Boiler wattage for use with unvented cylinder

[colin_]

Hi

The gas boiler is getting old and so at some point will need to be replaced and I am trying to calculate what size it needs to be for best efficiency.

The system is zoned with Zone 1 being 15KW of radiators, Zone 2 being 10KW of radiators, Zone 3 being a Centerstore Solar 300L unvented hot water cylinder (product code 176778) which is connected to the gas boiler (the solar side is no longer used).

I am trying to find out how much wattage the cyclinder requires from my gas boiler so I can size it and the radiators to work out what size the boiler needs to be.

I have read online that you need to allow 7KW for hot water but as far as I can find out from the product it says the coil is 18KW so I am confused as to what figure to use.

First question is what wattage do I need to use?

Secondly, if I had say a 30KW boiler (as hot water on only for 1-2 hours timed twice a day), then if all zones were on at once (say in winter) it would require 25+18=43KW which cannot be supplied, what would the system do? Would it simply take longer to heat the radiators and hot water or would heating the hot water drag the heat away from the radiators (the water from the boiler uses the same pipe for all zones)? Conversly in summer with just the hot water a larger boiler would need to get down to just 18Kw and not sure they can do that.

I would appreciate it if anyone has any experience of how I can get my head around all this.

Thanks
Colin

 

[John.g]

18kw will heat your 300 litre cylinder from a realistic 20C to 60C (13.95 kwh) in 47 minutes, allow 1 hour so you can time the HW zone to come on 1 hour before the CH required, if further heating required during the day then the coil will probably only input ~ 7/10kw so even with CH on and with house up to temperature shouldn't be a problem, don't think you really need a boiler bigger than 25/30 kw. No problem in the summer with HW only required as all gas boilers can now modulate down to ~ 5/8kw.
What is your present boiler output?.

 

[colin_]

Many thanks for the input.

The current boiler is a 11 year old Vaillant EcoTech 37KW system boiler and it takes ~45min to 1 hour to heat the water (depending upon how many people used the shower and for how long, timed to come on twice a day for up to 2 hours max) with no radiators on. Near the end of the time the boiler keeps going off and on doing a pump overrun, presumably as it cant't modulate low enough?

My concern with a smaller boiler is that in winter the demand would be (very worst case) 42KW which is a lot but the current one has coped well but then a bigger one is probably less efficient as most of time load would be ~10-15KW when running and so keep cycling on and off.

I was originally thinking about a Wocsester replacement, possibly 30KW or 35KW?, but looking online they do not appear to say they modulate (they do for some others but not for the system boiler sizes I looked at) so they may not be suitable as most of the time it would need to modulate down to ~10KW as zones not usually on together (except in winter when need the higher power)

 

[John.g]

There will only be a demand for 43kw if you demand all 3 zones on together say from cold in the morning, this can't happen if you time the HW to come on for that 1 hour, there may be a high demand again if both CH zones are on together and a demand for cylinder reheat if HW is programmed on all day.
You can get boiler/boiler controls which have hot water priority which will heat the cylinder only when programmed to do so and it raises the boiler water temp to 80C to give a very fast reheat and then revert to CH.
Your boiler I think can only modulate to 12kw which is the reason for the cycling.
Others on here may give you advise but I think a 30kw boiler is quite or more than sufficient and you should ensure getting one with the max turndown.

If you have one zone that's programmed off for one/two periods during the day then you can programme the HW on for those periods so effectively reduces the demand to a two zoned system.

 

[exedon2]

Is the cylinders solar coil disconnected or are the 2 coils now linked together?

 

[John.g]

That's a very good point, if solar coil not connected then the HW storage is 175 ltrs & with it connected, 295 ltrs.

 

[colin_]

There will only be a demand for 43kw if you demand all 3 zones on together say from cold in the morning, this can't happen if you time the HW to come on for that 1 hour, there may be a high demand again if both CH zones are on together and a demand for cylinder reheat if HW is programmed on all day.
You can get boiler/boiler controls which have hot water priority which will heat the cylinder only when programmed to do so and it raises the boiler water temp to 80C to give a very fast reheat and then revert to CH.
Your boiler I think can only modulate to 12kw which is the reason for the cycling.
Others on here may give you advise but I think a 30kw boiler is quite or more than sufficient and you should ensure getting one with the max turndown.

If you have one zone that's programmed off for one/two periods during the day then you can programme the HW on for those periods so effectively reduces the demand to a two zoned system.

Dear John/Exedon2

Thanks for the qucik replies and advice.

At the moment the radiators are on timed 24/7 and controlled from 2 thermostats and so keep the house at a residual temperature, so the boiler does not have to spend as much time continually on. I could possibly stop them for an hour or so during the night and get the hot water to come on then which means it should not need as much power later on. I am also thinking of getting an Evohome system installed and this may (or may not?) help with the power requirement - I will search the forum for peoples views on the Evohome as well.

I don't think I can get the hot water and radiators split as the current system seems to use the same output pipe for both but, when boiler changeover is being priced, I will ask for them to look at whether that can be easily changed as well.

Re the cylinder, there are two inlets at the bottom that have nothing going into them. I naively thought that the water in the cylinder was the full 300L and that the coils for hot water and solar were seperate but both put their energy into that as a whole, is this not the case? If it is only the equivilent of 175L then I am not sure why it would take an hour to heat it up on the boiler at 37KW.

I presume that if the boiler cannot give the output then all that happens is that the water tempertaure to the radiators and cylinder will not be as high as it should be and so things take longer to heat up, but the boiler should still be operating effeciantly (condensing?). Is there some kind of calculation that can be done to see how much longer things would take at lower temperatues?

I will have to contact Worcester to see if their 30KW/35KW boilers modulate down and to what as I cannot seem to find it online.

Thanks
Colin

 

[exedon2]

As you now probably realise to get full cylinder capacity you would need to use bottom coil. We have 'future proofed ' a number of properties by fitting 300 ltr solar cylinder ready for future extensions / roof alterations.
To give full capacity then linked the 2 coils together or fitted extra 2 port valve controlled by extra cylinder stat to give full capacity when required.

 

[John.g]

Dear John/Exedon2

Thanks for the qucik replies and advice.

At the moment the radiators are on timed 24/7 and controlled from 2 thermostats and so keep the house at a residual temperature, so the boiler does not have to spend as much time continually on. I could possibly stop them for an hour or so during the night and get the hot water to come on then which means it should not need as much power later on. I am also thinking of getting an Evohome system installed and this may (or may not?) help with the power requirement - I will search the forum for peoples views on the Evohome as well.

I don't think I can get the hot water and radiators split as the current system seems to use the same output pipe for both but, when boiler changeover is being priced, I will ask for them to look at whether that can be easily changed as well.

Re the cylinder, there are two inlets at the bottom that have nothing going into them. I naively thought that the water in the cylinder was the full 300L and that the coils for hot water and solar were seperate but both put their energy into that as a whole, is this not the case? If it is only the equivilent of 175L then I am not sure why it would take an hour to heat it up on the boiler at 37KW.

I presume that if the boiler cannot give the output then all that happens is that the water tempertaure to the radiators and cylinder will not be as high as it should be and so things take longer to heat up, but the boiler should still be operating effeciantly (condensing?). Is there some kind of calculation that can be done to see how much longer things would take at lower temperatues?

I will have to contact Worcester to see if their 30KW/35KW boilers modulate down and to what as I cannot seem to find it online.

Thanks
Colin

You will still only have one flow and return pipe from the boiler and any/all the zones are teed off these so you can get a motorized (zone) valve fitted to this but maybe you have a 3 way diverter valve only in your system, you can have a look for a electrical actuator mounted on a valve block with 3 pipes attached.
Its surprising your plumber didn't suggest linking the two coils to give you 300 litre HW capacity except he thought that you were installing solar at a later date.
The 1 hour heat up time has nothing to do with your boiler output, its the coil output that determines this, if it does take 1 hour then the coil is only outputting ~ 8kw, possible I suppose if the boiler SP temperature is set very low like 60/65C, theoretically a 18 kw coil should heat 175 ltrs from 20C to 60C in 27 minutes, the only other reasons are if the boiler temperature setpoint (SP) is at or only slightly > the cylinder SP of normally 60C or if the boiler is continually cycling on/off and the boiler flow temperature is fluctuating widely which some do.

For interest, the solar coil is mounted in the bottom of the cylinder and the auxiliary (oil/gas) above this, this means that in the "summer" the solar coil will heat all the cylinder and at other times the auxiliary coil will heat whatever volume of water its designed for, my 150 litre (dual coil) cylinder has the auxiliary coil heating 100 litres and the solar coil heating only 50 litres or the whole 150 lires, I often have 100 litres of water at 55/60c and 50 litres at 35/45C in the bottom, in the winter, and for 6 months of the year (no auxiliary heating on) the whole 150 litres at temperatures up to 85C, I also have a TMV on the cylinder outlet to limit the HW temperature to 60C.

 

[colin_]

You will still only have one flow and return pipe from the boiler and any/all the zones are teed off these so you can get a motorized (zone) valve fitted to this but maybe you have a 3 way diverter valve only in your system, you can have a look for a electrical actuator mounted on a valve block with 3 pipes attached.
Its surprising your plumber didn't suggest linking the two coils to give you 300 litre HW capacity except he thought that you were installing solar at a later date.
The 1 hour heat up time has nothing to do with your boiler output, its the coil output that determines this, if it does take 1 hour then the coil is only outputting ~ 8kw, possible I suppose if the boiler SP temperature is set very low like 60/65C, theoretically a 18 kw coil should heat 175 ltrs from 20C to 60C in 27 minutes, the only other reasons are if the boiler temperature setpoint (SP) is at or only slightly > the cylinder SP of normally 60C or if the boiler is continually cycling on/off and the boiler flow temperature is fluctuating widely which some do.

For interest, the solar coil is mounted in the bottom of the cylinder and the auxiliary (oil/gas) above this, this means that in the "summer" the solar coil will heat all the cylinder and at other times the auxiliary coil will heat whatever volume of water its designed for, my 150 litre (dual coil) cylinder has the auxiliary coil heating 100 litres and the solar coil heating only 50 litres or the whole 150 lires, I often have 100 litres of water at 55/60c and 50 litres at 35/45C in the bottom, in the winter, and for 6 months of the year (no auxiliary heating on) the whole 150 litres at temperatures up to 85C, I also have a TMV on the cylinder outlet to limit the HW temperature to 60C.

As you now probably realise to get full cylinder capacity you would need to use bottom coil. We have 'future proofed ' a number of properties by fitting 300 ltr solar cylinder ready for future extensions / roof alterations.
To give full capacity then linked the 2 coils together or fitted extra 2 port valve controlled by extra cylinder stat to give full capacity when required.

I am still having trouble understanding, mainly, the cylinder part of the system. I am trying to gather information before speaking to people who can supply and install it, as when left to the builder who arranged it 11 years ago, it is not the best solution and I do not want to get it wrong (and would like to have at least some basic understanding so I can understand what they are recomending as experience shows people recommend differently).

I originally had solar panels but had them removed and the solar coil was left unconnected. The system was insured using Homeserve and when when they had to change the cylinder they did so on a like for like basis (even though they were told and knew there was no longer any solar) and the plumbing people they contracted out to (as needed extra qualifications to work on unvented) used only the top coil, same as the one that was originally there.

I have looked at How a Twin Coil Cylinder Works - https://www.viridiansolar.co.uk/resources-3-7-1-twin-coil-cylinder.html and it appears that the whole 300 litres is used. The boiler part will heat up the part of it and above until it gets to temperature (which I think was set to 65 degrees, the boiler temperature is set to 70) but I would have thought (probably wrongly) that the water immediately below it would also be around the 65 degrees mark (and then gradually reduce as it gets lower and lower).

So possibly the 65 degrees will only apply to the top 175L (once heated) with the other 125L below it gradually reducing in temperature?

Is there any problem with having the two coils linked, presumably just longer to heat as now a larger volume to heat? Or would it have been better to just use the lower coil or leave it as it is now?

Listening to the boiler it appears that the shutting down/overrun starts after ~40 minutes and continues on/off for ~10-20 minutes doing so (depends upon how much hot water was drawn off beforehand). Presumably as the cylinder is at 65 and the the boiler at 70? Is there any issue with upping the boiler temperature to say 75 so a better differential (I asked the Homeserve engoineer who serviced it and he said that 70 is better as boiler more efficient and that the cylinder would heat up correctly even if just a degree or two higher than the cyclinder temperature that has been set).

Thanks
Colin

 

[John.g]

As you now probably realise to get full cylinder capacity you would need to use bottom coil. We have 'future proofed ' a number of properties by fitting 300 ltr solar cylinder ready for future extensions / roof alterations.
To give full capacity then linked the 2 coils together or fitted extra 2 port valve controlled by extra cylinder stat to give full capacity when required.

I am still having trouble understanding, mainly, the cylinder part of the system. I am trying to gather information before speaking to people who can supply and install it, as when left to the builder who arranged it 11 years ago, it is not the best solution and I do not want to get it wrong (and would like to have at least some basic understanding so I can understand what they are recomending as experience shows people recommend differently).

I originally had solar panels but had them removed and the solar coil was left unconnected. The system was insured using Homeserve and when when they had to change the cylinder they did so on a like for like basis (even though they were told and knew there was no longer any solar) and the plumbing people they contracted out to (as needed extra qualifications to work on unvented) used only the top coil, same as the one that was originally there.

I have looked at How a Twin Coil Cylinder Works - https://www.viridiansolar.co.uk/resources-3-7-1-twin-coil-cylinder.html and it appears that the whole 300 litres is used. The boiler part will heat up the part of it and above until it gets to temperature (which I think was set to 65 degrees, the boiler temperature is set to 70) but I would have thought (probably wrongly) that the water immediately below it would also be around the 65 degrees mark (and then gradually reduce as it gets lower and lower).

So possibly the 65 degrees will only apply to the top 175L (once heated) with the other 125L below it gradually reducing in temperature?

Is there any problem with having the two coils linked, presumably just longer to heat as now a larger volume to heat? Or would it have been better to just use the lower coil or leave it as it is now?

Listening to the boiler it appears that the shutting down/overrun starts after ~40 minutes and continues on/off for ~10-20 minutes doing so (depends upon how much hot water was drawn off beforehand). Presumably as the cylinder is at 65 and the the boiler at 70? Is there any issue with upping the boiler temperature to say 75 so a better differential (I asked the Homeserve engoineer who serviced it and he said that 70 is better as boiler more efficient and that the cylinder would heat up correctly even if just a degree or two higher than the cyclinder temperature that has been set).

Thanks
Colin

With the top coil only in use you have effectively a 175 litre cylinder, the water immediately beneath this coil will heat up a tiny amount and is not wasted but doesn't add to the 175 litres at 65C.
If you want to utilize the full 300 litres then you have 3 options, 1. connect the two coils in series where the hot boiler water goes in the top coil, down through both coils and exits the solar coil outlet, you effectively have one coil of ~ twice the heating power of one but heating ~ 300 litres of water, it certainly won't heat the 300 litres in the same time as one coil heating the top but will not take twice as long either, somewhere in between. 2. connect the two coils in parallel where the the hot water enters both coils separately and exits separately which theoretically should give a faster warm up than the two in series, in practice it won't because the circ pump would have to supply twice the flow to achieve this which it won't. 3. connect the bottom coil only which will heat the 300 litres but will take twice as long as heating the top half.
Choice 3 will only require the same boiler power as your existing top half heating, Choice 1 ~ 8/10 kw extra power & choice 2 ~ 12/15 kw extra.

Re the 70/65C boiler/coil differential, this will cause rapid cycling with a 12 kw turndown boiler as the HW temp approaches 60/65C, 75C boiler temp will help but will result in a slight loss of boiler efficiency.

 

[colin_]

With the top coil only in use you have effectively a 175 litre cylinder, the water immediately beneath this coil will heat up a tiny amount and is not wasted but doesn't add to the 175 litres at 65C.
If you want to utilize the full 300 litres then you have 3 options, 1. connect the two coils in series where the hot boiler water goes in the top coil, down through both coils and exits the solar coil outlet, you effectively have one coil of ~ twice the heating power of one but heating ~ 300 litres of water, it certainly won't heat the 300 litres in the same time as one coil heating the top but will not take twice as long either, somewhere in between. 2. connect the two coils in parallel where the the hot water enters both coils separately and exits separately which theoretically should give a faster warm up than the two in series, in practice it won't because the circ pump would have to supply twice the flow to achieve this which it won't. 3. connect the bottom coil only which will heat the 300 litres but will take twice as long as heating the top half.
Choice 3 will only require the same boiler power as your existing top half heating, Choice 1 ~ 8/10 kw extra power & choice 2 ~ 12/15 kw extra.

Re the 70/65C boiler/coil differential, this will cause rapid cycling with a 12 kw turndown boiler as the HW temp approaches 60/65C, 75C boiler temp but will result in a slight loss of boiler efficiency.

Thank you for clarifying.

I shall leave the system as it is as 175 lites should be sufficient as it has not cuased many issues over the last 11 years (if lots of showers/baths I can use the boost button).

Is the cycling something that will not affect the boiler itself (in terms of reliability) or would I be better upping the boiler temperature to 75 (and accepting extra gas useage/efficiency so boiler is reliable for longer)?

On a sperate note, the evohome cylinder control, as far as I can make out, wants the insulation removed and a sensor fitting which does not make sense to me as cylinders insulation loss will be worse. Is there a better way for it to be interfaced into the cyclinder that does not involve removing anything such as using the output from the cylinder that says it is not at the target temperature (which currently brings in a zone valve, when timer allows, to allow water to flow and the zone valve output brings the boiler on)?

Thanks
Colin

 

[John.g]

Increasing the temp to 75C IMO won't make any difference really to the boiler life/reliability, originally gas boilers didn't modulate but don't know if the modulating ones are more reliable.

If the original sensor is in a dry pocket then evohome might be able to supply a suitable sensor.

 

[Basher]

The Evohome tank thermostat kit ATF500DHW comes with both a contact sensor that can be strapped to the side of the tank, and also a pocket sensor. You can use whichever is appropriate for the tank.

From a fan of Evohome!

 

[EvilDrPorkChop]

Replace it with another EcoTec Plus 637 and use Vaillants Senso controls. Give you HW Priority along with setting the coil rating etc in the boiler settings.

 

[colin_]

Thank you all for your input, I have a better understanding now.

 

[John.g]

Colin,
Nothing to do with your problem but if you don't mind, you might sometime watch the boiler (flow) temp when it fires up while in cycling mode and see what temperature it reaches before boiler (burner) cut out. Most boilers don't cut out until the flow temperature is 5C higher than the setpoint so in your case 75C, there was a long running thread on here where a Vaillant (Ecotec 418) boiler is cutting out at only 2C above its setpoint and causing all sorts of problems. Thanks.

 

[colin_]

Hi John

Thanks, I will take a look later in the week and monitor it.

(today I have upped the set temperature of the boiler to 72 degrees and waiting to see what happens when the hot water come on)

 

[colin_]

Colin,
Nothing to do with your problem but if you don't mind, you might sometime watch the boiler (flow) temp when it fires up while in cycling mode and see what temperature it reaches before boiler (burner) cut out. Most boilers don't cut out until the flow temperature is 5C higher than the setpoint so in your case 75C, there was a long running thread on here where a Vaillant (Ecotec 418) boiler is cutting out at only 2C above its setpoint and causing all sorts of problems. Thanks.

Hi John

When it goes into the cycling mode it cycles as below.

The flame is on and the boiler temperature rises until it is about 4 or 5 degrees higher than the set point and then goes into pump overrun. The flame goes off and the system cools to around 8 or 9 degrees below the set point, pump overrun goes off and then flame on to start heating again.

This repeats continually until, in my case, the cyclinder stops calling for heat

 

[colin_]

Update: Just in case anyone has similar in future.

The solution to the excessive pump overrun was to up the boiler temperature by 3 degree to 72 degrees, as suggested above.

What I "think" was happening is that as the cylinder gets near its set temperature, the differntial between the boiler and the cylinder temperature was not high enough and so the returned water to the boiler was too hot. The boiler then tookpreventative measures by doing its pump overun and repeating.

Once again, many thanks for the replies and suggestions.