Author Topic: Is my condensing boiler working correctly?  (Read 417 times)

Pine

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Is my condensing boiler working correctly?
« on: February 10, 2019, 11:39:07 AM »
I have Glowworm 30cxi boiler that was in the house when I moved in but is coming up for 10 years old. I have the central heating output set to 70. Any lower the lounge, which is a large room with one radiator, would take all day to get to 21 degrees. The boiler quickly heats up to 70 and will run at this temp for a while then the output temp will increase to 71 and eventually to 72 which makes the burner shut down. I can hear the pump running as the thermostat is still demanding heat but it is quite a long time before the circulating water cools down and the boiler fires up again, so it takes an absolute age to heat the house in the morning.

So is this running of the central heating without the burner fired up normal operation for this type of boiler?

culzean

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Re: Is my condensing boiler working correctly?
« Reply #1 on: February 10, 2019, 11:57:15 AM »
I have Glowworm 30cxi boiler that was in the house when I moved in but is coming up for 10 years old. I have the central heating output set to 70. Any lower the lounge, which is a large room with one radiator, would take all day to get to 21 degrees. The boiler quickly heats up to 70 and will run at this temp for a while then the output temp will increase to 71 and eventually to 72 which makes the burner shut down. I can hear the pump running as the thermostat is still demanding heat but it is quite a long time before the circulating water cools down and the boiler fires up again, so it takes an absolute age to heat the house in the morning.

So is this running of the central heating without the burner fired up normal operation for this type of boiler?

Yes the pump has an overrun timer to keep water flowing through boiler after burner shuts off this is to stop boiler heat exchanger getting too hot and to cool water down by flowing it around radiators so that boiler can restart. There is also a timer in boiler to stop burner being started too soon after it has shut down to stop boiler cycling too fast. It is bad to have thermostatic valves on all radiators, and especially the radiator nearest the thermostat, as the thermostatic valves start shutting down flow through radiator and reduce its temperature, so thermostatic valve ends up fighting room thermostat.   With a condensing boiler it is recommended that only half of radiators have thermostatic valves.  If the return flow to boiler is above 55deg C the boiler will not condense ( which means it runs at 70% efficiency instead of 90% ), so the thermostatic valves reducing flow to some radiators effectively raise temperature of water returning to boiler.  They actually recommend to get best out of condensing boiler you fit larger radiators and turn the boiler temperature control down to  65 degrees C, this means hot water tank can reach over 50 degrees which is about right to not scald you or your family.

The lounge in most houses has another heat source ( gas fire ) so the radiator that is fitted is normally smaller than it should be.  Think about getting a larger or it is easier to get a double panel radiator fitted ( same size but heating area is doubled meaning twice as much heat output ).

http://www.yougen.co.uk/blog-entry/1946/Is+your+condensing+boiler+actually+condensing'3F/
« Last Edit: February 10, 2019, 12:15:02 PM by culzean »
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Rory

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Re: Is my condensing boiler working correctly?
« Reply #2 on: February 10, 2019, 01:17:24 PM »
Yes the pump has an overrun timer to keep water flowing through boiler after burner shuts off this is to stop boiler heat exchanger getting too hot and to cool water down by flowing it around radiators so that boiler can restart.

If there's a still a demand for heat then the pump will continue to run - certainly ours does, and the OP suggests his does.   The overrun timer only comes into play once the demand is satisfied.

Yes it's normal for the boiler to stay off for a while - ours is 3 mins.  Which doesn't seem long but if the house is coldish, it's enough to lose a lot of heat.

Daughter had this issue in her house and turned out the central heating was single pipe - common until not that long ago (her house was built 1970).   Water was whizzing around the single pipe and getting back to the boiler barely having cooled down, so the boiler shut off.  With the pump still running the whole system cooled down again.   House got warm eventually but in a lot of small steps.
« Last Edit: February 10, 2019, 01:18:55 PM by Rory »

culzean

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Re: Is my condensing boiler working correctly?
« Reply #3 on: February 10, 2019, 01:45:22 PM »
The biggest problem with condensing boilers is that if too many radiators have thermoststatic valves fitted then the water in the circuit is still too hot when it gets back to boiler.  Thermostat valves do not suddenly cut off flow through radiator when the set temperature is reached, they start cutting of water as temperature in room rises and cut it off completely at set temp.  I have opened up all the thermoststic valves on our radiators fully and water going back to boiler is noticeably cooler than flow, also as radiators need replacing I am fitting double panel radiators instead of single. 

If you look at the outlet to the flue on your boiler when it is running you should see a plume of white water vapour that starts directly at the outlet to flu, and if you hold your hand in the flue gas it should not be too hot to hold your hand there.   

The flames on a condensing boilers are upside down and go from top to bottom of the heat exchanger and are fan assisted to blow heat from top to bottom - the hot water flows out of top of heat exchanger and is returned at a cooler temperature to bottom of heat exchanger. When the burnt gasses pass over the lower part of heat exchanger they are supposed to condense and this condensing releases extra heat back into water,  if the lower part of heat exchanger is above 55deg C  (return water too hot ) then this condensing does not happen and the heat is blown out of flue instead.  This is the reason for having the water returning to boiler as cool as possible, to extract maximum heat from flue gas.  A boiler that is not condensing loses about 20% of its efficiency, which works out to a lot of money being blown out of flue, people who want their radiators red hot need to change their mindset for condensing boilers.

https://boilerfaultfinder.com/maximising-boiler-condensation-operation/

The condensate from heat exchanger drains into a sump at bottom of boiler and drains out through a bottle trap ( to stop air getting back into boiler ) this condensate in the past was often drained outside the house via a pipe, if it gets cold and the pipe would freeze and it will stop the boiler working, many people called out a repairman when this happens, I re-routed our boiler drain so it goes inside the house and down normal house drain ( and this is done by installers now that they know of the freezing problem ).
« Last Edit: February 10, 2019, 02:02:02 PM by culzean »
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Rory

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Re: Is my condensing boiler working correctly?
« Reply #4 on: February 10, 2019, 02:22:55 PM »
The biggest problem with condensing boilers is that if too many radiators have thermoststatic valves fitted then the water in the circuit is still too hot when it gets back to boiler.  Thermostat valves do not suddenly cut off flow through radiator when the set temperature is reached, they start cutting of water as temperature in room rises and cut it off completely at set temp.  I have opened up all the thermoststic valves on our radiators fully and water going back to boiler is noticeably cooler than flow, also as radiators need replacing I am fitting double panel radiators instead of single.

You've led on to it, but I'd say the biggest problem is installers putting in radiators that are too small.  The Worcester Bosch approved fitters who did daughter's house thought the whole notion of condensing was nonsense and the efficiency gains were grossly exaggeraged (which is true to some extent).

Their calculators produced radiator sizes that were absolutely tiny.  They thought the boiler should be run on max and looked gobsmacked when I pointed out the boiler instructions say run it at 1 to 2 (out of 6).

To get away with low flow temp, and produce the required temp drop (and necessary amount of heat!) rads have to be significantly oversized.  As you suggest, moving from single to same size doubles is probably about right.

culzean

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Re: Is my condensing boiler working correctly?
« Reply #5 on: February 10, 2019, 02:43:14 PM »
Yes the pump has an overrun timer to keep water flowing through boiler after burner shuts off this is to stop boiler heat exchanger getting too hot and to cool water down by flowing it around radiators so that boiler can restart.

If there's a still a demand for heat then the pump will continue to run - certainly ours does, and the OP suggests his does.   The overrun timer only comes into play once the demand is satisfied.

Yes it's normal for the boiler to stay off for a while - ours is 3 mins.  Which doesn't seem long but if the house is coldish, it's enough to lose a lot of heat.

Daughter had this issue in her house and turned out the central heating was single pipe - common until not that long ago (her house was built 1970).   Water was whizzing around the single pipe and getting back to the boiler barely having cooled down, so the boiler shut off.  With the pump still running the whole system cooled down again.   House got warm eventually but in a lot of small steps.

If your house has 10mm pipe going to radiators it will most likely be a more modern system than the older ones with 15mm pipe.  With 15mm they used to run a loop round house upstairs and downstairs and tap radiators off a various points,  with 10mm there will be a 'manifold' and each radiator will be run separately,  this is because of the carrying capacity ( flow rate ) of 10mm pipe is much less than 15mm,  but 10mm pipe is much easier to work with and can be easily bent.

Here is something to think about....  Once the walls get damp ( when heating goes off and the house cools water will condense into plaster and bricks) the insulating value drops and when the heating comes back on more heat has to be supplied to evaporate the damp.
This brings us back to the way condensing boilers work 'the latent heat of evaporation' ( the energy required to evaporate water which can be recovered again if you can condense the water ).  We run a dehumidifier with a built in humistat set at 55%  -  this runs 24/7 - as well as the heat from the motor ( 180watts) the system also recovers heat from the water it condenses which both go into the room giving a nett gain in energy.  The dehumidifier keeps the walls and ceilings dry ( maintains maximum insulation value ) and our heating bills are lower than our neighbours.  The dehumidifier switches itself on and off and as well as keeping windows and walls dry it also means no mould in bathrooms or damp smells,  costs about 50 a year to run and saves hundreds on heating bills as well as keeping fabric of house in tip-top shape. Bonus is it is made in UK,  when you first run it for maybe the first week or so you will be amazed how much water it collects and will probably have to empty it twice a day,  but the amount drops and ours only gets less than a litre a day now even in damp weather  ( 2.5 litre water container).

If you dry washing and clothes in the house the damp will go into walls as well,  we use our EBAC as a clothes drier ( it has a laundry drying setting with timer),  we just put spin-dried washing on a clothes horse in the room with EBAC and close the door,  it dries in a couple of hours and uses a tiny fraction of the energy of a tumble drier.

https://www.go-electrical.co.uk/ebac-3650e-dehumidifier-white.html?gclid=EAIaIQobChMI3v2QiLKx4AIV7bDtCh3w5Qu_EAQYBCABEgKVg_D_BwE

https://www.telegraph.co.uk/finance/property/advice/9831697/Jeff-Howell-can-keeping-the-heating-on-low-all-day-reduce-costs.html
« Last Edit: February 10, 2019, 02:50:43 PM by culzean »
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culzean

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Re: Is my condensing boiler working correctly?
« Reply #6 on: February 10, 2019, 03:05:09 PM »

You've led on to it, but I'd say the biggest problem is installers putting in radiators that are too small.  The Worcester Bosch approved fitters who did daughter's house thought the whole notion of condensing was nonsense and the efficiency gains were grossly exaggeraged (which is true to some extent).

Their calculators produced radiator sizes that were absolutely tiny.  They thought the boiler should be run on max and looked gobsmacked when I pointed out the boiler instructions say run it at 1 to 2 (out of 6).

To get away with low flow temp, and produce the required temp drop (and necessary amount of heat!) rads have to be significantly oversized.  As you suggest, moving from single to same size doubles is probably about right.

The efficiency gains of condensing boilers are real as long as they and the system is set up to make them condense properly, which as you point out most installers haven't got a clue and anyway will not want to spend money on larger radiators when they can charge the same price for fitting smaller ones and 'pocket the difference'.

Main problem with condensing boilers are that they are complicated and the acidic condensate tends to rot them away inside.   Where an older non-condensing boiler would easily last 40 years+ at say 60% efficiency the newer ones last 6 or 7 years then you fork out another 2K + to get it replaced ( although the boiler only costs the installer 500 at trade price and you can get it for 800)  - rip-off merchants. And if the boiler and system are set up wrong the new boiler will only be about 10% more efficient than the old one, and that is a hell of a payback period, you will need an expensive new boiler before the extra 10% saving has even paid for it, let alone made a profit.

Our Vaillant condensing boiler is like triggers broom, as far as I know it is 20 years old but I doubt any of the existing boiler is still there - this year I will fit another boiler myself and pay my mate who is a gas-safe engineer to come and sort the gas out and pass it off - probably save about 1500 over BG and Co prices .
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JimSh

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Re: Is my condensing boiler working correctly?
« Reply #7 on: February 10, 2019, 04:01:36 PM »
Yes the pump has an overrun timer to keep water flowing through boiler after burner shuts off this is to stop boiler heat exchanger getting too hot and to cool water down by flowing it around radiators so that boiler can restart.

If there's a still a demand for heat then the pump will continue to run - certainly ours does, and the OP suggests his does.   The overrun timer only comes into play once the demand is satisfied.

Yes it's normal for the boiler to stay off for a while - ours is 3 mins.  Which doesn't seem long but if the house is coldish, it's enough to lose a lot of heat.

Daughter had this issue in her house and turned out the central heating was single pipe - common until not that long ago (her house was built 1970).   Water was whizzing around the single pipe and getting back to the boiler barely having cooled down, so the boiler shut off.  With the pump still running the whole system cooled down again.   House got warm eventually but in a lot of small steps.

I had a similar problem to Rory's daughter which gave the same symptoms as described by OP . System worked OK with old Potterton boiler until parts no longer available but when changed to modern condensing boiler kept cutting out. Boiler temperature would rise to ~ 70 then  fall to ~40 before boiler cut in again. We too had a single pipe system. Check if radiators closer to boiler heat up faster than those further away.
We had the system changed to a flow and return system last year and it's now working fine.

John Ratsey

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Re: Is my condensing boiler working correctly?
« Reply #8 on: February 10, 2019, 04:58:45 PM »
It's not clear to me if the problem radiator is getting hot or not. If the radiator is getting hot (and 70C means about too hot to touch) without warming up the room properly then, as already suggested, the solution is a bigger radiator. If the radiator isn't getting very hot  then the problem is a constricted flow. Check the pipes going in and out. One (the supply) should be much hotter than the other (return flow).

First check if the radiators is properly bled and there's no air impeding the circulation of hot water (radiators which are warm at the bottom and cold at the top are a symptom). However, in my experience, it's usually an upstairs radiator which collects any air.

Then unscrew the thermostat off the thermostatic valve (if there is one) on the problem radiator. Sometimes the valves get sticky and the pin in the middle needs a bit of brute force (eg pulling up with pliars) to loosen it. Check that this pin moves up and down by a few mm freely (you need something solid to push it down but a spring should then push it up again).

The next step is to open the lockshield valve at the other end of the radiator by a couple of turns and see if that helps the water flow. More flow = hotter radiator. If you still aren't getting enough hot water coming into the radiator then open the lockshield valve some more. If the radiator isn't very hot with the valves fully open then most likely the pipes need a power flush. Not all homeowners (or plumbers) are diligent about keeping the system well dosed with inhibitor (eg Fernox).

I basically agree with all that's already been said about the need for big enough radiators with condensing systems as they are most efficient when running relatively cold as they can then extract a little more heat from the furnace. I've got an outside temperature sensor on my system which automatically changes the target boiler temperature according to the expected heating requirement based on the outside temperature (when the outside temperature is below zero the boiler temperature is about 60C but if it's 10C outside then the boiler temperature drops to about 45C. A consequence of this is that the heating water gets pretty tepid by the time it reaches the far end of the house and the problem is exacerbted by the burner cutting out once the circulating water in the boiler exceeding the target temperature. The burner won't restart until the circulating water was dropped by 10C which can take a while, particularly at the lower temperature end of the range.)

I suggest you get an infra-red thermometer eg https://www.ebay.co.uk/itm/Digital-Infrared-Handheld-Temperature-Gun-Thermometer-Non-Contact-IR-Laser-Point/251885627064 so you can quickly check the pipe (and therefore water) temperatures at various points in the system. When everything is heating up with a properly balanced system the temperature difference between inlet and outlet of each radiator should be the same (around 10C). The thermometer is also very useful for finding any cold spots in the house (eg poor insulation or drafty windows). Just point it at something and it will give the temperature.

ColinS

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Re: Is my condensing boiler working correctly?
« Reply #9 on: February 10, 2019, 05:36:50 PM »
So is this running of the central heating without the burner fired up normal operation for this type of boiler?

Simple answer to your original question is yes.

This has nothing to do with the overrun timer as this only kicks in when the the system switches off and you are describing a running system with the room stat demanding heat.

The basic principles are simple.  The boiler outputs hot water which is, initially, circulated evenly through all radiators.  The radiators remove the heat from the water and input it into the room.

If the radiators are not of sufficient size to remove the heat from the water, and hence heat the room, then the water is returned to the boiler too hot for the boiler to be bothered to heat it up again before passing it back to the radiators, so the burner doesn't cut in.  In essence if the room is not warm enough then the radiator should be hot.

Obviously a little bit more to it than that as has previously been said.  e.g. return flow should be 55 or less for condensing to take place, thermostatic valves will change the initial balance of the system etc.

culzean

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Re: Is my condensing boiler working correctly?
« Reply #10 on: February 10, 2019, 07:48:37 PM »
When you think about it the pump has to keep running as long as boiler demand is present to keep hot water flowing around the system, the overrun timer only kicks in when central heating thermostat is satisfied and boiler demand is removed, when it will keep pump running for several minutes.

Yours is a combination boiler so hot water is heated as required by a pressure switch triggering when a tap or shower is turned on and this fires burner up ( the pump still runs but a diverter valve directs the hot water that normally goes to radiators through a water / water heat exchanger to heat domestic hot water - on domestic hot water the pressure comes from incoming cold water mains ) - with a combi boiler when hot water is called the radiators cool down as water is diverted to heat tap or shower. 
« Last Edit: February 10, 2019, 07:58:50 PM by culzean »
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richardfrost

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Re: Is my condensing boiler working correctly?
« Reply #11 on: February 11, 2019, 10:31:41 AM »
The largest radiator (1800x500 single) in my house is in the lounge, and was at one end of a through lounge, dining room, conservatory combination. It is 21 years old and has a huge cold spot due to build up of sludge, but is coping with its job as the lounge is now separated off from the dining room and conservatory - which is now an extension.

Through a process of renovation and repair, it is one of only four original radiators in the house, and, as coincidence would have it, replacements for these four have arrived this morning to be fitted this week. I have gone for a slightly smaller radiator (1800x450) as they are more efficient these days and it has less of a job to do than the original design.

Whilst powerflushing might have cleared the sludge, it would cost me roughly the same as these 4 radiators. So for the cost of installing them, I will have a fresh and clear system again.

I guess the reason for my post is to watch out for the cold spots at the bottom of radiators due to this build up.

culzean

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Re: Is my condensing boiler working correctly?
« Reply #12 on: February 11, 2019, 11:21:50 AM »
I have always kept a good level of Fernox in our system and there is no sign of sludge ( i have removed quite a few single rads to fit doubles and the water that drained out of old rads was clean - even when the last dregs came out ).  Our system is pressurised so I have to add Fernox F1 Express through the filling loop ( much easier than trying to get it into system via the bleed valve on a radiator ), they say a good check of concentration is to decant some of the system water into a glass container and drop a couple of plain steel ( unplated ) nails into it, leave them and see if they tend to rust.   

If you regularly have to bleed radiators that is a sign that the system is corroding - it is hydrogen gas that comes out.  Our largest radiator is 1100 x 600mm double in kitchen and a 2000 x 300 double in lounge ( but if I replace that one I may go for same length and next deepest  ( 450mm ) - I have turned boiler flow temperature down to 60deg C make sure boiler is condensing ( but hot water tank still gets to 55deg C )  and radiator heat output drops according to how hot they are.

The builder fitted ridiculously small single panel radiators ( house if 20 years old ) and they were still imperial sizes,  they must have got a job lot..  I have replaced them over the years with double panels.
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richardfrost

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Re: Is my condensing boiler working correctly?
« Reply #13 on: February 11, 2019, 01:44:11 PM »
Forgot to say, I have one of those magnetic cleaning filters fitted just before the water goes back through the boiler. This gets cleaned once a year when the boiler has its maintenance visit.

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