Worcester Bosch Greenstar 37 CDi high gas consumption

[kancell10]

Hi guys,

Have recently had a new central heating system fitted to an old property I bought. The new system has approx 13kw of radiators at 80 deg flow, Danfoss TRVs and lockshields, and the existing Worcester Bosch Greenstar 37 CDi condensing combi boiler has been re-used as it is only 3 years old and still under warranty.

The system has been piped in two zones, upstairs and downstairs, with a Danfoss 2 port motorised zone valve on each branch in the ground floor boiler cupboard. The zone valves operate as part of a Danfoss wireless system, using a standard wiring centre, an RX2 wireless receiver, with a TP7000RF wireless programmable room thermostat in each zone.

I have just had my first gas bill since installation, and this was double what I expected. I have come to this conclusion as:

1) the weather has been very mild for the billed period
2) our house is south facing with a high amount of double glazing so gets a lot of solar gain with a natural unheated temp of about 13/14 degrees
3) we are out at work five days a week and have a low frost protection setpoint only
4) our programmer settings in each zone are very frugal on temp and on durations of operation i.e. boiler generally only operates in evening and at 20 degrees
5) a few radiators are turned down or off on TRV as room is not currently used,
6) we have been away on holiday for a couple of weeks with frost protection setting left on
7) we also have a wood burning stove which we run at least 2 nights a week which shuts the CH off in the ground floor zone

I observed the boiler operating recently, and noted that when the room stat is calling for heat as zone setpoint is not yet satisfied, the boiler will kick out 80 degree heat then suddenly the ignition indicator light will go out and the flow temp on the control panel will drop to approx 50 degrees. The boiler then seems to idle for a few mins then kicks back in to generate 80 degree heat before repeating the shut off process. I noted from the Danfoss RX2 receiver that the red light which indicates operation of the boiler is required is still lit and calling for heat, so it doesn't appear to be this which is causing the boiler to shut off. I just wondered whether this cycling/modulating should be expected as normal behaviour as I suspect this may be what has caused my high bill?

Thanks for any help!

 

[D_Hailsham]

I have just had my first gas bill since installation, and this was double what I expected.

What was the basis for your expectation?

 

[kancell10]

I have just had my first gas bill since installation, and this was double what I expected.

What was the basis for your expectation?

on the basis that in 10 years of house ownership, i have never used more than £45 a month of gas but have been averaging £100 for the last 3 months in the new property. Obviously bearing in mind prices have risen significantly lately, i only used to use £20 a month for first property with the £45 figure being my last property during the very cold (-15 deg) winters of 2009 - 2010 and 2010 - 2011. There are some circumstances to expect a rise in usage, new property is larger in footprint and volume and less well insulated, however to mitigate this the system has been zoned and temperatures dropped and on periods minimised. Also considering the 7 points i made in first post.

I have a feeling it may be to do with balancing of the system which is causing the cycling. i think the installer might have done this pretty crudely to be honest!

 

[Gasguru]

Turn the flow temperature down, unless the radiators are sized to old standards and it's snowing outside in which case you may need to run them at 80c. Hopefully with a new system the rads will have been a little overszied so the temp can be lowered with an increase in boiler efficiency.

Turn off the confort mode. Perhaps you have a water leak and the boiler is continually firing up to maintain internal temperarures.

There is a parameter to range rate the heating. Never tried it but worth looking at.

Balance the system.

I assume the radiators located where the room stats are have two lockshield valves on.

 

[kancell10]

Turn the flow temperature down, unless the radiators are sized to old standards and it's snowing outside in which case you may need to run them at 80c. Hopefully with a new system the rads will have been a little overszied so the temp can be lowered with an increase in boiler efficiency.

Turn off the confort mode. Perhaps you have a water leak and the boiler is continually firing up to maintain internal temperarures.

There is a parameter to range rate the heating. Never tried it but worth looking at.

Balance the system.

I assume the radiators located where the room stats are have two lockshield valves on.

thanks for your response, i previously had the flow at 70 deg and it was cycling, so yesterday i turned it up to 80 to see if this made any difference and stop the cycling, it's too early to say if it has made any difference though. i am satisfied with the heat loss and rad sizing calcs.

what is comfort mode?

i did have a leak previously, was losing system pressure (external to boiler leak as this was checked by WB) but of late the system has maintained pressure fine so i don't believe this will be an issue.

i will look into range rating, and have phoned the installer to get him to service boiler (which was needed anyway) and have a look at balancing also.

there is not a bypass radiator with lockshields only, as the wireless room stat is not in a fixed position and is taken with us to whichever room we are in for the night to ensure that is the reference point. we discussed this with installer and intended to fit auto bypass valve but on further inspection it had an ABV internal to the boiler so this was not needed

 

[Gasguru]

The lower the heating temperature the more heat energy will be extracted from the combustion products...so turn it down.

The room thermostats should be fixed to the wall. Otherwise they will fight with the TRV set temperature.

Where the room stats are located the radiator in that area should have two lockshield valves and be adjusted to make this the last area to come up to temperture. This will help prevent short cycling.

I'm talking about leaks on the domestic (hot) water side. Confort mode keeps the boiler warm for a faster draw-off. If there's a leak the boiler will be firing more often. See the manual for details..button on the front turns it off.

You need the service manual...it's on Worcesters website in the discontinued literature section. It will give details of the parameter to change to range rate.

Make sure the system is balanced properly.

 

[benwhite]

This won't help you but might make you feel better

I to recently moved into a new place with one of these boilers and my gas bill is well over £200 per month! It's eye watering when we came from a place where it was all in leccy + gas for £70 per month before.

The worst thing is the house is still chilly even for all that cost!

In my case it's because the house has solid walls and very patch loft insulation. Loft is being done soon (electrics first), hopefully followed by external wall insulation.

 

[D_Hailsham]

Gong back to your opening post, you say you have 13kW of new Rads and retained the existing WB 37CDi boiler.

1. How was the 13kW arrived at?
Did you do a room by room heat-loss calculation? If so what were you using?
Or did you use the Whole House Boiler Size Calculator, which is more reliable.

2. The WB 37CDi gives CH output of 9.8kW - 30.9kW. Assuming the 13kW is correct, you boiler will only work at its most efficient when the outside temperature is below approx 4°C. Above that temperature, it will not be able to modulate low enough, so it will run in on/off mode, which is not so efficient. It would help if the max output was reduced to, say 15kW, as this will reduce the cycling when the boiler tried to put 30kW through 13kW of rads, finds that the water temperature has risen to quickly, and goes into anti-cycle mode. This is how you do it:

Press the Central Heating Boost Button for approximately 10 seconds (the
display shows 88 ). When the button lights up orange, release it.
Turn the CH control knob on the right and adjust maximum heat output (the display shows alternatively the CH flow temperature and % output).
Press the Central Heating Boost Button to exit.

3.You seem to be continually turning the heating on and off. This may not be the most efficient way of heating the house, particularly if the 'on' times are short.

4. You say that you are turning off the rads in unused rooms. Do you keep the doors closed.

5. Did you find out from the previous owners how much they were spending on gas?

 

[kancell10]

Gong back to your opening post, you say you have 13kW of new Rads and retained the existing WB 37CDi boiler.

1. How was the 13kW arrived at?
Did you do a room by room heat-loss calculation? If so what were you using?
Or did you use the Whole House Boiler Size Calculator, which is more reliable.

2. The WB 37CDi gives CH output of 9.8kW - 30.9kW. Assuming the 13kW is correct, you boiler will only work at its most efficient when the outside temperature is below approx 4°C. Above that temperature, it will not be able to modulate low enough, so it will run in on/off mode, which is not so efficient. It would help if the max output was reduced to, say 15kW, as this will reduce the cycling when the boiler tried to put 30kW through 13kW of rads, finds that the water temperature has risen to quickly, and goes into anti-cycle mode. This is how you do it:

Press the Central Heating Boost Button for approximately 10 seconds (the
display shows 88 ). When the button lights up orange, release it.
Turn the CH control knob on the right and adjust maximum heat output (the display shows alternatively the CH flow temperature and % output).
Press the Central Heating Boost Button to exit.

3.You seem to be continually turning the heating on and off. This may not be the most efficient way of heating the house, particularly if the 'on' times are short.

4. You say that you are turning off the rads in unused rooms. Do you keep the doors closed.

5. Did you find out from the previous owners how much they were spending on gas?

1. room by room heat loss, and a double check against rule of thumb figures, happy that radiators are sized correctly, certainly aren't undersized at all.

2. i entered this setting, but wasn't clear how to set the rating. the only parameter showing was the flow temp which was increasing rapidly.

3. i now have it set to maintain a minimum day set point of 12 degrees (was previously 15) whilst we are at work and during the night. i was off today and checked out temperatures, high of 18 and low of 13, so heating has not been on at all. the main system on time is 5pm - 10pm downstairs, and 9pm - 11pm upstairs, where it maintains a 20 deg setpoint in both. i have option of chrono proportional control on danfoss controls but having talked to danfoss technical we figured on/off control would suit better.

4. yes generally speaking doors are kept closed.

5. no unfortunately not, elderly lady owned it and she has moved on.

i have been monitoring gas usage over last few days, and will get a weeks worth of data so i can check any trends.

i also checked the lockshields, and the system has not been balanced at all, so i have been back onto installer. as suggested previously (or another thread) i think the return is getting too hot and boiler is shutting off prematurely whilst stat is still calling for heat. i have ordered an IR thermometer so i can hopefully get some idea what is going on with temperatures in meantime.

 

[D_Hailsham]

room by room heat loss, and a double check against rule of thumb figures, happy that radiators are sized correctly, certainly aren't undersized at all.

Worth comparing with the result from the Whole House Calculator.

i entered this setting, but wasn't clear how to set the rating. the only parameter showing was the flow temp which was increasing rapidly.

The display should alternate between flow temperature and boiler output.

the main system on time is 5pm - 10pm downstairs, and 9pm - 11pm upstairs, where it maintains a 20 deg setpoint in both.

What about the mornings?

I also checked the lockshields, and the system has not been balanced at all, so i have been back onto installer. as suggested previously (or another thread) i think the return is getting too hot and boiler is shutting off prematurely whilst stat is still calling for heat. i have ordered an IR thermometer so i can hopefully get some idea what is going on with temperatures in meantime.

Balancing should help!

Your boiler is designed to work with a 20C differential between low and return. If you have the same differential across the rads their output will be reduced to 85% of the stated figure.

The return is getting too hot because the boiler is pumping out more heat than the rads can handle; so it closes down and goes into an anti-cycle mode.

IR thermometers can give misleading readings as it assumes a certain 'shininess' on the surface (technically the emissivity - &#949. Most IR thermometers are calibrated against a 'black body' with ε value of 0.98. Shiny copper pipe has an ε value of approx 0.05.

The safest way to use an IR thermometer is to wrap some black insulating tape round each pipe and measure off that. You also need to hold the thermometer against the pipe as it averages the temperature over an area, like a torch beam. The spec will give the spot/distance ratio.

 

[kancell10]

room by room heat loss, and a double check against rule of thumb figures, happy that radiators are sized correctly, certainly aren't undersized at all.

Worth comparing with the result from the Whole House Calculator.

i entered this setting, but wasn't clear how to set the rating. the only parameter showing was the flow temp which was increasing rapidly.

The display should alternate between flow temperature and boiler output.

the main system on time is 5pm - 10pm downstairs, and 9pm - 11pm upstairs, where it maintains a 20 deg setpoint in both.

What about the mornings?

I also checked the lockshields, and the system has not been balanced at all, so i have been back onto installer. as suggested previously (or another thread) i think the return is getting too hot and boiler is shutting off prematurely whilst stat is still calling for heat. i have ordered an IR thermometer so i can hopefully get some idea what is going on with temperatures in meantime.

Balancing should help!

Your boiler is designed to work with a 20C differential between low and return. If you have the same differential across the rads their output will be reduced to 85% of the stated figure.

The return is getting too hot because the boiler is pumping out more heat than the rads can handle; so it closes down and goes into an anti-cycle mode.

IR thermometers can give misleading readings as it assumes a certain 'shininess' on the surface (technically the emissivity - &#949. Most IR thermometers are calibrated against a 'black body' with ε value of 0.98. Shiny copper pipe has an ε value of approx 0.05.

The safest way to use an IR thermometer is to wrap some black insulating tape round each pipe and measure off that. You also need to hold the thermometer against the pipe as it averages the temperature over an area, like a torch beam. The spec will give the spot/distance ratio.

it doesn't come up with a temperature, it shows what would be an 88 but with the vertical bits removed, so basically i end up with 3 stacked horizontal lines, twice over. i just tried it again but had to abort again as the flow temp rose to 95 deg and it sounded like boiler was literally boiling the water!

we don't use heating in the morning, just evenings and the odd weekend afternoon. we get so much solar gain we have no need really.

i would expect/aim for a delta t of 10 (or 11) degrees across each radiator and delat t of 20 across boiler as you state. will push to get installer back to balance and will use IR thermo to check his work! had seen that tip about emissivity and using black tape somewhere, great idea.

thanks for your help so far, hopefully get to the bottom of this!

 

[benwhite]

I am watching this thread with interest as I have one of these boilers too, also with seriously old rads (nowhere near 30kw).

I tried to figure out what my %output was and got exactly the same result - alternating between the flow temp and 88 with the vertical bars removed.

Would be interested in how to do this - seems like the instructions are not quite right.

 

[kancell10]

I have just spoken to WB technical support, and the chap was very helpful, he talked me through range rating the boiler, which we dropped from 88% to 50%. I have been monitoring boiler since the change but unfortunately it is still cycling whilst room temperature is nowhere near setpoint temperature.

I also have a WB tech coming out tomorrow for a look as boiler is still under warranty, so will advise any findings.

 

[benwhite]

Did you get to the bottom of this? I'm in a similar situation (cycling) and would be interested to know what happened after the WB tech visit.

Ta

 

[kancell10]

Hi, it's a yes and no really!

I have had WB out, they hooked a computer up and inserted probe into flue and measured a few things. Result of which appears that boiler is working fine. We range rated the boiler to a more suitable level for the output (50%) and this stopped the cycling but increased the warm up time. I have since then been stepping up the range rating by 10% until i have found the balance between cycling and heat up period of about 80%. if you phone up WB technical they will explain how to do this, or worst case i can give you second hand instructions.

however, my problem still has not been solved unfortunately. after range rating and a few modifications to temperatures and time on/off, my usage has dropped from an average of 150 kw/hr a day over a 6 week period to 115 kw/hr a day over a 2 week period. I still feel the consumption is excessive for the little time the boiler is on, so i have heating engineer who installed the system hopefully coming over tonight to have a look at the system and the gas meter. saying that the weather has been pretty cold lately, and my cavity wall and loft insulation has not been fitted yet so this should hopefully make further improvements to the efficiency.

can i ask what symptoms and issues you are having? i take a meter reading every night in m3 and convert it to kw/hr on spreadsheet so i can track usage, if you don't already do this i suggest you should start there.

i have also been looking into a smart gas meter, and it turns out my work (SSE) will be fitting these to some staff members houses as a trial, so i will be applying for that also!

p.s. i also bought an IR thermometer to check flow and return temps, i have a delta t of about 10 degrees across boiler flow (75) and return (65), not quite the 20 degrees to get it further into the condensing zone and balancing will probably help but just making small changes at a time.