Explanation of thermostat vs control temp

[Dylan T]

Well I don't know if anyone saw, but Heat Geek just premiered a new video on youtube with a head guy from Tado announcing they'd brought back opentherm support for their wireless model, although they announced it was available now and seems to be sold out. Still seems most promising option for me to get some nice compensation controls without having to get too funky with DIY solutions, but the question is whether a few more years in this house sees enough savings to make it worthwhile, and whether the wait is long for it to come back into stock.

 

[vulcancontinental]

What's your boiler, what's your annual gas consumption? Currently NG is around 8.5p/kW, what do you anticipate your saving to need to be to make it worthwhile subtracted from any improvement in usability or convenience or comfort?

 

[karlbbb]

What's your boiler, what's your annual gas consumption? Currently NG is around 8.5p/kW, what do you anticipate your saving to need to be to make it worthwhile subtracted from any improvement in usability or convenience or comfort?

I'm struggling to work this out myself. My basic on/off thermostats are doing the job right now - I'm doing a bit of manual "OpenTherm" by keeping the flow temperature artificially low while the temperatures are still mild. But I also know my boiler is vastly oversized and doesn't modulate down below 7.5kW, in a fairly new house which is likely suffering <1kW heat loss at the moment, and likely not much higher even in to the colder temperatures.

Most estimates say around 2-3% saving by getting proper modulating controls, but for this house I'm guessing that's at best a saving of £20/year, and I'd be looking at £400 install to get OpenTherm as my house is zoned upstairs/downstairs and very few thermostat systems do OpenTherm for multiple zones. I'm sure it'd be more comfortable having the flow temperature properly modulating, but that's going to come at the expense of the boiler cycling even more than it would left on the "ECO" setting of 68ºC flow temp - how quickly would the wear and tear add to the payback period?

 

[Harry Bloomfield]

I'm sure it'd be more comfortable having the flow temperature properly modulating, but that's going to come at the expense of the boiler cycling even more than it would left on the "ECO" setting of 68ºC flow temp - how quickly would the wear and tear add to the payback period?

That reads as if you are confused. Modulation is where the heat created by the boiler is reduced to try to match a less than 100% demand for heat. The design of the boiler dictates how much the boiler can modulate it's output down, without having to switch off, the lower they can modulate, the better, in as much as they will need to start and stop less, which can make them more efficient.

A boiler just working on an on/off demand system, when there is demand, can only modulate down, based on it's flow thermostat.

Opentherm can improve on that, by telling the boiler to modulate down it's output, based on knowing the demand for heat has almost been met. It can also have the boiler output it's minimum output, when there is an initial call for heat, because it can predict just how much heat might be needed.

The net result is much more stable room temperatures, and some saving in gas use and wear and tear from cycling.

For example - I put my heating on at lunch time, by tweaking the room stat up by 2C, with an Opentherm type system. The boiler fired up, modulated down to it's minimum output. I have just fitted pipe thermostats to the flow and return pipes, out of curiosity. Flow got to 35C, return to 29C, as the boiler cut off completely, despite the room temperature not being quite there. 15 minutes later, the room temperature hit the 18C I had set, without the boiler firing again.

Prior to my installing my Opentherm type controls, the boiler would have fired at maximum output, until limited by the boiler flow temperature, or the stat clicking off the demand.

 

[karlbbb]

That reads as if you are confused. Modulation is where the heat created by the boiler is reduced to try to match a less than 100% demand for heat. The design of the boiler dictates how much the boiler can modulate it's output down, without having to switch off, the lower they can modulate, the better, in as much as they will need to start and stop less, which can make them more efficient.

A boiler just working on an on/off demand system, when there is demand, can only modulate down, based on it's flow thermostat.

Opentherm can improve on that, by telling the boiler to modulate down it's output, based on knowing the demand for heat has almost been met. It can also have the boiler output it's minimum output, when there is an initial call for heat, because it can predict just how much heat might be needed.

The net result is much more stable room temperatures, and some saving in gas use and wear and tear from cycling.

For example - I put my heating on at lunch time, by tweaking the room stat up by 2C, with an Opentherm type system. The boiler fired up, modulated down to it's minimum output. I have just fitted pipe thermostats to the flow and return pipes, out of curiosity. Flow got to 35C, return to 29C, as the boiler cut off completely, despite the room temperature not being quite there. 15 minutes later, the room temperature hit the 18C I had set, without the boiler firing again.

Prior to my installing my Opentherm type controls, the boiler would have fired at maximum output, until limited by the boiler flow temperature, or the stat clicking off the demand.

I'm not too confused, but perhaps I worded it badly - my boiler is so oversized that getting OpenTherm controls will make it cycle more often. The minimum output is 7.5kW. I doubt my house would ever lose that amount of heat even at -3ºC, so this means even when the boiler modulates the flame down to that minimum, it's adding far, far too much heat into the system, therefore starting the flame on/off cycle (not a boiler/heating on/off cycle).

Adding OpenTherm will exacerbate this even more as lowering the flow temperature will mean even less heat needs to go into the system, so the boiler will be cycling the flame on/off a lot, as when the flame first fires, it's not at the lowest modulation, so to start with it could be putting 18-22kW of heat into the system. This will make the flow temperature rise very rapidly above the set point, at which time the boiler will then shut off the flame, wait for the flow temperature to reduce to set point minus 5ºC, then fire, then it'll quickly reach set point plus 5ºC and shut off, and on and on we go...

 

[Harry Bloomfield]

Adding OpenTherm will exacerbate this even more as lowering the flow temperature will mean even less heat needs to go into the system, so the boiler will be cycling the flame on/off a lot, as when the flame first fires, it's not at the lowest modulation, so to start with it could be putting 18-22kW of heat into the system.

I doubt Opentherm would exacerbate the issue, but I'm not sure it would improve it any. Why is your boiler so oversized?

 

[karlbbb]

I doubt Opentherm would exacerbate the issue, but I'm not sure it would improve it any. Why is your boiler so oversized?

The only answer to this is "ask Persimmon Homes" . I imagine they just bought a job lot of this boiler to cover every house in the estate, using the largest demand as the benchmark. The model's heating output range is 7.1-25.6kW depending on temperature (and 35kW output for DHW). All for a modest, 3-bed detached home with approx 90m2 covered by the radiators (a garage conversion has an electric radiator).

I think we'd be better served by a boiler that could do something more in the range of 1-10kW for heating, but I'm not sure they exist, at least not as a Combi that can supply the hot water on demand.

 

[Harry Bloomfield]

I think we'd be better served by a boiler that could do something more in the range of 1-10kW for heating, but I'm not sure they exist, at least not as a Combi that can supply the hot water on demand.

Your limitation there, is the amount of heat you need to produce your hot water. Reduce the Kw rating and it will not heat your hot water - it's always a problem with a combi system and one of the reasons I refuse to have my open vented changed to a combi.

 

[karlbbb]

Your limitation there, is the amount of heat you need to produce your hot water. Reduce the Kw rating and it will not heat your hot water - it's always a problem with a combi system and one of the reasons I refuse to have my open vented changed to a combi.

Exactly. I'll just make the best of what we have for now, and when the time comes to replace the boiler, I'll get the smallest one we can get away with based on DWH demand.

I'd certainly like to add some kind of proper modulating (flow temperature) controls, but as I mentioned a few posts above, I'm hitting a high buy-in cost thanks to the house being zoned by law so payback period is very long. I'm sure the increased comfort will certainly help but even then it's a lot of money for feeling slightly more comfortable

 

[JonathanM]

I'm not too confused, but perhaps I worded it badly - my boiler is so oversized that getting OpenTherm controls will make it cycle more often. The minimum output is 7.5kW. I doubt my house would ever lose that amount of heat even at -3ºC, so this means even when the boiler modulates the flame down to that minimum, it's adding far, far too much heat into the system, therefore starting the flame on/off cycle (not a boiler/heating on/off cycle).

Adding OpenTherm will exacerbate this even more as lowering the flow temperature will mean even less heat needs to go into the system, so the boiler will be cycling the flame on/off a lot, as when the flame first fires, it's not at the lowest modulation, so to start with it could be putting 18-22kW of heat into the system. This will make the flow temperature rise very rapidly above the set point, at which time the boiler will then shut off the flame, wait for the flow temperature to reduce to set point minus 5ºC, then fire, then it'll quickly reach set point plus 5ºC and shut off, and on and on we go...

I've been wondering myself, when a boiler is oversized even when at its lowest modulation, whether running at lower flow temperatures increases the amount of cycling. It seems intuitive that it would, because it can only burn so long before that lower temperature is reached. It's the final part of the puzzle I've been trying to get my head around, following previous discussions on here about range rating, modulation, cycling and flow temperature.

Do you have any settings on the boiler to ameliorate cycling, like adjusting anti-cycle time? or adjusting the initial burn rate and the time it takes to ramp up? Some boilers have more options than others.

 

[vulcancontinental]

I've been wondering myself, when a boiler is oversized even when at its lowest modulation, whether running at lower flow temperatures increases the amount of cycling. It seems intuitive that it would, because it can only burn so long before that lower temperature is reached. It's the final part of the puzzle I've been trying to get my head around, following previous discussions on here about range rating, modulation, cycling and flow temperature.

Do you have any settings on the boiler to ameliorate cycling, like adjusting anti-cycle time? or adjusting the initial burn rate and the time it takes to ramp up? Some boilers have more options than others.

Yes, the above plus hot water delay, heating delay, soft start

 

[karlbbb]

I've been wondering myself, when a boiler is oversized even when at its lowest modulation, whether running at lower flow temperatures increases the amount of cycling. It seems intuitive that it would, because it can only burn so long before that lower temperature is reached. It's the final part of the puzzle I've been trying to get my head around, following previous discussions on here about range rating, modulation, cycling and flow temperature.

Do you have any settings on the boiler to ameliorate cycling, like adjusting anti-cycle time? or adjusting the initial burn rate and the time it takes to ramp up? Some boilers have more options than others.

As far as I can see, the boiler can't fire up on the lowest modulation, and so as the boiler cycles the flame it - very briefly - dumps a lot of energy into the loop. At lower flow temperatures it seems to exceed the set point very, very quickly due to this, hence my thinking the problem will grow with a "proper" OpenTherm setup.

I've not investigated those options actually. I imagine if I pull up the service manual for the boiler I may be able to get into some options. I certainly saw how to get into the maintenance options such as firing tests, so I'll give it another look through. It's an Ideal Logic Combi C35, for reference (https://idealheating.com/products/logic-max-combi).

 

[vulcancontinental]

As far as I can see, the boiler can't fire up on the lowest modulation, and so as the boiler cycles the flame it - very briefly - dumps a lot of energy into the loop. At lower flow temperatures it seems to exceed the set point very, very quickly due to this, hence my thinking the problem will grow with a "proper" OpenTherm setup.

I've not investigated those options actually. I imagine if I pull up the service manual for the boiler I may be able to get into some options. I certainly saw how to get into the maintenance options such as firing tests, so I'll give it another look through. It's an Ideal Logic Combi C35, for reference (https://idealheating.com/products/logic-max-combi).

Some can but I dont think the logic can. There are posts elsewhere where boilers with huge modulation ranges will microfire so after a DHW reheat cycle at high temp it fires for heating at the ignition fan speed, a mid-output, and doesn't even have time to modulate down before the modulated setpoint is reached.

 

[karlbbb]

Some can but I dont think the logic can. There are posts elsewhere where boilers with huge modulation ranges will microfire so after a DHW reheat cycle at high temp it fires for heating at the ignition fan speed, a mid-output, and doesn't even have time to modulate down before the modulated setpoint is reached.

I found the service/install manual for the boiler, and it certainly looks like there's nothing that can be set. Service mode only allows for testing of pump speed and max/min firing rates, with no settings available, so all control looks to be firmly baked into the boiler. Never mind!

I feel like I need two boilers. A small one for CH, and a larger one for DHW.

 

[JonathanM]

I found the service/install manual for the boiler, and it certainly looks like there's nothing that can be set. Service mode only allows for testing of pump speed and max/min firing rates, with no settings available, so all control looks to be firmly baked into the boiler. Never mind!

I feel like I need two boilers. A small one for CH, and a larger one for DHW.

Ideal boilers seem to have very few options.

Very short cycles reduce efficiency, but I don't know in practice how much of a problem short cycling is, as regards wear and tear. I know that most modern thermostats use a TPI algorithm to control the temperature, and they fire the boiler in short cycles on purpose. Also, a poster pointed out to me that oil boilers cycle a lot more without apparently causing a problem.