Boiler short cycling with low flow temperatures

[seanblee]

I've got a Worcester 38 CDI Classic combi boiler supplying a small 90s 3 bedroom house (8 radiators, 2 towel rails, UFH in kitchen extension). Pipework is 22mm primaries at the boiler with 10mm microbore drops to each radiator. I've set up some automation in Home Assistant to provide a degree of weather compensation and lower the flow temperature on warmer days, but I'm finding the boiler is struggling with this - unless the flow temperature is 65 or above, it hits target after about 30-60 seconds then shuts down the burner. I can see from the metrics that it's modulating down to 28%, which appears to be the minimum. I think my problem is the microbore pipework (so small water volume) and the rubbish modulation ratio of the boiler - I think my heat loss is probably around 6kW and the lowest modulation is over 10kW. Is there anything I can do to improve this (buffer/volumiser, increasing radiator sizes, ...), or am I fighting a loosing battle? Plan is to replace the boiler with an ASHP in the future but it's only about 6 years old so I was hoping not to do this just yet. Many thanks!

[JohnMo]

Boiler and equally so heat pumps, just produce heat, if the heat cannot be dispersed to the house, return temp stays too high and you get what you are seeing. Splitting the system with zones will make life hard for the boiler.

 

Are you running with lots of thermostats, TRVs etc. micro bore may not be the issue, actual flow rate may be. Remember the microbore is only carrying the flow to a radiator which will have a decent volume.

 

 

[JohnMo]

11 minutes ago, seanblee said:

I've set up some automation in Home Assistant to provide a degree of weather compensation

HA has it's merits, until your internet stops, or you have a power cut and the server doesn't automatically restart - been there. I have just moved everything away from HA for heating, so nothing is reliant on internet based services.

 

Think your boiler does Opentherm, so I would look at an Opentherm Tado to manage the boiler for you. Then dump HA from the heating system.

[seanblee]

I've got Evohome with smart TRVs but I'm starting to phase this out - I've put two zones back on "dumb" TRVs and synchronised the schedule for the other downstairs zones to try and make sure most of the system is "open" at the same time. The area I'm struggling with the most is the lounge - this has two vertical aluminium radiators (Milano Skye 1800x375, 840W at T50) but these have very low water volume and I get a lot of cycling when it's the only zone calling for heat, so I think I may need to swap them for something bigger.

[seanblee]

I did try Opentherm with Evohome using the Nefit adapter that was suggested but it didn't work properly - the boiler didn't fire most of the time and just ran the pump so the house got very cold.

[JohnMo]

1 hour ago, seanblee said:

I get a lot of cycling when it's the only zone calling for heat,

That's to be expected. One zone and 10kW output stands no chance of running well. Putting bigger rads in, is unlikely to help if only that zone is allowed to call for heat. Would really try to get it all on or all off, nothing in between. Dumb TRVs that don't fully close, maybe helpful in a small number, not everywhere.

 

You may need to balance the system so room comes up in temperature at a fairly equal rate and use a single thermostat to bring all on and all off together. Keep it simple is generally the only way.

 

[SimonD]

In my conversions, the nefit opentherm ems converter works well, but when boilers are more closely matched to heat loss.

 

The Evohome system may be a problem here too. In this regard, I have to apologise to both @PeterW and @Nickfromwales because I now agree with them that it isn't great - well actually it's shite, especially for the price. I've seen problems, and have one installation  I'm dealing with where it also has the tendency to short cycle the boiler under certain circumstances and its temperature management of the house is rubbish. I'm about to throw it in the bin and use EPH controls that will support proper pdhw and opentherm too. At least with EPH you've got access to the Opentherm configuration settings, unlike Evohome which likes to hide everything away.

 

Do keep in mind that some boilers may not like running at low flow temperatures, which means that settling for 65 degrees if that is the minimum you've found the boiler to happily run at. I've found a few where this is the case because of the boiler and system design. As long as you're getting a low enough return temps, try it out for a while to see what happens. As a note currently I have exactly this problem with an Ideal Vogue max that will not run below 55 flow Temp without short cycling.

 

With your tall rads, it's worth also double checking that they do indeed have a 20degree drop - I've come across many that due to their design can't be properly balanced because flow & returns short circuit. And this in turn plays havoc with the boiler.

 

So a few things to ponder

[marshian]

3 hours ago, seanblee said:

I've got a Worcester 38 CDI Classic combi boiler supplying a small 90s 3 bedroom house (8 radiators, 2 towel rails, UFH in kitchen extension). Pipework is 22mm primaries at the boiler with 10mm microbore drops to each radiator. I've set up some automation in Home Assistant to provide a degree of weather compensation and lower the flow temperature on warmer days, but I'm finding the boiler is struggling with this - unless the flow temperature is 65 or above, it hits target after about 30-60 seconds then shuts down the burner.

 

Sounds to me like the boiler isn't seeing enough flow to keep it happy in the initial purge and high output phase unless the flow temp is set high enough that it doesn't overshoot (most boilers initial fire/purge is at 75% of max before they start modulating down)

 

I had similar issues with a Glow Worm 24kW boiler which had a similar 10kW min - my resolution to run lower flow temps was to sacrifice the delta T at the rads - ie increase the pump speed and or and free off the rads will still maintaining the balance in the system.

 

If you have an ABV in the circuit you could try increasing the flow thro it a little but ultimately returning flow back to the boiler via an ABV isn't great for efficiency

 

3 hours ago, seanblee said:

I can see from the metrics that it's modulating down to 28%, which appears to be the minimum. I think my problem is the microbore pipework (so small water volume) and the rubbish modulation ratio of the boiler - I think my heat loss is probably around 6kW and the lowest modulation is over 10kW.

 

Microbore to the rads isn't necessarily a problem (if it's just a short section from main flow and return that are 22mm)

 

If your heat loss at around -2 is 6kW and the min modulation is 10kW it should be able to cope with that

 

10kWh with 6kWh requirement is going to run for ~30 mins in the hour with all rads in circuit provided you have the flow thro the boiler above the min which will be 10 lpm

 

However once TRV's start to close the circuit size is going to shrink quite quickly

 

3 hours ago, seanblee said:

Is there anything I can do to improve this (buffer/volumiser, increasing radiator sizes, ...), or am I fighting a loosing battle? Plan is to replace the boiler with an ASHP in the future but it's only about 6 years old so I was hoping not to do this just yet. Many thanks!

 

Increasing rad sizes might make things worse - will result in rooms getting to temp faster if you can't get the flow temps down to work with the larger rads.

 

A volumiser might help - it was in my plan to deal with the glow worm I had but as it was 14 years old I chose to replace it instead. I can understand you not wanting to replace a boiler that is only 6 years old

 

I'd try increasing the flow rate thro the circuit first and see if you can get the boiler to cope with a lower flow temp and not overshoot on the initial fire/purge

 

 

 

[SimonD]

1 hour ago, marshian said:

Sounds to me like the boiler isn't seeing enough flow

 

Yes, that's pretty much what seems to happen and thus no heat distribution either.

 

The 38CDi classic should theoretically be able to run 40/30 flow and return, so I think there is a control issue.

 

Would be interesting to test the system on relay control only and then set flow temp at boiler with system all open.

[marshian]

1 hour ago, SimonD said:

With your tall rads, it's worth also double checking that they do indeed have a 20degree drop - I've come across many that due to their design can't be properly balanced because flow & returns short circuit. And this in turn plays havoc with the boiler.

 

Made me chuckle because I'd just written a post about just that situation - rads with inbuilt diverters which don't

 

 

[marshian]

7 minutes ago, SimonD said:

 

Yes, that's pretty much what seems to happen and thus no heat distribution either.

 

The 38CDi classic should theoretically be able to run 40/30 flow and return, so I think there is a control issue.

 

Well I defer to your "Boiler Knowledge" cos I only have good knowledge about Glow Worm boilers (although that crosses over to Valliants)  but if a 38CDi Classic can run 40/30 then I'd say the system is too restrictive right now to allow it to do that with a 10kW min

[seanblee]

Thanks folks. The Evohome is currently switching the boiler by relay and I've got a spare round thermostat, so I could try taking Evohome out of the picture and pairing that directly instead, but I think it does TPI control too. I'm not sure about the heating flow and return, it's possible it reduces to 15mm later in the run. When I balanced radiators at 65deg flow, I was going for a 10 degree drop across the rad - should I be aiming for more than that? 

 

I've got monitoring of the boiler through an EMS-ESP and that shows it dropping the burner to 28% within a few seconds but the flow temperature continues to climb rapidly (and it continues to climb for a bit after the burner is turned off too). I've tried the pump on proportional low but I'm currently running it on fixed speed step 3 (of 7) - any faster and the radiators get quite noisy.

[seanblee]

Oh, and there's no ABV on the system (one of the towel rails has no TRV). I also struggled a lot with balancing one of the tall radiators - I got to about 6 degrees drop, but if I closed the lock shield any more, the flow temperature started dropping too. I think it's near the end of the system, so maybe not enough residual head? However, it's also the noisiest if I turn the pump speed up...

[Nickfromwales]

3 hours ago, SimonD said:

In my conversions, the nefit opentherm ems converter works well, but when boilers are more closely matched to heat loss.

 

The Evohome system may be a problem here too. In this regard, I have to apologise to both @PeterW and @Nickfromwales because I now agree with them that it isn't great - well actually it's shite, especially for the price. I've seen problems, and have one installation  I'm dealing with where it also has the tendency to short cycle the boiler under certain circumstances and its temperature management of the house is rubbish. I'm about to throw it in the bin and use EPH controls that will support proper pdhw and opentherm too. At least with EPH you've got access to the Opentherm configuration settings, unlike Evohome which likes to hide everything away.

 

Do keep in mind that some boilers may not like running at low flow temperatures, which means that settling for 65 degrees if that is the minimum you've found the boiler to happily run at. I've found a few where this is the case because of the boiler and system design. As long as you're getting a low enough return temps, try it out for a while to see what happens. As a note currently I have exactly this problem with an Ideal Vogue max that will not run below 55 flow Temp without short cycling.

 

With your tall rads, it's worth also double checking that they do indeed have a 20degree drop - I've come across many that due to their design can't be properly balanced because flow & returns short circuit. And this in turn plays havoc with the boiler.

 

So a few things to ponder

Thanks for this feedback.
 

No need for apologies, folk comment here on their current experiences, if stuff changes then what you’ve done is spot on, eg come back and tell us 😎😉🫡
 

Not just me that thinks they’re boat anchors. Defo oversold in the ‘bumf’ and failed to deliver big time for me (certified Passivhaus project where I was attempting to shine). Cheers HW 😤. 
 

Since replaced with Heatmiser by a local sparky, and I wish I’d have not stuck my neck out with HW EvoHome, hindsight being a wonderful thing etc etc. 
 

HW are great for just about everything else, but here, sadly, a complete flop. When you boil the HW EH down, they’re just a really flashy (and expensive) on / off switch, with very few strings to its bow. 

[marshian]

58 minutes ago, seanblee said:

Thanks folks. The Evohome is currently switching the boiler by relay and I've got a spare round thermostat, so I could try taking Evohome out of the picture and pairing that directly instead, but I think it does TPI control too. I'm not sure about the heating flow and return, it's possible it reduces to 15mm later in the run. When I balanced radiators at 65deg flow, I was going for a 10 degree drop across the rad - should I be aiming for more than that?

 

Depends on the rads - running 60 deg flow on my old glow worm I was struggling to better 8 deg on a T11 and 14 on a T22 rad

 

I gave up worrying about getting rads to the highest possible delta T and just concentrated on getting the right mean water temps to heat the rooms

 

So in responding to your last question a higher than 10 deg delta is going to make life worse for your boiler so no don't aim for more aim for less

 

 

58 minutes ago, seanblee said:

I've got monitoring of the boiler through an EMS-ESP and that shows it dropping the burner to 28% within a few seconds but the flow temperature continues to climb rapidly (and it continues to climb for a bit after the burner is turned off too). I've tried the pump on proportional low but I'm currently running it on fixed speed step 3 (of 7) - any faster and the radiators get quite noisy.

 

The rads get noisy because you are restricting the flow so generating system noise

 

Open the lockshields a little and lower the pump speed and keep it on a fixed speed - proportional will do the opposite of what you want

 

Just for an example I'm running a ~30 deg flow temp at around zero deg outside air temp - I get a delta at the boiler of 7 deg

 

Pump speed is 2 (fixed) and is around 0.5 m3/hr - there are 13 rads in the circuit

 

My biggest rad 700 x 1400 K33 has a flow in of 30 deg and a return of 24 deg and it's getting nearly 20% of the total circuit flow but it's a big room and one rad has to heat that room

 

[marshian]

1 hour ago, seanblee said:

Oh, and there's no ABV on the system (one of the towel rails has no TRV). I also struggled a lot with balancing one of the tall radiators - I got to about 6 degrees drop, but if I closed the lock shield any more, the flow temperature started dropping too. I think it's near the end of the system, so maybe not enough residual head? However, it's also the noisiest if I turn the pump speed up...

 

Verticals are a pain - you have to sacrifice some delta T at the rad - they aren't as good as heat emitters and need a good bit of flow - restricting them doesn't work very well

 

Ladder type towel rails are the work of the devil - they don't balance very well - they are rubbish as heat emitters and generally they live to short circuit!!!

 

My bypass rad is a 1400 x 500 T22 in the hallway - it has a flow rate of 7% of the total - keeps the hall at 18 deg and allows the boiler to fire if the circuit shrinks to just a few rads

[SimonD]

16 hours ago, marshian said:

Made me chuckle because I'd just written a post about just that situation - rads with inbuilt diverters which don't

 Yes, I saw that. Good modification you made. Something I warn all my customers about when they buy them but most don't listen.

[marshian]

7 minutes ago, SimonD said:

 Yes, I saw that. Good modification you made. Something I warn all my customers about when they buy them but most don't listen.

 

Funny that - customers* are strange!!!

 

* it's why I don't have any

 

I have every sympathy for those that do!!!

Edited February 4 by marshian
Clarify

[seanblee]

I've removed the Evohome valves from all but the lounge radiators and put everything else back on TRV4s - that leaves Evohome controlling the two vertical rads, the boiler and the UFH (via another BDR91 to power up the zone valve, pump and actuators). I'll let you know if that helps! Should I be considering an ABV, or some sort of flow rate monitoring?

[seanblee]

19 hours ago, marshian said:

 

Made me chuckle because I'd just written a post about just that situation - rads with inbuilt diverters which don't

 

 

When I get a moment, I'll drain and remove the vertical rads and check the diverters - mine are a bit more awkward because I've got central flow and return via a H block valve, but there are bungs at each end of the bottom tube so I'll remove those and check.

[seanblee]

No obvious improvement yet, but I'll see what it's like tomorrow morning on warm-up...

[marshian]

9 cycles an hour between 12 and 1? And in the afternoon way more - I’d not be happy with that at all

[MrPotts]

27 minutes ago, seanblee said:

No obvious improvement yet, but I'll see what it's like tomorrow morning on warm-up...

The boiler is being cycled 6 times an hour by Evohome. It is not the boiler initiating the cycle. My Drayton Wiser does exactly the same. I think you can change the cycling frequency within the Evohome settings.

[seanblee]

8 minutes ago, MrPotts said:

The boiler is being cycled 6 times an hour by Evohome. It is not the boiler initiating the cycle. My Drayton Wiser does exactly the same. I think you can change the cycling frequency within the Evohome settings.

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...

[marshian]

1 hour ago, MrPotts said:

The boiler is being cycled 6 times an hour by Evohome. It is not the boiler initiating the cycle. My Drayton Wiser does exactly the same. I think you can change the cycling frequency within the Evohome settings.

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