Boiler short cycling with low flow temperatures

[marshian]

1 hour ago, seanblee said:

I understand that Evohome turns the boiler on and off a few times an hour, but that's not the issue I'm having here - even when Evohome is continuously calling for heat, the burner is firing, the flow temperature is rapidly rising until it crosses the setpoint, then the burner is turned off until the flow falls to the hysteresis threshold of 10 degrees below the setpoint, then the cycle repeats...

so you need to increase the flow rate thro the boiler to stop this overshoot - open all the lockshields 1/2 a turn (except the towel rails) see if that stops it - if it does wind them back a 1/4 and see how it reacts - if it doesn’t add another 1/4 turn and see if that helps 

 

if you get to a point where the boiler isn’t overshooting then you can drop the flow temp and you’ll get longer burns at lower flow temps but you’ve got to get the boiler happy on initial purge and burn 

[seanblee]

No improvement this morning. I'll try opening the lockshields half a turn as you suggest and see what that does...

[MrPotts]

11 hours ago, marshian said:

Mine (wiser hub) doesn’t any more - I set it to oil boiler and that halved the number of cycles -then set it on open therm boiler settings (even though it’s not wired for open therm) - that fixed it when I was running scheduled heating slots. 

 

Noe it doesn’t do it at all but I’ve set the majority of  TRV’s to 0.5 deg above actual room temps and that can’t be achieved with WC flow temps

 

But that isn’t @seanblee‘s issue I’m sure of it and it’s a bit off topic

Thanks for the info but I have already tried setting my Wiser hub to oil boiler but the setting doesn’t work, the boiler still fires 6 times an hour. I’ve reported it to Drayton tech support.

Edited February 5 by MrPotts

[marshian]

3 hours ago, seanblee said:

No improvement this morning. I'll try opening the lockshields half a turn as you suggest and see what that does...

Just to explain why you don’t see the same thing on initial heat up 

 

On initial heat up you have a cold circuit even if the flow is a little low (and I think it is) the water has a lot more capacity to take the heat from the boiler so it doesn’t overshoot in the same way - once the circuit has some temperature in it the capacity of the circuit and flow to take the initial purge and hi level burn before the boiler modulates down is much reduced

[marshian]

30 minutes ago, MrPotts said:

Thanks for the info but I have already tried setting my Wiser hub to oil boiler but the setting doesn’t work, the boiler still fires 6 times an hour. I’ve reported it to Drayton tech support.

Then it must be because that’s what your house needs to keep it warm 😉

[MrPotts]

1 hour ago, marshian said:

Then it must be because that’s what your house needs to keep it warm 😉

No, I can monitor the hub directly and it is the hub forcing the cycling.

[marshian]

5 minutes ago, MrPotts said:

No, I can monitor the hub directly and it is the hub forcing the cycling.

OK so how close are the rooms to target temp?

 

Is it wiser trying to minimise overshoot in room temps?

[Nickfromwales]

19 minutes ago, MrPotts said:

No, I can monitor the hub directly and it is the hub forcing the cycling.

I’m confused.

 

The thermostat onboard the boiler manages that and the 3rd party controls just give a call for heat signal? 
 

Are you saying the 3rd party controls give 6 on signals per hour and the boiler doesn’t get to satisfy its own onboard stat?

 

Sounds like this needs a buffer of some sort? 

[marshian]

1 hour ago, Nickfromwales said:

I’m confused.

 

The thermostat onboard the boiler manages that and the 3rd party controls just give a call for heat signal? 
 

Are you saying the 3rd party controls give 6 on signals per hour and the boiler doesn’t get to satisfy its own onboard stat?

 

Sounds like this needs a buffer of some sort? 

 

OK let me help @Nickfromwales

 

The Wiser Hub has some "smart" functionality inbuilt into the software where it tries to save energy

 

It's looking at the delta between "actual room temp" the TRV is feeding back to the hub and the individual target temp set point if the delta between them both is a very small the Wiser hub can turn off the call for CH.

 

The boiler as a result shuts down and waits for the return temp to reach a point where it can fire again

 

Trouble is this "smart" approach can result in additional cycling where none would actually be required so maybe "not so smart"

 

It's bloody annoying to have a few rooms not yet up to temp and the boiler switching off as soon as it's just fired just because one or two rads as saying they are close to target.

 

If it's installed in a house with a traditional higher flow temp (60/70/80) this sort of intervention will actually work quite well - it did when I was using higher flow temps and scheduled heating

 

However in a lower flow temp system 30/40/50 it's very much an unwanted interruption to the boilers normal cycles

 

There are a number of ways you can limit the impact

 

1. You can tell the Hub it's managing an oil boiler - that should drop the hourly cycle limit from a gas boilers 6 to an oil boilers 3 per hour (other options include telling wiser it's connected to an "Opentherm" managed boiler - it really cops the hump with that one and gives you a warning triangle on the app but it worked for me

 

2. You can restrict a couple of rads in areas not used all the time to never achieve the target room temp (thereby ensuring that there are always a number of rooms where the set point hasn't been achieved and this minimises interventions for the hub

 

3. You can set some rooms to have a higher target temp than can be achieved - your room temps in the app will go bright red cos of "overheat potential" but this is the "(expletive deleted) you wiser" option but in reality has same impact as 2.

 

 

[Nickfromwales]

3 minutes ago, marshian said:

 

OK let me help @Nickfromwales

 

The Wiser Hub has some "smart" functionality inbuilt into the software where it tries to save energy

 

It's looking at the delta between "actual room temp" the TRV is feeding back to the hub and the individual target temp set point if the delta between them both is a very small the Wiser hub can turn off the call for CH.

 

The boiler as a result shuts down and waits for the return temp to reach a point where it can fire again

 

Trouble is this "smart" approach can result in additional cycling where none would actually be required so maybe "not so smart"

 

It's bloody annoying to have a few rooms not yet up to temp and the boiler switching off as soon as it's just fired just because one or two rads as saying they are close to target.

 

If it's installed in a house with a traditional higher flow temp (60/70/80) this sort of intervention will actually work quite well - it did when I was using higher flow temps and scheduled heating

 

However in a lower flow temp system 30/40/50 it's very much an unwanted interruption to the boilers normal cycles

 

There are a number of ways you can limit the impact

 

1. You can tell the Hub it's managing an oil boiler - that should drop the hourly cycle limit from a gas boilers 6 to an oil boilers 3 per hour (other options include telling wiser it's connected to an "Opentherm" managed boiler - it really cops the hump with that one and gives you a warning triangle on the app but it worked for me

 

2. You can restrict a couple of rads in areas not used all the time to never achieve the target room temp (thereby ensuring that there are always a number of rooms where the set point hasn't been achieved and this minimises interventions for the hub

 

3. You can set some rooms to have a higher target temp than can be achieved - your room temps in the app will go bright red cos of "overheat potential" but this is the "(expletive deleted) you wiser" option but in reality has same impact as 2.

 

 

Ah, I didn’t realise (read) that the TRVs fed back too. That’s a lot of info for the system to juggle!!

[marshian]

So in a nutshell "smart" can be "really bloody dumb" - Anyway this is OT for the original post and I'm damn sure it's been discussed elsewhere

[MrPotts]

1 hour ago, marshian said:

OK so how close are the rooms to target temp?

 

Is it wiser trying to minimise overshoot in room temps?

Exactly that. It’s pretty annoying for Wiser to be close to temp with say a 20% heat demand to then fire the boiler for 2mins in every 10min cycle. At least with the “oil” setting Wiser will fire the boiler for 4mins but this is either a 20min cycle. I have removed all the independently controlled Wiser rad stats as they just add an unnecessary complication.

[marshian]

21 minutes ago, MrPotts said:

Exactly that. It’s pretty annoying for Wiser to be close to temp with say a 20% heat demand to then fire the boiler for 2mins in every 10min cycle. At least with the “oil” setting Wiser will fire the boiler for 4mins but this is either a 20min cycle. I have removed all the independently controlled Wiser rad stats as they just add an unnecessary complication.

 

Whoa!!! You witnessed it doing that??

 

That's bloody nuts!!!

 

I never got those symptoms - it would always switch off the call for heat (like a single house room thermostat reaching temp) - you would see the CH light go out on the hub - the pump over run would kick in and then 2 mins later the CH light on the hub would come on again and it would re-fire if the return/flow temps had dropped enough to call for heat.

[marshian]

36 minutes ago, Nickfromwales said:

Ah, I didn’t realise (read) that the TRVs fed back too. That’s a lot of info for the system to juggle!!

 

Yeah 13 TRV's managed by the hub and when a number of them get close to target the system goes into save energy mode and actually uses more

 

For me on the old boiler (Glow worm) I'd dialed in 10 mins of pump over-run and in the shoulder seasons nearly 30 mins of anticycle time at the lowest flow temps I could realistically run (So as the POR runs concurrently with ACT a max of 30 mins between burns)

 

This was down to 10 kW min of the boiler and a heat loss requirement of max 1.5kWh - I wanted to do everything I could to maximise the burn time and allow the system to coast for as long as possible

 

And then when the boiler finally fires up Wiser says "oh no you don't"

 

It nearly went in the bloody sea!!!

[JohnMo]

Sounds like almost every thermostat that runs TPI control is just rubbish.

 

Maybe better just getting an adjustable hysteresis thermostat and make the hysteresis wide, but not uncomfortably wide to give the boiler something to work against.

[marshian]

8 hours ago, seanblee said:

No improvement this morning. I'll try opening the lockshields half a turn as you suggest and see what that does...

 

Can I have a bit more detail on this end of the graph please??

 

[JohnMo]

Just looking at the graph.

 

I would operate a little more dumb and at a higher temp.  Do you need heating on at 3.30am? If not leave for as late as possible to give boiler way more work to do flow at 60+. As long as your return temp is below 53/54 deg you are in condensing mode, only marginal gains are made after that.  It's going to way more efficient than your current operation.  Does your boiler control have its own thermostat? or if you want to, do through Home Assistant for now just use the generic thermostat set the hysteresis to 0.5 or more

[marshian]

4 minutes ago, JohnMo said:

Just looking at the graph.

 

I would operate a little more dumb and at a higher temp.  Do you need heating on at 3.30am? If not leave for as late as possible to give boiler way more work to do flow at 60+.

 

I think that's what he is doing - he's running it at the minimum elevated temp he can - he wants to run the boiler at lower flow temps but can't because it cycles like heck (more than it's doing now)

 

4 minutes ago, JohnMo said:

As long as your return temp is below 53/54 deg you are in condensing mode, only marginal gains are made after that. 

 

Marginal??

 

The difference between the red arrow and the green arrow is marginal??

 

 

That's like saying the difference between running a heat pump at 55 versus 45 is marginal in terms of COP!!!

 

4 minutes ago, JohnMo said:

It's going to way more efficient than your current operation. 

 

On this we agree

[MrPotts]

1 hour ago, marshian said:

 

Whoa!!! You witnessed it doing that??

 

That's bloody nuts!!!

 

I never got those symptoms - it would always switch off the call for heat (like a single house room thermostat reaching temp) - you would see the CH light go out on the hub - the pump over run would kick in and then 2 mins later the CH light on the hub would come on again and it would re-fire if the return/flow temps had dropped enough to call for heat.

Yes, its how Wiser works.  I use the Drayton Wiser integration in Home Assistant which directly interrogates the hub.

 

For those that aren't familiar with the Wiser system the heating cycle works like this (I think Evohome works the same way, it looks like it from the OP's graphs).....

 

Configured within the hub is the boiler type so gas boiler, oil boiler, electric boiler or heat pump. Mine is configured for gas boiler which means that the Wiser hub divides an hour into six 10min segments.  From cold in order to reach target temperature the Wiser hub will set a heat demand of 100%, this means in a 100% heat demand situation Wiser will request the boiler to fire for the full 10mins and will continue to do this for each subsequent 10min segment until the actual temperature and target temperature get closer.  The heat demand gradually reduces as target temperature gets closer so an 80% demand fires the boiler for 8mins, a 60% demand for 6mins etc. until the heat demand reaches 20% which is the lowest I have seen.  

 

There must be some clever algorithms within the Wiser software that takes in all the different temperature setpoints and actual temperatures from all the thermostats to come up with the heat demand percentage.  Sounds complicated doesn't it! This is why I have replaced almost all my radiator thermostats so the hub now only takes information from one room thermostat.

 

 

Edited February 5 by MrPotts

[marshian]

27 minutes ago, MrPotts said:

Yes, its how Wiser works.  I use the Drayton Wiser integration in Home Assistant which directly interrogates the hub.

 

For those that aren't familiar with the Wiser system the heating cycle works like this (I think Evohome works the same way, it looks like it from the OP's graphs).....

 

Configured within the hub is the boiler type so gas boiler, oil boiler, electric boiler or heat pump. Mine is configured for gas boiler which means that the Wiser hub divides an hour into six 10min segments.  From cold in order to reach target temperature the Wiser hub will set a heat demand of 100%, this means in a 100% heat demand situation Wiser will request the boiler to fire for the full 10mins and will continue to do this for each subsequent 10min segment until the actual temperature and target temperature get closer.  The heat demand gradually reduces as target temperature gets closer so an 80% demand fires the boiler for 8mins, a 60% demand for 6mins etc. until the heat demand reaches 20% which is the lowest I have seen.  

 

There must be some clever algorithms within the Wiser software that takes in all the different temperature setpoints and actual temperatures from all the thermostats to come up with the heat demand percentage.  Sounds complicated doesn't it! This is why I have replaced almost all my radiator thermostats so the hub now only takes information from one room thermostat.

 

 

 

Interesting but my set up never did that - it would just intervene when rads were close to temp - knock the boiler off for anywhere between 2 and 8 mins and then call for heat again like nothing ever happened.

 

That was enough to annoy the heck out of me hence the various approaches I took to limit it's influence over the heating circuit

 

I'm almost 100% sure it hasn't intervened at all in the last 2 months since I've been heating 24/7 with TRV's set to elevated room temps (that the weather compensated flow temp from the boiler cannot hope to achieve)

 

If it had intervened then I'm very sure I'd be removing it all and going back to a simple clock timer set to a heating schedule for the shoulder months and switched to constant "24/7 On" for the winter months

[JohnMo]

1 hour ago, marshian said:

he wants to run the boiler at lower flow temps but can't because it cycles like heck (more than it's doing now)

The only way to do that is way bigger radiators, big buffer or volumiser.

 

Just keep increasing set point temp until you get the the equilibrium point where the boiler can get heat away and you have a relatively long run time.

 

2 degs on mine make the difference between short run times and running for hours. TPI control makes the boiler go daft.

1 hour ago, marshian said:

That's like saying the difference between running a heat pump at 55 versus 45 is marginal in terms of COP!!!

Not really, long run times have way more impact on gas or electricity consumption than lots of short cycling at a lower than optimal flow temperature. Also 10 degs difference in flow temp, is only a couple of percent reduced gas consumption. Long runs win everyday.

[seanblee]

18 hours ago, marshian said:

so you need to increase the flow rate thro the boiler to stop this overshoot - open all the lockshields 1/2 a turn (except the towel rails) see if that stops it - if it does wind them back a 1/4 and see how it reacts - if it doesn’t add another 1/4 turn and see if that helps

 

I increased by half a turn this morning and another quarter this afternoon (leaving some lockshields now wide open) and it hasn't made any real difference to warm-up this afternoon. The setpoint was 18 for the lounge during the day, increasing to 20 at 5pm - Evohome decides when to start warm-up so it hits this temperature at this time, and it looks like it kicked off just after 3pm today:

 

 

1 hour ago, marshian said:

 

Can I have a bit more detail on this end of the graph please??

 

 

Overnight setback for the lounge is 17 (dropping from 20 at 10pm), so there are a few spikes from 3am onwards where I guess Evohome is firing the boiler for short durations to maintain temperature. Warm-up proper started at 3:30am in order to hit 20 by 5:30am (my kids wake up annoyingly early!):

 

 

8 minutes ago, JohnMo said:

Just keep increasing set point temp until you get the the equilibrium point where the boiler can get heat away and you have a relatively long run time.

 

I think I'm going to try that - increasing pump speed is out because it makes the whole system noisy (even with lockshields wide open), so the only logical thing to try is raising the flow temperature. I think I'll put the lockshields back where they were this morning, turn off the HA automation then bump the flow dial up to 60 and see how that runs. If it's stable, I'll start incrementally decreasing it until I get back into cycling.

 

It does make me worry about a potential future ASHP - I know they don't need such a big delta between flow and return, but I don't see a way around a buffer without a full repipe, which would be a nightmare...

[JohnMo]

5 minutes ago, seanblee said:

does make me worry about a potential future ASHP

Bigger radiators are needed. And dT is about 5 degs.

[Nickfromwales]

1 hour ago, MrPotts said:

This is why I have replaced almost all my radiator thermostats so the hub now only takes information from one room thermostat.

So all the way around the gardens and back to basics then? KISS lol. 👌

[Nickfromwales]

3 hours ago, marshian said:

However in a lower flow temp system 30/40/50 it's very much an unwanted interruption to the boilers normal cycles

Very much unsuited to how an oil boiler wants to work. Exactly why in the last job with mixed temps flow vs oil boiler, I did a thermal store to provide temp & hydraulic separation and provide a wallop of DHW too. 
 

I’ll stick to my guns, this needs to be set up for the boiler to heat a buffer 100% on / 100% off, for more set durations, and the lot will run more efficiently.

 

Then these ‘smart-arse’ controls will come into their own. 
 

👌.