Central Heating Struggling in the Cold

[DIYDisaster987]

Hi,
I have a 30 yr+ Potterton Flamingo boiler with a hot water tank in an airing cupboard with a header tank and a header tank for the central heating. The boiler gets serviced every year.
With the cold weather the radiators are taking a while (30 mins or so) to start getting warm and then arent heating the house up to the point where the thermostat will turn off the system - its set to 18 degrees.
I did notice that the upstairs radiators were much hotter than the downstairs ones so balanced the system by only opening the non-trv valve a 1/4 of a turn (upstairs) which does seem to even them out. The boiler thermostat was set to 3/5 and ive put that up to the maximum now. Ive bled all the radiators. The heat is pretty even amongst them all but I have one towel rail upstairs that is now cold. The output from the boiler seems OK as the pipe is too hot to hold and I can measure about 55 deg C.
The hot water seems OK but I think im adjusting the thermostat in the shower more towards the hot side. The pump isn't noisy and the 3 way valve moves where it should.

Does anyone have any ideas of what to start with? I think it could be sludge in the system. If so is it feasible to flush without special equipment?

 

[MeldrewsMate]

To start....

Does your boiler stay lit continuously when heating (only) is selected?
If not, for how many minutes out of an hour is the flame lit?
Is one pipe to each radiator so hot that you can't hold onto it for more than a few seconds?
Is the other pipe only luke warm?

MM

 

[Johntheo5]

Can you measure the flow & return temps at the boiler and the same for the rads as the dT across the rad makes a huge difference to its output.
Rads are based on "50deg" rating which is the difference between the mean rad temperature and the required room temperature.
In your case here are the % outputs based on your 55C flow temp
Flow/return/dT/output
55C/40C/15C/50.4%
55C/49C/6C/60.6%
If you can increase the pump speed to give a dT of 6C instead of 15C (if this is the present dT) then there is a 18% gain in output

If you can increase the boiler flow temp to 70C then for the same dTs you get a massive increase in outputs.
Flow/return/dT/output
70C/55C/15C/85.9%
70C/64C/6C/92.3%

The easiest way is to increase the boiler temperature if boiler suitable for a 70C flow temperature.

 

[DIYDisaster987]

To start....

Does your boiler stay lit continuously when heating (only) is selected?
If not, for how many minutes out of an hour is the flame lit?
Is one pipe to each radiator so hot that you can't hold onto it for more than a few seconds?
Is the other pipe only luke warm?

MM

So I just ran the system for a while on heating only (hadn't done this before) and it all works perfectly. Every radiator is too hot to hold your hand on for mor than about 5 secs, even the towel rail which was cold before.
The boiler is lit continuously and one pipe is too hot to handle the other pipe slightly less but certainly not luke warm.

 

[DIYDisaster987]

Can you measure the flow & return temps at the boiler and the same for the rads as the dT across the rad makes a huge difference to its output.
Rads are based on "50deg" rating which is the difference between the mean rad temperature and the required room temperature.
In your case here are the % outputs based on your 55C flow temp
Flow/return/dT/output
55C/40C/15C/50.4%
55C/49C/6C/60.6%
If you can increase the pump speed to give a dT of 6C instead of 15C (if this is the present dT) then there is a 18% gain in output

If you can increase the boiler flow temp to 70C then for the same dTs you get a massive increase in outputs.
Flow/return/dT/output
70C/55C/15C/85.9%
70C/64C/6C/92.3%

The easiest way is to increase the boiler temperature if boiler suitable for a 70C flow temperature.

Boiler flow and return is 59 and 42 and a radiator is 56 and 42. The pump and the boiler are at their maximum settings.
But its working fine while just on heating.

 

[robinbanks]

And it's not working fine when on.. ?

 

[DIYDisaster987]

Will try on heating and hot water for a while and see what changes...

 

[terryplumb]

Sounds like the rads are fine when heating only is selected ,but when domestic hot water is called for as well ,the rads don't heat well. Is that so ?
If so ,throttle down the supply to the coil

 

[DIYDisaster987]

And it's not working fine when on.. ?

Wasnt working fine with heating and hot water or at least having just heating on is the only thing thats changed.

 

[DIYDisaster987]

Sounds like the rads are fine when heating only is selected ,but when domestic hot water is called for as well ,the rads don't heat well. Is that so ?
If so ,throttle down the supply to the coil

Exactly that. How do I throttle down the supply to the coil?

 

[terryplumb]

Show us pics of pipework into and out of the cylinder ,you may have a gate valve on one of them.
If not ,you would need to fit one.

 

[Johntheo5]

Yes, 14C is a fair dT but because of the low boiler temperature, the rad(s) is "only" emitting 53.7% of its rated output at a room temperature of 18C.

 

[JohnD]

When I had a Flamingo, I think it was 15kW which was scarcely enough to heat the house on a frosty night.

If you had a bath and it was trying to reheat the cylinder at the same time, the house went cold.

However

After fitting CWI the heat loss reduced enough that the boiler was adequate

 

[robinbanks]

I was gonna say, if your house is losing so much heat that 15kW is inadequate, it points more to excessive losses than insignificant input..

 

[DIYDisaster987]

Show us pics of pipework into and out of the cylinder ,you may have a gate valve on one of them.
If not ,you would need to fit one.

There's not much space to take pictures but here's what I could do. I'm pretty sure there isnt a gate valve there though.