a glance. (Stupid boy) In reality, it is 'economy mode' which is lower down the scale than the graphic on the boiler suggests. It is the point at which the boiler uses all of its bag of tricks to keep itself in 'condensing mode' (modulating the flame and keeping the outgoing temperature at a level which keeps the return low enough for that 10% efficiency boost) for that 95% of the time.

Hopefully that clears up something. I have been typing fast and trying to put it all down sensibly to try and 'fix' something that has caused confusion for others. Sorry about that!

CashStrapped

From what I have read, without citing any sources, I was always under the impression that for a condensing boiler to be in "condensing mode" the return temperature of the water had to be around the dew point of water which in that situation would be approximatly around 55 degrees celsius or below.

This requires a heating set up that makes use of lower overall heating temperatures. So oversized radiators and/or underfloor heating system (which is the ideal combination).

However condensing boilers can often be retrofitted to existing systems, or installed with normal sized radiators, and run as a conventional boiler at much higher temperatutes.

This means the return temperature to the boiler may be as high as 70 degrees. Water will not condense at this temperature and the heat will not be extracted via the second heat exchanger.

There will be a lot of variables involved so it will require an installer to really know their stuff to ensure a system is installed and setup correctly to fully exploit the potential of a condensing boiler.

I have no doubt that the efficiency ratings for boilers are measured under ideal conditions though. Probably similar to mpg measurements for road cars, where they basically strip the car down and put it on a rolling road indoors to get the figures they require.

lstar337

CashStrapped wrote: »

From what I have read, without citing any sources, I was always under the impression that for a condensing boiler to be in "condensing mode" the return temperature of the water had to be around the dew point of water which in that situation would be approximatly around 55 degrees celsius or below.

Yes. The returning water temperature must be at least low enough to turn the water vapour to condensation to recover its latent heat. Not sure if I put that in my post above or not.

Andy_WSM

I have found mine puffs like a steam engine only if I keep the temperature setting down low. 55C-60C max. Any higher than that and once the system has warmed it stops condensing.

I really don't get how you can keep a condensing boiler effective or even efficient.

To make it condense you keep the temperatures low, keeping the temperatures low means it takes longer (a lot) to warm the house so you end up having the boiler on for a lot longer than you might if the system was running at 80C+

I am home all day now so happy for the boiler to chug along at a low temperature puffing condense out all day long, rather than a few short sharp blasts of high heat.

A word of warning to others though - if you do as I do, make sure the condensate drain pipe is well insulated otherwise you will join the hundreds of people that were posting on here during the winters of 2011/2012 saying their boilers had stopped as the pipes had frozen!

lstar337

Should it be puffing in condensing mode?

If effective, the water vapour should condense and run out the bottom through the condensate drain.

If water vapour is puffing out, then I don't think it is condensing at all.

Unless I am missing something?

CashStrapped

lstar337 wrote: »

Not sure if I put that in my post above or not.

If your making the point that I repeated some of what you asserted in your post, if you look at the posting time, were were writing a reply at pretty much the same time.

Don't worry, I did not copy your homework.:D

lstar337

CashStrapped wrote: »

If your making the point that I repeated some of what you asserted in your post, if you look at the posting time, were were writing a reply at pretty much the same time.

No not at all. It was just good info about the dew point of water that I wasn't sure if I mentioned as I was typing so fast and didn't read back before hitting the submit button!

I know we posted at the same time as your post appeared when the page refreshed after my submission.

CashStrapped wrote: »

Don't worry, I did not copy your homework.:D

Better not be! I worked hard on that. :rotfl:

CashStrapped

lstar337 wrote: »

Should it be puffing in condensing mode?

If effective, the water vapour should condense and run out the bottom through the condensate drain.

If water vapour is puffing out, then I don't think it is condensing at all.

Unless I am missing something?

This is usually seen as a good thing as it means the exhaust gas has already had heat extracted from it. As a result it is at a much lower temperature when it leaves the flue and therefore condenses (turns to steam) immediately.

If there were no steam, this would usually mean that the exhaust has was still a very high temperature.

lstar337

CashStrapped wrote: »

This is usually seen as a good thing as it means the exhaust gas has already had heat extracted from it. As a result it is at a much lower temperature when it leaves the flue and therefore condenses (turns to steam) immediately.

If there were no steam, this would usually mean that the exhaust has was still a very high temperature.

Ok. So presumably not a lot of the exhaust vapour is actually condensed then, because if it were wouldn't it run out as liquid?

Andy_WSM

lstar337 wrote: »

Ok. So presumably not a lot of the exhaust vapour is actually condensed then, because if it were wouldn't it run out as liquid?

Yes, a LOT of liquid is produced when it's puffing away, which is why I warned people to keep their drains from freezing.