Boiler wattage for use with unvented cylinder

[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.