Boiler pump replaced but keeps coming on when room temp already reached

Soconfused14

Had anew pump put in my 20 year old boiler on Saturday as it was leaking.
Since then I have noticed that boiler coming on even though room temp already above that set, ie set for 19c keeps coming on til nearly 20c.
Obviously Im concerned that this is going to end up costing me a fortune.

What is the issue, the boiler or the thermostat?? And why has changing the pump caused this??

Albermarle

Are you actually measuring the room temperature with something else than the thermostat, or is the thermostat telling you the room temperature as well as being a thermostat?
Does it switch off when it reaches 20?

Soconfused14

@Albermarle. Yes checked with different room thermometer and its accurate.
Yes the boiler cuts out at about half/one degree above that set and then doesnt come back on til gets below the set temp. The issue there is that Ive set it lower to stop the temp getting too high and  so gets quite cold before it come son again. 
Not sure what to do as its only started since had new pump on Saturday.

FreeBear

Soconfused14 said:

@Albermarle. Yes checked with different room thermometer and its accurate.
Yes the boiler cuts out at about half/one degree above that set and then doesnt come back on til gets below the set temp. The issue there is that Ive set it lower to stop the temp getting too high and  so gets quite cold before it come son again. 
Not sure what to do as its only started since had new pump on Saturday.

Do you have data to confirm that this behavior has started since the new pump was fitted ?

Make and model of the thermostat ?

All thermostats have what is called hysteresis - The upper point where it turns off, and the lower point where it turns on. With some thermostats, this property can be set via a menu option, on others, you are stuck with what ever the manufacturers have designed in. However, most will drift as they age, as will the absolute set point.

ThisIsWeird

As FreeBear asks - make and model of thermostat.
Meanwhile, don't assume it has anything to do with the pump swap

Soconfused14

The thermostat is one British gas gave me, I believe a rebranded Drayton Digistat.
I made connection with pump replacement as was fine til Saturday when pump replaced. Only since Saturday has this issue happened

ThisIsWeird

Soconfused14 said:

The thermostat is one British gas gave me, I believe a rebranded Drayton Digistat.
I made connection with pump replacement as was fine til Saturday when pump replaced. Only since Saturday has this issue happened

Totally understand being suspicious about the seeming coincidence, but just suggesting caution. Also, it's hard to see an obvious connection.
Is this pump part of the boiler, or an external component? Make and model of boiler, please? 
The Drayton is digital. As mentioned above, it will have a built in 'hysteresis', which is a kind of temperature lag around the set temperature. If the stat was 'perfect' in its temp control, it would be turning on and off every minute or so as it measures the room temp and tries to maintain that temp. This would soon wear out the boiler! So, these digital 'stats have an artificial hyst built in which should control the room temp to within acceptable levels, say within a half degree overall. So, it might allow the temp to go up fractionally above the set temp before going off, and then allow it to fall fractionally below the set temp before it comes back on. Ie, a half-degree hysteresis around the set temp.
Yours appears to have a full degree or so hysteresis, which will indeed be considered 'uncomfortable'. Ie, you'd notice it, and find it unpleasant.
Any chance of a photo of the 'stat, please? And can you hear it 'click' as it goes on and off? If so, what sort of time spans between them? And do you have the instructions that came with it?
Did the guy go anywhere near the 'stat?

Soconfused14

@ThisIsWeird
Worcester bosch 28i junior, about 20 years old.
He went nowhere near the thermostat control unit. No click when comes on just a flame sign on the display. Its a Drayton Digistat + Rf .

ThisIsWeird

Soconfused14 said:

@ThisIsWeird
Worcester bosch 28i junior, about 20 years old.
He went nowhere near the thermostat control unit. No click when comes on just a flame sign on the display. Its a Drayton Digistat + Rf .

Thanks.

Is it this model?: https://www.draytoncontrols.co.uk/products/thermostats/digistats-thermostats/digistat-rf

If so, I can't see any mention of adjusting the hysteresis. It seemingly claims to have:

Temperature resolution:	0.5°C
Control accuracy:	+ 0.5K @ 20ºC

I'm guessing that the typical room temp should therefore not fluctuate from its set point by more than a half-degree. 

Could you explain, exactly, what you find, please? What temp do you set it to? How do you monitor the actual room temp? At what point does the 'flame' symbol go out, and when does it come back on?

And, does the boiler appear to respond correctly to the Digistat? Ie, when the flame is on, the boiler is ditto? And vice versa? Or, does the Digi seemingly switch on and off correctly, but the boiler doesn't follow its direction?

The pump is integral to the boiler. I cannot see what the plumber could have done to change the boiler's behaviour by the swap, but that's not to say it isn't possible; the pump might have an adjustable flow rate, for example, and if this was turned right down, it could make the whole system more 'sluggish', tho' not obviously in the way you are experiencing. 
Q: When the CH is turned on, do the rads all become hot just as they did before?
There's two separate units here - the boiler and the stat. It should be fairly easy to determine whether the stat is working correctly as I suggested a couple of paragraphs above. So, eliminate the easiest as a cause?

Soconfused14

So for instance on an evening I set it to come on and the temp is 19c. The flame is on, the boiler is responding fine.
The temp is reached and boiler stops/flame out. Then the flame comes on again and boiler on again . This continues til heat is around 19.7 on "stat". However because heat isnt instantly felt, the room temp rises as radiators are now hot and temp can end up over 20c . If I set at 18.5, the room temp ends up at 19.5 BUT as room cools, the boiler/stat wont come on again til temp below 18.5 so its cold.

The stats not that one, its more loke the Drayton Digistat +1 RF but cant be certain of model as its rebranded with British Gas.

Just a pain altogether.

ThisIsWeird

Thanks.

Does it look like:

That's useful, as it shows both the set and actual temps. So, assuming it has a hysteresis of around a half-degree, then the flame symbol should correspond nicely with the difference between the two readings. Eg, the flame goes off when the 'actual' reaches 'set', and the actual could then well creep up a small amount - say 0.2 degree more. And the flame should remain off until the actual drops to around 0.2 below the 'set'. Yours appears to be swinging a lot more.

Ok, two possibilities come to mind. One is that the 'stat is faulty. The other is that something else has changed...

Is the location of the stat the same? Is hasn't been moved?

Have the rads in that room been altered - their valves adjusted?

And - importantly - what type of valves do the rads have?

I'm also wondering if the old pump was so weak that it delivered the CH water so slowly that the system was naturally 'slow'! Now it has a new pump, the rads in that room heat up super-quickly, become very hot very quickly, and over-shoot? In which case the solution could be to turn down the pump speed - if that is a boiler option - or to tweak the rads in that room down - via their lockshield valves only - so that they don't receive too much water for their size. 

Anyhoo, photos of the valves, please, and have they been adjusted at all?