Downstairs rads cold when hot water on.

[ads1]

Your cylinder is an old Yellow Foam one. Around 30 years I think. It has a smaller heat exchange coil inside than a modern cylinder so takes longer to heat up.

What is the power of your boiler?

Long ago I had a 15kW boiler, which was only just enough, and if you had a bath on a frosty night, you could feel the house cool down.

Installing CWI reduced the heat loss and removed that problem, but the new boiler is 24kW so is ample anyway.

I calculate heat loss for whole house at 0C outside and 20C inside as 12kW now I have CWI and thicker loft insulation. It uses more at first when it is heating from cold.

The modern (blue) cylinder heats in 30minutes.

As Terry says, if you can set your timer to heat the cylinder earlier or later than the CH start, it will reduce the simultaneous load.

Yes correct, I believe it is old and plan to upgrade it in the near future. (only been in the house just over a week).
The Boiler is a Glowworm Flexicom 18hx. So 18 Kw. The house has 14 rads in total.
Surely I should be able to have hot water and heating together though? so would rather get the issue resolved if I can, but may have to change the timer for now.

Many thanks

 

[JohnD]

See if you can estimate the heating demand

https://www.heat-engineer.com/heat-loss-estimator

 

[ads1]

The difference is pretty marginal really. Do you use a lot of hot water ,baths ?? Looks like you have a shower from that cylinder ,is it pumped ?

Yes correct we have a Mira digital shower which is pumped, but only 2 people in the house that use it, the 3 year old has a small bath with only 3 inches of water in (or he splashes it everywhere )
Wouldn't say we use a lot of hot water, but I guess if I'm comparing it to our old house where I didn't have a shower, so only a bath maybe we are using more. Although I thought showers should use less.

 

[terryplumb]

Yes showers use a lot less.

 

[ads1]

See if you can estimate the heating demand

https://www.heat-engineer.com/heat-loss-estimator

Thanks, Just done this and obviously had to guess at some of the inputs, but it seems to suggest that the boiler is under sized and that I should have a 30 KW boiler, Maybe my guesses would have influenced this a bit but I was generally putting in what I know. So maybe a 24 KW boiler is required. Although I would have thought British gas would have put the right size in.

Would you think getting a bigger boiler and new heating cylinder would cure my issues?

As a side note I went around the pipes in the airing cupboard with a magnet to make sure there was no sludge build up causing any issues, I had this on my old house before I replaced the pipes. Good news that it didn't stick to anything.

 

[ads1]

Yes showers use a lot less.

Generally before the HW comes on the tank is about 30 degrees, so not all used up completely. I think maybe the HW cylinder is old and maybe needs changing for a more efficient one.

 

[JohnD]

If your boiler is continually running at full power, but the house and cylinder are not getting enough heat, your boiler is undersized.

However, if your boiler is modulating down, or turning off, then its heat is not being correctly delivered.

 

[rbranco]

Thanks, yes I will try opening a bit more and see what happens, I was thinking that if I have CH and HW on together then it may be more efficient, rather than separate, but maybe I am thinking of it wrong.

Actually it can be worse as you can not heat up your tank to 60c if you have set low flow/return temps to make sure you are always condensing and getting most efficiency.
To get a tank to heat up to 60c you really need the flow temp to be 75-80c but you only want CH flow to be around 65 or lower, there is a conflict, also boilers can be de-rated for the CH load, but for HW it may take more.
Some boilers will complain when the flow and return temp difference is too large, this can happen when CH is on then suddenly a cold HW tank comes on and the return temp is say 30c but flow is 70c+ then it can go into low fire mode and stay there for ages. Again dependent on boiler.

On my Vaillant boiler it has HW priority, it runs 12kw for the CH (flow 65c max but on WeatherComp) and 16kw for the HW (flow 80c), but it will make sure the tank is up to temp before doing any central heating. This is the most efficient way to run the boiler. But to do this as mine is a regular ecotec boiler ive had to put in a Vaillant wiring centre with NTC in the HW tank, if it was a system boiler then it would have been simpler.

My boiler is 18kw (derated to 12kw) in a old 1910 house, all Type10 radiators x 10 radiators with normal copper tank only with jacket. It works ok, would have been better with a 15kw so it modulates even lower.

 

[rbranco]

If your boiler is continually running at full power, but the house and cylinder are not getting enough heat, your boiler is undersized.

However, if your boiler is modulating down, or turning off, then its heat is not being correctly delivered.

This is why HW priority may be better so you do not oversize the boiler as modulation only has a certain range and when the HW is satisfied the lower end of the modulation might still be too high then the boiler cycles.

 

[ads1]

If your boiler is continually running at full power, but the house and cylinder are not getting enough heat, your boiler is undersized.

However, if your boiler is modulating down, or turning off, then its heat is not being correctly delivered.

I think the boiler modulates down / turns off, especially when only HW is on. Boiler is set to 77 degrees C and when on CH it displays 55-60 degrees C. So I am guessing my boiler is ok and so is the CH, but the HW Cylinder is the issue where the boiler cycles and maybe the least efficient part of the system as the rads are only approx 3-4 years old.

 

[ads1]

Actually it can be worse as you can not heat up your tank to 60c if you have set low flow/return temps to make sure you are always condensing and getting most efficiency.
To get a tank to heat up to 60c you really need the flow temp to be 75-80c but you only want CH flow to be around 65 or lower, there is a conflict, also boilers can be de-rated for the CH load, but for HW it may take more.
Some boilers will complain when the flow and return temp difference is too large, this can happen when CH is on then suddenly a cold HW tank comes on and the return temp is say 30c but flow is 70c+ then it can go into low fire mode and stay there for ages. Again dependent on boiler.

On my Vaillant boiler it has HW priority, it runs 12kw for the CH (flow 65c max but on WeatherComp) and 16kw for the HW (flow 80c), but it will make sure the tank is up to temp before doing any central heating. This is the most efficient way to run the boiler. But to do this as mine is a regular ecotec boiler ive had to put in a Vaillant wiring centre with NTC in the HW tank, if it was a system boiler then it would have been simpler.

My boiler is 18kw (derated to 12kw) in a old 1910 house, all Type10 radiators x 10 radiators with normal copper tank only with jacket. It works ok, would have been better with a 15kw so it modulates even lower.

Thanks for the useful info, from the help on here it would seem that a new HW cylinder would be the way to go as they are more efficient. At the moment though I think I will have the HW on separately from the CH until I have a bit more money, (have a leak in the roof too, so that's priority) then get it looked at / cylinder changed. Many thanks all.

 

[rbranco]

Thanks for the useful info, from the help on here it would seem that a new HW cylinder would be the way to go as they are more efficient. At the moment though I think I will have the HW on separately from the CH until I have a bit more money, (have a leak in the roof too, so that's priority) then get it looked at / cylinder changed. Many thanks all.

You have a Glowworm Flexicom 18hx, ive just looked at the manual and seems its ebus capable, you should be able to add a baxi smart wiring center to it so it can give HW priority and control the temps separately. This would be your cheapest solution and does not require a new tank, just rewiring.

 

[JohnD]

I think the boiler modulates down / turns off, especially when only HW is on. Boiler is set to 77 degrees C and when on CH it displays 55-60 degrees C. So I am guessing my boiler is ok and so is the CH, but the HW Cylinder is the issue where the boiler cycles and maybe the least efficient part of the system as the rads are only approx 3-4 years old.

Somebody will know how many kW a yellow cylinder can absorb through its coil, and how low your boiler can modulate to.

If changing the cylinder, I'd go for the biggest you can, then you may run two baths and still have hot water.

 

[ads1]

You have a Glowworm Flexicom 18hx, ive just looked at the manual and seems its ebus capable, you should be able to add a baxi smart wiring center to it so it can give HW priority and control the temps separately. This would be your cheapest solution and does not require a new tank, just rewiring.

Thanks, I'll look in to that, although the issue is the HW already gets priority and so I lose heat in the downstairs rads if I have both HW and CH on. or am I missing something from your post?

Also thinking out loud, could the pump be on the way out and cannot cope with the demand to circulate the water?

 

[rbranco]

Thanks, I'll look in to that, although the issue is the HW already gets priority and so I lose heat in the downstairs rads if I have both HW and CH on. or am I missing something from your post?

Also thinking out loud, could the pump be on the way out and cannot cope with the demand to circulate the water?

This would be priority by making sure the valves are closed and it will then tell the boiler to fire at max temp to get cylinder to satisfy quickly, then when it reverts back to CH it will lower the flow temp.
Manual controls can not do this, normally it requires a boiler specific wiring center.