Explanation of thermostat vs control temp

[ScottishGasMan]

On/off controls, generally refers to your typical clock/programmer and room thermostat arrangement, where it either sends a signal saying have the heating on, or dont have the heating on. The boiler will run with a fixed target flow temperature, but will still modulate the flame size depending on how near or far it is from that flow temperature.

Take this example. The boiler can modulate its output, but is fitted with standard on/off controls.

Room at 15°C, You turn upto 20°C. Boiler fires up, with a fixed target flow temp of 75°C (very common), Way more heat than is required will be put into the rooms, so the room tempearture goes above 20°C and the thermostat will cut the demand for heating off. After a little time, the room cools, and the thermostat clicks and sends a demand for heat again, boiler fires up, aims for 75°C on the flow, room overheats, cuts off again. The "modulation" capabilities of the boiler in this case, only refer to the burner, so it will get to about 70ish and reduce the flame to try maintain 75°C in the flow pipework as that is how it has been told to run.

This description is probably about 99% of UK heating systems. (On gas boilers)

 

[JonathanM]

If the boilers minimum modulation is higher than what is needed, it will inevibetly keep increasing the flow temperature as it cant adjust the power any lower, and will cut off to cycle and allow the system water to cool a bit, before coming back on, other wise it would get too hot.

I see that happening with my uncle's new boiler. It has great modulation, but still it creeps up every so slowly, 0.1C at a time, until it is 5C over set point, and then it turns off.

 

[JonathanM]

Take this example. The boiler can modulate its output, but is fitted with standard on/off controls.

Room at 15°C, You turn upto 20°C. Boiler fires up, with a fixed target flow temp of 75°C (very common), Way more heat than is required will be put into the rooms, so the room tempearture goes above 20°C and the thermostat will cut the demand for heating off. After a little time, the room cools, and the thermostat clicks and sends a demand for heat again, boiler fires up, aims for 75°C on the flow, room overheats, cuts off again. The "modulation" capabilities of the boiler in this case, only refer to the burner, so it will get to about 70ish and reduce the flame to try maintain 75°C in the flow pipework as that is how it has been told to run.

This description is probably about 99% of UK heating systems. (On gas boilers)

Thanks. So, different types of modulation, burner and controls.

 

[ScottishGasMan]

hanks. So, different types of modulation, burner and controls.

Yep. Pretty much any mains gas boiler will have a modulating burner in it, unless its pretty old (think back to the likes of cast iron boilers, the backboiler that hid behind the fireplace, or the big floor standing units)

Where as modulating control is very uncommon. Its gained a lot of popularity in last few years, but still a tiny minority of installs out there have it. If and when you ever read into the likes of Heat-Pump systems, it is essential they have some form of modulating control, as the low temperature giving higher efficiency is multiplied by massive factors for them.

 

[ScottishGasMan]

 

[JonathanM]

Yep. Pretty much any mains gas boiler will have a modulating burner in it, unless its pretty old (think back to the likes of cast iron boilers, the backboiler that hid behind the fireplace, or the big floor standing units)

The main reason I've started looking into all this is because here there is still a floor standing Baxi from 1990 and I was wondering if it made sense to replace it. But with the energy freeze it's much more marginal.

 

[ScottishGasMan]

Thats the readings of my flow and return temperatures for today. High spike in middle was a legionella disinfection cycle the system runs once per week to ensure no microbial build up in the hot water cyclinder.

The rest of the time the boiler is responding to the room and outside temperature. Thats holding the house at 17.5 degrees during the day and 20 from about 5pm

 

[JonathanM]

Is that your system? Is the big spike the hot water?

EDIT posts crossed!

 

[JonathanM]

Thats the readings of my flow and return temperatures for today. High spike in middle was a legionella disinfection cycle the system runs once per week to ensure no microbial build up in the hot water cyclinder.

The rest of the time the boiler is responding to the room and outside temperature. Thats holding the house at 17.5 degrees during the day and 20 from about 5pm

Amazing control. This is what I want eventually.

 

[ScottishGasMan]

The main reason I've started looking into all this is because here there is still a floor standing Baxi from 1990 and I was wondering if it made sense to replace it. But with the energy freeze it's much more marginal.

It will only have so much life left in it, but with those old boilers it can be 10months or 10 more years! They are pretty wastefull energy and CO2 wise when looking at environmental impacts, but there also near bulletproof.

If it were mine, I'd put a bit of money aside and wait till I'm doing work in the house, and look at my options when there will be upheaval. The best thing you can do is make sure your system is good, i.e.: radiators that have been sized to work at low temperatures, pipework thats sized to suit whatever technologie goes in in future, as in heatpumps need bigger pipes usually. And when its time to replace hopefully you can do more.

The boiler is a small part of the equation, the system and controls are what makes it.

FYI, dont try to run your current boiler at low temperature, as they do not like condensing and it actually damages them.

 

[ScottishGasMan]

Is that your system? Is the big spike the hot water?

Normally I heat my hot water using 55°C flow temp from boiler, and store the water at 48°C, but there is a risk (albeit very very small) of legionella bacteria building up at that temperature, so once a week the control will raise the cylinder above 60°C to kill any potential build up.

 

[JonathanM]

FYI, dont try to run your current boiler at low temperature, as they do not like condensing and it actually damages them.

I have actually turned it down the past two weeks, not to save money, because I know it doesn't, but just for comfort.

What would be a minimum safe flow and return?

 

[ScottishGasMan]

If the return is above 55-60 then you shouldnt have any chance of condensation forming when its running at normal pace.

Older units like that and the radiator systems associated with them were typically designed to run around 80°C flow and 70°C return, although they all had a thermostat that you could turn down at this time of year so its not like a self destruct button turning it down a bit, but long term on a system designed for lower running temperatures a boiler like that wouldnt thank you.

 

[JonathanM]

If the return is above 55-60 then you shouldnt have any chance of condensation forming when its running at normal pace.

Older units like that and the radiator systems associated with them were typically designed to run around 80°C flow and 70°C return, although they all had a thermostat that you could turn down at this time of year so its not like a self destruct button turning it down a bit, but long term on a system designed for lower running temperatures a boiler like that wouldnt thank you.

I'm going to turn it up a bit! It's literally only fired for about five hours so far since the summer.

 

[vulcancontinental]

When the water hits 100°C, the heat that's being put into the water is no longer increasing its temperature, it is not breaking the bonds that hold the water molecules together as a liquid and allowing them to turn to a gas. To boil (evaporate) the whole pot of water, you need to put an "amount" of heat into the pot, the equivalent to what it would take to heat the water to over 500°C if it stayed a liquid.

I think you meant to write 'the energy put into the liquid at 100 degrees is breaking the bonds that hold the molecules together'

I'd just add that energy cannot be destroyed, the heat energy that is necessary to break the bonds is converted into kinetic energy keeping the molecules apart, bouncing off each other. They want to join together in their 'natural state' but the environment is too hot. If they come into contact with a surface cool enough they will condense and the kinetic energy is released as heat again hence the desire to reduce the surface temperatures within the boiler heat exchanger. The plume outside the flue shows the inefficiency of the condensing boiler, there would be none is 100% efficient, just stick a hygrometer in the flue to check the moisture content of combustion air and combustion products..

The efficiency of the heat exchanger is important because it might be the plan to bring return water back at a temperature below the water vapour dew point but if the exchanger is not so good then the area from the return port at cool temperatures will be lessened. Sensible heat has to be removed before you can harvest latent. Old school but simple physics, it's how close the return and flue temperatures are that tells you how good the exchanger is and the return to flow delta T tells you how well the system is set up. The first two the closer the better, the second two optimum 20 degrees in a condensing boiler though this gets more of a challenge as operating temperatures are reduced.