Condensing: or not condensing?

[Jaded]

Hi. Ideal 'Icos HE15', I was told by a plumber of many years experience, that my boiler would not work in condensing mode unless the the rotary heat setting control was left fully clockwise (max or 5?). I have recently been told that condensing mode is operating also at any lower setting. Would anyone like to comment? please.
T I A
J

 

[stevetheplumber]

Hi. Ideal 'Icos HE15', I was told by a plumber of many years experience, that my boiler would not work in condensing mode unless the the rotary heat setting control was left fully clockwise (max or 5?). I have recently been told that condensing mode is operating also at any lower setting. Would anyone like to comment? please.
T I A
J

not true boilers will condense at any temperature from what i know they actually condense better at lower temperature

 

[Jaded]

Steve, thanks for comment. How does an observer know if/when the boiler is operating in condensing mode?
J

 

[doitmyself]

not true boilers will condense at any temperature from what i know they actually condense better at lower temperature

Condensing is dependent on the return temperature. This has to be below 55 deg C. It does not matter what the flow temp is.

Some boilers are still designed to run with an 11C differential, so if the flow temperature is above 66C the boiler will rarely condense. Other boilers now use a 20C differential, so you can be sure of condensing, provided the flow temp is below 75C. The problem with using a 20C differential is that most older heating systems have radiators which have been sized assuming an 11C differential. If the differential is 20C, the rad output will be about 15% lower. Of course, if the house had been upgraded with better insulation, this may not matter as the rads will probably now be oversized.

 

[Jaded]

'doitmyself'. Thanks for your contribution. My original informant seemed like a decent, hard-working (and knowledgeable) plumber. If he was wrong, I can understand why others can also be wrong.
J

 

[Bernie2]

Yes its the low temperature below 55C that does the condensing. That is the temperature at which flue gases condense and turn into water.

So the idea is to get the temperature in the secondary heat exchanger as low as possible. That way the hot flue gases coming off the primary heat exchanger give off their heat to the cooler secondary heat exchanger.

Basically hot goes to cold is one of the principles at work.

Most UK c/h systems are designed for a return temperature of 11C lower than the flow.

But that means you can only turn the temperature up to 67C for it to stop condensing. The thing is, most people want it at about 82C in really cold weather. However that would mean a return temperature of about 71C too high for condensation to form.

The thing is though, its said that these boilers are still more efficient because of their design than a none condenser.
And its said that a flow temperature of 60 - 65C would be okay in a modern house that was well insulated. Also the higher flow temperature requirement would only be for 30 or so days a year.

 

[nzmanc]

Condensers are a little pointless in this country as the return temp is never low enough

 

[migoplumber]

hang on! if you have a rad with hot water in inside it at return temp of 55 degrees for example, (plenty enuf for condensate!!) the room in which it is mounted will eventually reach a maximum of 55 degrees.
weather compensators measure the outside temperature and change the boiler flow/return temps to suit, with an outside temp of 20 degrees the boiler flow could be set at 30 degrees, your condensing boiler would love that
with an outside temp of 5degrees your boiler could flow at 60 degrees and work perfectly well.

an understanding that boiler flow temperatures are directly linked to radiator temps is needed.
u/floor heating is designed for flow temps of 60 degrees.
why would you need any more than this.

my companies consultant engineers who earn ten times my wage save the taxpayers fuel bill millions work all this out, and is simple to understand if you think about about it.

 

[doitmyself]

hang on! if you have a rad with hot water in inside it at return temp of 55 degrees for example, (plenty enuf for condensate!!) the room in which it is mounted will eventually reach a maximum of 55 degrees.

Oh no it won't!

The return temp has to be higher than the room temperature. Let's say you need 1kW to heat room to 55C. If you install a 1kW radiator and have a return temp of 56C, then your flow temperature will have to be 335C to produce the 1kW of heat.

If you think a flow temp of 335C is a tad too high and want to limit it to 75C, still with a return temp of 55/56C, you would have to install a radiator with a nominal output of 19-20kW to provide a room temperature of 55C.

weather compensators measure the outside temperature and change the boiler flow/return temps to suit, with an outside temp of 20 degrees the boiler flow could be set at 30 degrees, your condensing boiler would love that
with an outside temp of 5degrees your boiler could flow at 60 degrees and work perfectly well.

You have to size the radiators correctly as well.

There is no reason why a heating system cannot work with flow temps of 60C and return of 40C when the temp is -1C. But you would have to install a 2kW radiator to provide the same heat as a 1kW running with 75/65C temperatures.

an understanding that boiler flow temperatures are directly linked to radiator temps is needed.

I would have thought that was obvious.

 

[migoplumber]

doitmyself, my above quote is a theory quote as anybody can see.
if you place a constant heat source into a room (i never mentioned radiator sizes) the room will eventually arrive at that temperature (minus a bit for losses)
basic physics.
of course radiator sizes can be made larger to allow for cooler boiler primaries, but that was not part or the original question.
thats obvious as well.

 

[radswilldo]

75-55 or 70-50 with radiators or the customer will be ringing. Good thing is when doing most boiler replacements or conversions (as long as the rads are under 20 years/not severely corroded) the existing rads are oversized due to double glazing and the installation/improvement of loft insulation. New installation I get 20 diff, existing 20 - 16. It is impossible to get more than 11(sometimes only 9 or eight) diff on small single rads.

 

[doitmyself]

if you place a constant heat source into a room (i never mentioned radiator sizes) the room will eventually arrive at that temperature (minus a bit for losses)

But a radiator is not a constant heat source, unlike an electric fire.

75-55 or 70-50 with radiators or the customer will be ringing.

Ah! The person who decides how warm the house is by feeling the radiator.

It is impossible to get more than 11(sometimes only 9 or eight) diff on small single rads.

The problem lies in the design of the typical LS valve, which may have 3-4 turns from open to closed but is fully open 1½ turns from closed. Most balancing takes place within one turn from closed and is very sensitive to change in valve opening. An eighth of a turn can make a difference.

The velocity of the water through a 15mm pipe will be about 1m/s, which means it can carry about 12kW -assuming a 20C differential. But a 500w radiator only requires a flow rate of 0.04 metres/sec, so the velocity has to be reduced by 96%. The LS valve will have to be virtually closed.

 

[Jaded]

All. Please take time for a breather. The last thing I want is any trouble. I would still be interested in an answer to my question as to whether it is possible for an observer (standing near to the boiler) to know when the boiler enters condensing mode. For example, does it make a slightly different noise; or can one hear the condensate trickling out. Any sign.

J