Boiler Temperature Setting

[Diablo]

Now that my hot water issue has been solved, it has got me thinking whether my boiler thermostat is set correctly.

 

The plumber that installed it reckoned it should be set to near enough max (75 deg) on the basis that the boiler will run for less time to get the UFH up to temperature and will get a faster response.  The UFH is set to 45 deg and the manifold pump is set to max flow (setting 3 on the pump).

 

Also, any thoughts on what a typical temp setting is on the UVC stat?  The showers are now so hot that we have the mixer set at just over 50%, so leads me to believe we might be heating water unnecessarily.

 

Thanks in advance.

Edited December 7, 2017 by Diablo

[Jeremy Harris]

Is it a condensing boiler?

 

If so, then it should be set so that the return temperature is always below 56 deg C, and setting it so that the return is lower than this will improve efficiency a fair bit.  I doubt that a condensing boiler set to flow at 75 deg C is condensing at all, so will be consuming way more fuel than it needs to.  When a condensing boiler goes into condensing mode there is a step change in efficiency that makes a big difference.

 

I did some experiments when we first fitted a condensing boiler to our old house, measuring the temperature and reading the gas consumption for different flow temperature settings.  I found that I could run the heating flow temperature down to around 55 deg C, even in the coldest weather and the return temperature would then be around 46 to 48 deg C.  At that temperature it's fully condensing all the time, and probably operating at at an efficiency in the high 80's of %.  When it was set to 65 deg C flow temperature it was barely ever condensing, as the return temp was almost always above 56 deg C, and the efficiency was down around 75%.

 

The UFH response rate won't be the slightest bit affected by the boiler temp, as it never sees flow temperatures above whatever the mixer valve is set to, which will be way lower than the boiler output.

Edited December 7, 2017 by JSHarris

[Diablo]

Hi,

 

Yes, it is a condensing boiler, but I should add that it is oil fired.

 

What is the best way of measuring return temperate?  I have an infra-red thermometer gun? Also, should the measurement be taken when the heating system is pretty much up to temperature?

[Jeremy Harris]

Being oil fired makes no real difference, it still won't start condensing until the return temperature is below about 56 deg C.  The IR thermometer will be fine for measuring the return temperature, with one proviso.  It won't give an accurate reading from a copper pipe, but will if the pipe has a bit of paint on it.  It's to do with infra red emissivity and the way these things are calibrated.  They are set to read reasonably accurately from a matt light grey to white painted surface, with an emissivity of around 0.9, and a shiny copper pipe will have an emissivity of less than 0.01, so will give a big error.

 

Measure the return temperature when the boiler has been running for some time, so the temperatures have stabilised, and try and get a measurement on the return pipe as close to the boiler as you can.

Edited December 8, 2017 by JSHarris

[Trw144]

One question I have regarding condensing a boiler - you always see that you require a low return temperature, but is it dependent on boiler/flow temp also. For instance, I run my boiler at a temperature dependent upon demand, so my boiler temp tends to vary from anywhere between 40 degrees and 70 degrees (when recharging the cylinder). So most of the time I would have a return temp below 56 degrees (probably more like 30), but at the same time the flow would be at a lowish temp.

[Jeremy Harris]

7 minutes ago, Trw144 said:

One question I have regarding condensing a boiler - you always see that you require a low return temperature, but is it dependent on boiler/flow temp also. For instance, I run my boiler at a temperature dependent upon demand, so my boiler temp tends to vary from anywhere between 40 degrees and 70 degrees (when recharging the cylinder). So most of the time I would have a return temp below 56 degrees (probably more like 30), but at the same time the flow would be at a lowish temp.

 

 

The return temperature is directly linked, in a non-linear way, to the flow temperature.  The flow temperature determines the heat output of radiators, and these will cool the water as the give out that heat by an amount that depends on their area, airflow around them, the room temperature, as well as the flow temperature.  If the system has radiators with TRVs, then the return temperature will also vary depending on how many radiators are actually delivering heat, and at what rate.

 

As the flow temperature is reduced, the return temperature will also reduce, with the difference between the two being higher with a high flow temperature than with a low flow temperature.

 

The lower the return temperature, the more efficient a condensing boiler will be, and it carries on getting more efficient as the return temperature drops, although the efficiency gain with reducing return temperature falls off a bit as the return drops below about 40 deg C, as by then the boiler will be near enough 100% condensing, and so there's little gain below that.

[Trw144]

Thanks @JSHarris so I should be condensing a lot of the time then, how is this affected with a narrow delta t when not much heat is being pulled by the ufh (say one or two small circuits open)?

[Jeremy Harris]

1 minute ago, Trw144 said:

Thanks @JSHarris so I should be condensing a lot of the time then, how is this affected with a narrow delta t when not much heat is being pulled by the ufh (say one or two small circuits open)?

 

 

Doesn't make any real difference, other than the question as to whether the boiler can actually modulate down to the lower power needed at the lower flow temperature without tripping the anti-short cycle timer, by shutting down.  Generally, boilers of all types (and heat pumps) work more efficiently and tend to be more reliable, if they are not short cycled.  Starts and stops have a significant impact on both efficiency and reliability.

 

One way around this if you only need a low flow temperature  for UFH is to run the boiler into a buffer tank at a reasonable temperature, say 50 to 60 deg C, then let the UFH draw off water from this, via it's thermostatic mixer, as required.  The boiler can then run for a longer time, both running the UFH directly and slowly recharging the buffer tank, then shut down and stay off for a fairly long time whilst the UFH draws the heat it needs from the buffer tank.

[Diablo]

A quick update:  Whilst I have not got around to taking a reliable temperature reading from my copper flow pipes (for reasons explained by Jeremy), I have turned the boiler stat down from just below max.

 

I initially set it half way (60 deg flow temp) and guesstimated that the return temp must be 56 deg or lower. I still had plenty of DHW and the heating seemed to run fine. 

 

I noticed that my shower mixer was delivering hot water when I had it at about 60% of max temperature.  This got me thinking whether about further efficiency gains and the fact that DHW was still being heated to a temperature far higher than I needed in the shower so, I turned the boiler stat down a little more...I now have to turn my shower mixer to max to have a nice hot shower.

 

I am wondering whether in my simplistic thinking I have hit the efficiency 'sweet spot' for my system?  Certainly, my Oil Watchman gauge shows no movement in the last three weeks!  

[Nickfromwales]

If it shows no movement in 3 weeks then I'd check the float isn't stuck. ?

I can only assume your showers are mixers rather than thermostatic mixers? If the stored DHW is at or above 38-40oC then the cartridge should compensate for any further fluctuations. Seeing such big changes in your DHW tells me your cylinder stat may not be set correctly too, eg getting too hot. The water temp in your tank should ideally be at around 60-65oC, and should never be any lower than 50oC in normal service. 

Do you know what the cylinder stat is set at ? Remember if the boiler flow is lower than the cylinder stat the system will continuously circulate with the boiler cycling unnecessarily as the cylinder stat will never get satisfied.  

[Jeremy Harris]

I bet you're saving a fair bit of oil!  We had our old gas combi temp turned down to a lower temperature than the shower thermostatic mixer was supposed to work at.  All that happened was that we needed to turn the temperature up to near maximum on the shower to get a nice comfortable temperature.  We ended up saving a fair bit of gas by doing this, as the boiler was always running in condensing mode.  A 10 to 15% efficiency improvement may not sound much, but it does knock a fair bit off your fuel bill!  The annoying thing was that the guys that fitted our boiler recommended that we run it at much higher temperatures - not sure why, but they left it with the heating set to 75 deg C and the hot water set to 65 deg C, both a lot hotter than needed and those temperatures meant the thing was never condensing in heating mode at all.

[SteamyTea]

@JSHarris Is there any science behind the 56°C temperature, or is it an arbitrary set point that 'just seems to work'.

 

@Nickfromwales ° instead of o, alt plus 248.  Just get a PC and stop using a kiddies iPad.

 

@DiabloI have got my photography grey cards out and my IR thermometer. 

White is at 22°C, Black at 21°C and Grey at 20.6°C.  Probably within the accuracy of the thermometer.
 

 

 

 

 

[Jeremy Harris]

19 minutes ago, SteamyTea said:

@JSHarris Is there any science behind the 56°C temperature, or is it an arbitrary set point that 'just seems to work'.

 

It depends to some extent on the design of the condensing heat recovery system in the boiler, but the figure of 56 deg C is straight from the manual, and it seems that this varies from around 55 to 56 deg C depending on which set of boiler instructions you loom at.

 

In reality, this is almost certainly the temperature at which the onset of useful condensing flue gas heat recovery starts, so is probably variable.  When I measured ours I found that gas consumption reduced as the return temperature was reduced below this temperature, but that by the time the return temperature was down to about 45 deg C there was not much to be gained from it being any lower - it seemed that the boiler design was limiting it and it was recovering all the heat is usefully could by that point. 

 

19 minutes ago, SteamyTea said:

@DiabloI have got my photography grey cards out and my IR thermometer. 

White is at 22°C, Black at 21°C and Grey at 20.6°C.  Probably within the accuracy of the thermometer.
 

 

Visual colour make little difference to long wavelength IR emissivity.  However, a shiny copper pipe will have an emissivity that is massively lower than any painted surface, typically around 0.01 versus around 0.85 to 0.9 for a painted surface, hence the big error when trying to measure the temperture of unpainted copper pipes with an IR thermometer.

Edited January 5, 2018 by JSHarris

[Diablo]

3 hours ago, SteamyTea said:

@JSHarris Is there any science behind the 56°C temperature, or is it an arbitrary set point that 'just seems to work'.

 

@Nickfromwales ° instead of o, alt plus 248.  Just get a PC and stop using a kiddies iPad.

 

@DiabloI have got my photography grey cards out and my IR thermometer. 

White is at 22°C, Black at 21°C and Grey at 20.6°C.  Probably within the accuracy of the thermometer.
 

 

 

 

 

I think I'll go with a nice Gunmetal Grey! Thanks for the info.

[Diablo]

13 hours ago, Nickfromwales said:

If it shows no movement in 3 weeks then I'd check the float isn't stuck. ?

I can only assume your showers are mixers rather than thermostatic mixers? If the stored DHW is at or above 38-40oC then the cartridge should compensate for any further fluctuations. Seeing such big changes in your DHW tells me your cylinder stat may not be set correctly too, eg getting too hot. The water temp in your tank should ideally be at around 60-65oC, and should never be any lower than 50oC in normal service. 

Do you know what the cylinder stat is set at ? Remember if the boiler flow is lower than the cylinder stat the system will continuously circulate with the boiler cycling unnecessarily as the cylinder stat will never get satisfied.  

Hi,

Not sure about the shower mixers.  They are concealed Hansgrohe mixers - thermostatic?

I just checked and the cylinder stat is set to about 60oC.

 

The boiler flow is less than 60o, as it is below half way (which is 60o) in effort to ensure my return is below 56o.  I guess the issue is that the boiler will cycle unnecessarily for the 2 hrs a day we have it heating DHW? Is it more critical to get the boiler return temperature below 56o, even if this means the boiler flow is below the cylinder stat?

 

Many thanks.