Why are my radiators now heating slowly?

[jedzi]

I've started using my heating after the summer and the radiators are
taking ages to heat-up (eg about an hour to get to 52 deg). I
don’t have a baseline for comparison, but the boiler is probably 25
yrs old & small, so it’s never been quick.

It had a good service in Sept & was declared in good order. The
engineer said that he’d adjusted something (burners?). IIRC he said
he’d got it down to something like 5 (not sure what unit of
measurement). I haven’t noticed hot water taking longer to heat,
although it may be.

I drained & refilled the system a week or so later and, while it was
the first time I’ve done it, all seemed to be fine (must admit I
didn’t test exhaustively).

I’m wondering if the changes made during the service are the most
likely cause or if it’s to do with when I drained & refilled?

Are there any adjustments I can make myself, I’m thinking of things
like: just run the boiler for longer, increase the pump speed,
increase the boiler thermostat?

Jedzi

 

[gblades]

Airlock in the system?
Did you bleed the air out of the pump after refilling the system?

 

[stem]

My first guess would be that you have a by-pass valve in the pipework somewhere that you opened too much during the draining & refilling process so that most of the hot water is now by-passing the radiators. BTW Hope you added corrosion inhibitor when you refilled them

If it's not that more information will help, for example

What is the boiler thermostat set to? has it been inadvertantly turned down?

Does the boiler burner operate continually, or does it go off for a while and then re-light?

Can you get a clamp on pipe thermometer to measure what the temperature of the water leaving the boiler is. This will show if the boiler is doing its job of getting the water to the set temperature, or if the problem is with the equipment connected to it.

Pipe Thermometer

 

[jedzi]

Apols it's taken me a while - I've been away. To address the points:

"My first guess would be that you have a by-pass valve in the pipework somewhere that you opened too much during the draining & refilling process so that most of the hot water is now by-passing the radiators."
- I did have a small problem removing the mid-position actuator & snapped a small bit of the seating, but it appeared to be working fine.

"BTW Hope you added corrosion inhibitor when you refilled them"
- Yep

"What is the boiler thermostat set to?"
- It's set to 3 (out of 5). It hasn't really been increased, AFAIK

"Does the boiler burner operate continually, or does it go off for a while and then re-light?"
- I've just run it for an hour & it was operating continually.

"Can you get a clamp on pipe thermometer to measure what the temperature of the water leaving the boiler is. This will show if the boiler is doing its job of getting the water to the set temperature, or if the problem is with the equipment connected to it."
- The outward flow pipe got up to 53 deg & the return was 47. I checked one of the rads & it was at 52.

Hope this gives some clues to the problem.

Jedzi

 

[kevplumb]

6 deg diff that don't sound right for a start

 

[bengasman]

Sounds like bypass is too far open.
53 C flow is too low too.
Why did you drain the system?