Explanation of thermostat vs control temp

[ScottishGasMan]

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

Nice spot! I meant to write "it is now breaking the the bonds" not "it is not breaking the bonds"

 

[ScottishGasMan]

edited it to save confusion for readers

 

[JonathanM]

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.

This is really interesting, and it leads to something I want to ask you, ScottishGasMan (and anyone else) about my boiler which is 32 years old and non-condensing. At the moment it only comes on for about fifteen minutes before room temperature satisfied, because the weather is mild, and right now the return never really gets above 55C. I've known for a long time there is the issue here with unwanted condensation causing corrosion, and we discussed that a bit above. But it's made me think overnight. For most of this first fifteen minutes, the return is below 45C. But even then, the flue is over 100C (measured with IR thermometer). I'm guessing the flame is about 2000C, so the gases are really hot when they set off. By the time they exit the flue they are still over 100C. When do they actually condense (unintentionally!) in a non-condensing boiler? I'm guessing it's during the off cycle?

EDIT: Asking because worried these short cycles will cause corrosion.

 

[ericmark]

I have read through the thread, and there are a number of items which don't seem to add up. We have in the main 4 controls, the lock shield valve, the TRV, the wall thermostat, and the boiler thermostat. In my case boiler does not modulate so wall thermostat is set to on/off, but this is how I understand how the system works.

Boiler and pump fire up at set time and water is circulated around the house.
The lock shield valves control how fast the radiator heats up, so the TRV can control the room temperature without over shooting.
When the water reaches a set temperature the boiler switches off, but pump continues so radiators stay hot, and there is a mark/space ratio which controls how often the boiler fires to maintain the circulating water temperature, between two points.
As the house warms up slowly the circulation in radiators is reduced, so radiator temperature reduces, to just enough to maintain the room at temperature set on the TRV, however even when all TRV's are closed, the boiler will continue to fire up every so often to maintain the water temperature, so we fit a wall thermostat in a lower floor, in a room with no outside doors, or alternative heating, which normally cools fast to turn off the boiler rather than leaving it cycling off/on.
That room rarely exists, so a compromise is to fit both TRV and wall thermostat in the hall, careful matched so whole of house warm before the wall thermostat turns off boiler to stop it cycling.

So far so good, I get that, and setting TRV to 17ºC and wall thermostat to 19ºC because the wall thermostat is higher on the wall, and closer to centre of house, it switches off boiler once whole house warm.

But can't get how the boiler temperature affects heating or efficiency. I don't know why I am given to option to alter it? Clearly the hotter the radiator the faster it heats the room, and the hotter the boiler circulating water is the hotter the DHW is, so a balance between DHW being too hot, and radiators taking too long to heat the rooms. So to keep the DHW cool enough not to burn, I keep boiler temperature down as low as possible.

Cure would be some temperature regulations for the DHW, I suppose a motorised valve in the airing cupboard connected to a tank thermostat? But then really need some method to tell the boiler not required as well, and there is no cabling between airing cupboard and boiler. (C Plan)

Other option is some mixer valve at the taps, and if I can stop the DHW getting too hot, then boiler thermostat could be set a lot higher, which would speed up the rooms reheating. But the radiators are not in a protective box, TABLE 42.1 BS7671 gives the maximum temperatures for items electrical controlled which can be touched, A part (Metallic) which need not be touched for normal operation 80°C and a part intended to be touched but not hand-held 70°C, and hand held 55°C.

So it seems we can go to 80°C, but the water (non metallic) 65°C. Which is just about warm enough to protect from legionnaires, so boiler temperature permitted some where between 65°C and 80°C to comply with BS7671. Only 15°C so why give the user the option to adjust?

OK last house did have a fan assisted Myson radiator, but only one, and that was only radiator in a box so you could not touch it.

So why have the option to adjust boiler water temperature? And what difference will 15°C make?

 

[ScottishGasMan]

Cure would be some temperature regulations for the DHW, I suppose a motorised valve in the airing cupboard connected to a tank thermostat? But then really need some method to tell the boiler not required as well, and there is no cabling between airing cupboard and boiler. (C Plan)

I think your issue is the system you describe is pretty old. And designed when gas was less than a tenth of the price it is now. So paying for extra control would not have necessarily saved money.

Any system fitted in the last 15 years should have had a Cylinder thermostat and electronic valve to switch boiler off and stop it heating the cylinder once the cylinder got to the desired temperature (boiler interlock)

There was a thermostat on the boiler, more so for decreasing the temperature of the radiators during the autum/spring months to reduce overheating in the house, as again, some years back, there wernt TRVs and often not even a room stat.

Remember the earlier systems would have been gravity hot water, so the room stat turned the pump on and off, the hot water was heated by the natural gravity convection of the hot water without the pump. Whether the cylinder got to 50 or 70 was irrelevant as health and saftey was nothing like it is today with regards to legionella (which by the way there is no regs for domestically, legionella regulation is a work place regulation)

Also systems in other countries could be quite different, UK heating systems are different to German ones, which are different to American ones, so a boiler maker would make a boiler that could work on multiple system types and use cases, not all would be the one in your house.

 

[ScottishGasMan]

When do they actually condense (unintentionally!) in a non-condensing boiler? I'm guessing it's during the off cycle?

While we talk about flow and return temperatures as they are easily and regularly measured, you are correct in asking about flue gas, as that is where the water vapour exists and that is the actual temperature that needs to be below a certain figure before "condensing" takes place.

On older boilers, I used to see a little condensation when they first fired up, when the heat exchanger and system water was very cool, although most of the vapour went up the flue, that initial cold heat exchanger allowed a little condensate to form before drying off, once there was a little heat in the water there was virtually none visible due to the flue gas temperature.

On older commercial boiler systems, sometimes you had a thermostatic mixing valve, so if the flow was cold, it would run the heating water only round the boiler and valve to get it warmed up, then as the water heated the valve would expand and allow water into the heating circuit, the reason for this was to ensure there was always a highish return temperature to the boiler to stop "back end rot" as where the return water came into the boiler could have a little condensation which over the years would rot them out.

Like I say, a boiler thats 30 years old has already paid its dues, and running it at lower temperatures isn't going to save much gas as they cant use the condensing heat the way a modern unit can. If its sprin/autum Id have no issue running one at a reduced rate then turning it up a bit when it gets colder, its had 30 years of wear and tear already, it will give up when its ready to.

 

[JonathanM]

Like I say, a boiler thats 30 years old has already paid its dues, and running it at lower temperatures isn't going to save much gas as they cant use the condensing heat the way a modern unit can. If its sprin/autum Id have no issue running one at a reduced rate then turning it up a bit when it gets colder, its had 30 years of wear and tear already, it will give up when its ready to.

That's very sensible advice, again!

I seem to be noticing the uncontrollability of my big old boiler more this year, because I'm trying to have the house two to three degrees cooler. And generally save money by keeping warm room doors closed, but then you really notice the heat.

 

[vulcancontinental]

If a cast iron 32 year old SE boiler is around 50 - 60% efficient changing to a HE boiler that doesn't condense will bring you up to the 80's with controls. If you get the boiler to condense you can add a few more percentage points and modulating flow temps a couple more maintain a wide delta T across the exchanger another.

Just a point about Viessmann mentioned earlier, they produced a lab test showing the deterioration in the efficiency of aluminium to stainless when running continual condensing temps 24 hrs a day. Less apparently was that it also showed that the Ali running at non-condensing temps was more efficient than the stainless condensing. It's how stuff is interpreted that's important.

 

[ScottishGasMan]

That's very sensible advice, again!

I seem to be noticing the uncontrollability of my big old boiler more this year, because I'm trying to have the house two to three degrees cooler. And generally save money by keeping warm room doors closed, but then you really notice the heat.

Yea, I think everyones paying more attention this year to these things, which as much as energy crisis and high bills is no fun, it is at least getting people to look at their heating systems, wastage, CO2 emmissions etc.

On what we would reffer to as High Temp systems like yours, lowering the flow temperature is more of a comfort thing vs efficiency thing, of course the less heat you use the less you burn, that always remains true, but your system were what traditional TRVs and room stats were made for. If you have them turn down the the rooms you use less, close doors and stay comfortable.

On condensing boilers, lowering flow temps is the first priority, then using external controls to limit heat in other rooms is the way to go.

For heatpumps it should be all about the flow temp, without touching the rooms or circulation systems

 

[ScottishGasMan]

Viessmann mentioned earlier, they produced a lab test showing the deterioration in the efficiency of aluminium

Is that available to general public do you know? I've heard about it and understand it, but would be nice to have a read through.

 

[JonathanM]

Just a point about Viessmann mentioned earlier, they produced a lab test showing the deterioration in the efficiency of aluminium to stainless when running continual condensing temps 24 hrs a day. Less apparently was that it also showed that the Ali running at non-condensing temps was more efficient than the stainless condensing. It's how stuff is interpreted that's important.

I did actually wonder with that graph whether they were trying to promote extreme modulation ratios. I read other articles saying too much modulation can lower efficiency.

As regards the aluminium vs stainless steel, that is fascinating about non-condensing Al being more efficient than condensing SS. I would like to see that report, too.

I've only recently started looking at boilers, but many manufacturers seem to be flagging up SS as a selling point. But I read an article by a man from Ideal, who said they sell both, and that Al is more tolerant for retrofit on old systems.

 

[ericmark]

I look at the price of a boiler at around £2000, and then it needs fitting, although some are now condensing many are not, I note new boilers are around 92% efficient, no idea how efficient old one is, however at around £400 now for a fill up, over 10 years, it would need to be down to below 50% to be worth replacing.

So in real terms looking at better control, already have 9 programmable TRV heads, so next step is linked TRV heads, at £50 each plus a new hub/wall thermostat looking at over £600, not convinced it would save enough oil to be worth while.

So looking at new patio doors, and windows, rather than the central heating system. Bedroom today at 23.5ºC with no heating running.

I look at the energy history and it shows when the thermostat was calling for heat, not when the boiler was running, so it shows very little as boiler may have been running 100% of the time, but more likely it was cycling and only runs for 10% of times shown, as can't use full 20 kW to heat one room, it will always cycle.

 

[Dylan T]

Wow thanks so much for the replies everyone - I don't feel so stupid now because it seems I'm not the only one who finds it complicated. I've got the baby brain going on at the moment, so I'm going to have to read these responses several times for it to sink in and might come back to individual ones, but thanks so much for all the help so far - hopefully its helpful to some others too.

I have a Worcester 37CDi boiler that has a vert basic MT10RF transmitter thermostat, which there's not a lot of info about it online. I'd love to replace it with something more intuitive but I'm not entirely sure yet what is compatible, if anything.

I have ready a bit previously about modulation and from comments from others it seems my boiler does some sort of pre-programmed modulation (I'm not sure if this is done by the plumber based on size of home when originally commissioning or if this is pre-set and unchangeable - there's nothing about it in the manual). From what I understand the more economical form is when there is an external temperature probe that allows the boiler to modulate to accordingly - there are even some folks who have created their own microcontrollers (raspberry pi/arduino) that connect to a weather service, but annoyingly I can't find these articles which I was sure I bookmarked.

My house is an old 1920's stone home and in reality we aren't going to be here for a long time; the current property market and our new arrival has delayed our plans to move but it seems a bit backward to spend too much money on a house with such bad heat loss. My priority at the moment is getting the current system running as efficiently as I can.

 

[Johntheo5]

The name of the game in achieving best boiler efficiencies is getting the boiler return temperature as low as possible, TVRs can achieve this with relatively high boiler flow temperatures, Flow temperature Control differently obviously.
I have TVR control on all my rads with a flow temp of 70C and I regularly see return temps down to 40/45C, the challenge for a oil fired condensing boiler (which I don't have) is to maintain the return not lower than 43/45C to avoid corrosion, and with gas boilers its to keep the dT below 30C, in both cases either a LLH or some other means of by pass should be used. If I ever install a HE boiler I am going to install a Tapstat sensing the boiler return temp to automatically control the by pass to give a return of 43/45C.
The attached gives some idea of the various temps and by pass required to keep a oil fired boiler return to ~ 43C and a gas fired to a dT of 28C at a rad demand of 50%.

 

[JonathanM]

Wow thanks so much for the replies everyone - I don't feel so stupid now because it seems I'm not the only one who finds it complicated. I've got the baby brain going on at the moment, so I'm going to have to read these responses several times for it to sink in and might come back to individual ones, but thanks so much for all the help so far - hopefully its helpful to some others too.

I just want to say that I think that was largely my fault.

What everyone who responded agreed on, is that if you run your boiler at low temperature, it will condense more, and that makes it more efficient. So, it will produce the same KWh output, but use less gas to do so.

One of my replies started a technical discussion about modulation. Basically about whether modulation is inherently more efficient, or is it just a mechanism to help increase condensing. And a lot of the other posts came as a result of that. And then there were posts about the science of condensation.

But I just want to make sure I haven’t obscured the simple answer for you, which others gave much more clearly than me, which is that low temperature increases condensing!

As regards your boiler, it has a maximum 30 KW output for the central heating, which is factory set to modulate down to 9.4 KW. Here in the installation manual:

Greenstar 25-42 CDi Combi Installation and Servicing Instructions | Worcester Bosch

central heating boilers, combi boilers, gas boilers, oil boilers, gas central heating, heating engineers, boiler installation, boiler repair

www.worcester-bosch.co.uk