Thinking of adding the hot water kit to my Evohome

[SimonW500]

On the Potterton Suprima the manual says the temperature range is from 50-82 degrees but there are no markings around the adjustment knob at all. I usually leave it in the mid-way setting (about 66 degrees), except in very cold winters where I up it to 2/3 of its range. I was told as a rule of thumb, a boiler should never be operated on its maximum temperature but 82 degrees sounds like a fairly normal boiler temperature to me.

Also my boiler is well known for short cycling due to problems with it's control boards. But I don't think that is what I am having as if the system is completely cold, the cycles are long and fine and the radiators get toasty like they are meant to. My short cycling only happens on a hot water only cycle once it has been heating for some time and I suspect this is due to a sizing issue with the hot water loop vs the boiler sizing, or an undersized differential in the thermostat. I am confident the flow rate is fine.

I did see the overheat stat operate twice in a strange circumstance where hot water was off, heating was off, but evo still throttled the heating relay for several minutes. In my boiler this results in a flashing red light but does not require a reset. It recovers by itself. Since I rebooted the EVOHOME controller by removing the batteries, it hasn't happened again since.

Again exactly the same experience..

Perhaps worth pointing out for others the short cycling cut out we are talking about is indicated by a flashing GREEN light. A flashing green means either: a) call for heat ended so boiler no longer firing however pump overrun for 2 minutes, b) call for heat still there however boiler already up to target temp therefore not firing but instead just running the pump to circulate heat away. Key thing is here both these are entirely expected operations (as opposed to a flashing red which is a more drastic overheat, sometimes recovering itself and sometimes requiring a reset) - the only question in my mind is just however often these short cycles are to be expected / non-damaging. Suspect true answer is these boilers weren't really designed with TRVs in mind and further compounded by smart(ish!) heating systems = plays havoc with what was originally an appropriately sized boiler.

[erik]

Modern boilers are able to burn at much lower power (modulating), which should prevent a lot of short-cycling. When burning at low power, there's a better chance of the radiators being able to output all the heat generated by the boiler and not increasing the return temperature. This means the boiler can keep running on low power for quiete a while before reaching its maximum temperature. However, when the outside temperature isn't much lower than setpoint and you're only heating 1 room, even modern boilers are going to be way over-sized. And they will short-cycle.

Short-cyling in itself should not be a problem. It just means the boiler will be running less steady. More switching on/off. But your radiators shouldn't really care about it. The pump will often keep running when the boiler shuts off. This means that the water running through the radiators will be of a pretty constant temperature. The boiler puts in a lump of heat every X minutes, keeping it at pretty constant temperature in the end.

[G4RHL]

Modern boilers are able to burn at much lower power (modulating), which should prevent a lot of short-cycling. When burning at low power, there's a better chance of the radiators being able to output all the heat generated by the boiler and not increasing the return temperature. This means the boiler can keep running on low power for quiete a while before reaching its maximum temperature. However, when the outside temperature isn't much lower than setpoint and you're only heating 1 room, even modern boilers are going to be way over-sized. And they will short-cycle.

Short-cyling in itself should not be a problem. It just means the boiler will be running less steady. More switching on/off. But your radiators shouldn't really care about it. The pump will often keep running when the boiler shuts off. This means that the water running through the radiators will be of a pretty constant temperature. The boiler puts in a lump of heat every X minutes, keeping it at pretty constant temperature in the end.

Thank you Erik, that's helpful.

[jonstatt]

Modern boilers are able to burn at much lower power (modulating), which should prevent a lot of short-cycling. When burning at low power, there's a better chance of the radiators being able to output all the heat generated by the boiler and not increasing the return temperature. This means the boiler can keep running on low power for quiete a while before reaching its maximum temperature. However, when the outside temperature isn't much lower than setpoint and you're only heating 1 room, even modern boilers are going to be way over-sized. And they will short-cycle.

Short-cyling in itself should not be a problem. It just means the boiler will be running less steady. More switching on/off. But your radiators shouldn't really care about it. The pump will often keep running when the boiler shuts off. This means that the water running through the radiators will be of a pretty constant temperature. The boiler puts in a lump of heat every X minutes, keeping it at pretty constant temperature in the end.

That is indeed very helpful. In fact I was just talking with my Dad over lunch today wondering how difficult it would be for boilers to have adjustable flame/burn power to do exactly that. I didn't realise the latest boilers can indeed do this. I think that the concern is that a boiler that is constantly igniting will put greater stress/wear and tear on it, and therefore fail more quickly.

[jonstatt]

Again exactly the same experience..

Perhaps worth pointing out for others the short cycling cut out we are talking about is indicated by a flashing GREEN light. A flashing green means either: a) call for heat ended so boiler no longer firing however pump overrun for 2 minutes, b) call for heat still there however boiler already up to target temp therefore not firing but instead just running the pump to circulate heat away. Key thing is here both these are entirely expected operations (as opposed to a flashing red which is a more drastic overheat, sometimes recovering itself and sometimes requiring a reset) - the only question in my mind is just however often these short cycles are to be expected / non-damaging. Suspect true answer is these boilers weren't really designed with TRVs in mind and further compounded by smart(ish!) heating systems = plays havoc with what was originally an appropriately sized boiler.

Yes indeed. Thanks for posting your experience. By the way, the Potteron Suprima had 3 or 4 iterations of it's electronic board. The final version was created by Siemens. It does have an issue that causes short cycling but it is not like what we are seeing, but actually stopping the boiler heating up anything properly. You know you have this fault if the radiators only become slightly warm instead of hot. It requires yet another replacement of the board.

[erik]

My boiler, 15 years old, for some reason, always fires at full power the first 15 seconds after being fired. After 15 seconds, it switches to lower modes (I think it can do 40% as a lowest). But these 15 seconds of full power sometimes causes short cycling as well, especially when the radiators are already hot. Those 15 seconds are then enough to make it hit its maximum water temp of 60 that I have set. (it actually reaches 65 before stopping)

[SimonW500]

Yes indeed. Thanks for posting your experience. By the way, the Potteron Suprima had 3 or 4 iterations of it's electronic board. The final version was created by Siemens. It does have an issue that causes short cycling but it is not like what we are seeing, but actually stopping the boiler heating up anything properly. You know you have this fault if the radiators only become slightly warm instead of hot. It requires yet another replacement of the board.

My Potterton is of 1999 vintage however sometime between install and 2007 (when I bought the property) the board has been replaced (noted by the front panel now looking slightly different and where the model/serial number etc should be now handwritten!) but nothing since then. Don't suppose you have any information re. the various dates these replacement boards were released or a more even perhaps a more definitive way of determining which variation is installed (other than "if you have this fault it's probably an older known-dodgy one!")?

[SimonW500]

My boiler, 15 years old, for some reason, always fires at full power the first 15 seconds after being fired. After 15 seconds, it switches to lower modes (I think it can do 40% as a lowest). But these 15 seconds of full power sometimes causes short cycling as well, especially when the radiators are already hot. Those 15 seconds are then enough to make it hit its maximum water temp of 60 that I have set. (it actually reaches 65 before stopping)

Interesting - I had assumed my 15yr old boiler would be non-modulating simply due to its age (modulating models not around then) - is this assumption mistaken?

More specifically.. the Potterton Suprima 60 = non-modulating, right?

I was in two minds whether to replace it with a modulating model, largely due to the fact my home rarely has more than a third of its rads firing at any one time therefore for the vast majority of the time is oversized for the application.. yet my heating engineer was of the opinion it wasn't really worth the cost replacing until it actually failed (refreshingly honest advice!) - therefore just live with the short-cycling it will rightfully do to compensate (perhaps annoying me now more than it should due to a separate problem: crap in the system = noisy = waiting for a powerflush).

[erik]

My boiler is a modulating boiler. But it can't be controlled by a modulating thermostat. Maybe that causes some confusion? They're seperate things.

[SimonW500]

My boiler is a modulating boiler. But it can't be controlled by a modulating thermostat. Maybe that causes some confusion? They're seperate things.

No confusion - fully understand what you're saying on this: the control system simply calls for heat, how the boiler provides this (on/off all or nothing cycling or modulating the heat source) is not relayed back nor important. I just for some reason thought modulating boilers were a relatively new thing and not be a feature likely to be found on something 15 years old. Obviously not (perhaps just more common now?).

[jonstatt]

No confusion - fully understand what you're saying on this: the control system simply calls for heat, how the boiler provides this (on/off all or nothing cycling or modulating the heat source) is not relayed back nor important. I just for some reason thought modulating boilers were a relatively new thing and not be a feature likely to be found on something 15 years old. Obviously not (perhaps just more common now?).

While I am not in a position to be definitive on the matter, from my research and understanding the boiler that you and I both share is very simple...on or off. No modulation.

[SimonW500]

While I am not in a position to be definitive on the matter, from my research and understanding the boiler that you and I both share is very simple...on or off. No modulation.

+1 here. Unfortunately.

[sharpener]

On the Potterton Suprima the manual says the temperature range is from 50-82 degrees but there are no markings around the adjustment knob at all. I usually leave it in the mid-way setting (about 66 degrees), except in very cold winters where I up it to 2/3 of its range. I was told as a rule of thumb, a boiler should never be operated on its maximum temperature but 82 degrees sounds like a fairly normal boiler temperature to me.

Helpful aren't they? I used to have an Ideal Mexico with a scale of 1 to 7(!).

82 degrees is the Celsius equivalent of 180 F which for many many years was the flow temperature assumed when sizing radiators (160F return). So it should be safe to use this setting indefinitely.

But the boiler efficiency will be significantly higher if you can turn it down to the lowest setting you can get away with for the time of year, because the cooler water means more heat is extracted from the flue gases. This is what Weather Compensation does, by using an outside air temperature sensor typically the boiler temperature is reduced by 3 degrees for every degree it gets warmer outside.

If you can get it below the dew point (~55 C) then you get condensing action and the flue gases give up even more heat to the water from the latent heat of condensation. (Condensing boilers will only condense if the control system will allow this to happen, often it doesn't).

Modulating boilers are a further improvement because when they are running at say 20% of full power the heat transfer is much better. As the temperature differential across the heat exchanger is only one fifth as much it's as though the heat exchanger were suddenly five times as big, and consequently you are more likely to achieve condensing action than when a boiler is operating at full power for 20% of the time.

All of which explains why the evohome system will give the best thermodynamic efficiency when driving a modulating boiler using the OpenTherm bridge not the BDR91 (assuming of course that the control algorithmns are sensible)

HTH

[jonstatt]

Helpful aren't they? I used to have an Ideal Mexico with a scale of 1 to 7(!).

82 degrees is the Celsius equivalent of 180 F which for many many years was the flow temperature assumed when sizing radiators (160F return). So it should be safe to use this setting indefinitely.

But the boiler efficiency will be significantly higher if you can turn it down to the lowest setting you can get away with for the time of year, because the cooler water means more heat is extracted from the flue gases. This is what Weather Compensation does, by using an outside air temperature sensor typically the boiler temperature is reduced by 3 degrees for every degree it gets warmer outside.

If you can get it below the dew point (~55 C) then you get condensing action and the flue gases give up even more heat to the water from the latent heat of condensation. (Condensing boilers will only condense if the control system will allow this to happen, often it doesn't).

Modulating boilers are a further improvement because when they are running at say 20% of full power the heat transfer is much better. As the temperature differential across the heat exchanger is only one fifth as much it's as though the heat exchanger were suddenly five times as big, and consequently you are more likely to achieve condensing action than when a boiler is operating at full power for 20% of the time.

All of which explains why the evohome system will give the best thermodynamic efficiency when driving a modulating boiler using the OpenTherm bridge not the BDR91 (assuming of course that the control algorithmns are sensible)

HTH

That does help...thank you. It also validates what I have been doing whereby I increase the boiler temperature a bit in the coldest weather but lower it when the weather allows. However I have to keep it above 60 degrees otherwise the hot water won't get to its target.

In the case of the Potterton Suprima, it is a conventional boiler without condensing action. By law now I believe new boilers in the UK have to be condensing one. Despite the theory, the reality is that the efficiency gain is less than 10% over a conventional boiler so I don't believe many will rush to upgrade until they are forced to

[SimonW500]

In the case of the Potterton Suprima, it is a conventional boiler without condensing action. By law now I believe new boilers in the UK have to be condensing one. Despite the theory, the reality is that the efficiency gain is less than 10% over a conventional boiler so I don't believe many will rush to upgrade until they are forced to

I'm with you on this: while I think there will be an efficiency gain replacing the 'dumb' Potterton with a condensing modulating alternative, I don't think this will be anything close to a cost effective replacement until such a point whereby the Potterton requires an expensive repair (££££). Which is a shame really as I generally like to have the latest tech.. that said I'm not a fan of throwing significant money after disproportionate gain so pretty much rules this one out.