How to optimise your combi boiler

[dirtychinchilla]

Currently optimising my system before the winter.

I've fitted a tempetature sensor to the output and return to my combi boiler - I'm looking to balance the radiators so that the return temperature is as low as possible.

Approx. efficiency on a condensing gas combi is:

> 54 = non-condensing mode = 87% or less
50 = 90%
40 = 94%
30 = 97%
20 = 99%

Sadly, my system seems pretty poor with the return temperatures, but that's with all the radiators full open. I had hoped that shutting off the bathroom towel rail / radiator would help (they're known to give off little heat, therefore return high temperatures and destroy the condensing efficiency).

Currently = 52'C output and 48 'C return!

Now to hunt for the radiators that are flowing too fast and returning high water temperatures...

Found the main culprits - small radiator in the single bedroom is pretty much returning water at the same temperature, as is the one in the main bedroom.

"Best" one so far is returning 44'C with 54'C input, but even that's pretty poor! It ought to be at least 12 degrees delta, but I am doing this test with an ambient of 22'C..

I've tightened down the lockshields on the offending rads and will re-test when they settle.

Radiators fully open are unlikely to give you a low return as the flow rate will be pretty high. Unfortunately balancing radiators is a task in pain - works fine in full load but when you have part load, it'll be borked.

Glad you found the bad seed though. That whole setup sounds like a big improvement. Remember, you won't get a good dT until the dT between your room temperature and the radiator temperature is larger.

I'm going to get a clip on temperature sensor for my radiators. My work sell these wonderful self-balancing radiator valves, but they're expensive. For my house it'd be £200 to fit it out, and I'm not sure that's worth it. But I will get a couple for the towel rails as towel rails are notoriously awful.

I've actually turned our heating on this weekend. It'll only trigger if the temp in the house gets too low though. I could really do with the towel rails coming on for a little bit as our towels aren't drying properly now.

A couple of years ago we had our boiler and rads replaced. A few months later I found 'urban plumbers' youtube channel where he talks about the things being mentioned in here. It annoys me that I didn't take an interest when the system was being installed because I would have made better design discissions than our plumber did.

I would have chosen a smaller boiler and openthrerm compatible (this should be a mandatory feature for all boilers fitted), I would have gone combi instead of a system boiler, but at the very least fitted a hot water diverter kit so I could run a different flow temperature for the radiators and a higher temp to fill the water cylinder (difficult to retrofit this now). I would have wanted a weather compensation sensor (again this should be mandatory) and bigger radiators. I've reduced the pump speed from highest to lowest but even so my flow/return delta is hilariously bad and there isn't really much I can do about it.

It's sad to think that this plumber was very experienced but wasn't aware, or didn't care, about designing a system to get the best out of the technology available. It's not even that hard. Think of all the energy we've wasted as a country over the decades.

Unfortunately, plumbers are generally not trained in heating system design. I had one service my boiler years ago who told me a high return temperature was good ffs! I don't think my new house even had its system balanced. Weather comp should be mandatory, absolutely.

What have you done to improve your dT?

 

[robj20]

All my rads are fully open, system won't work of I set them up like the old days. Large majority of the time only one or two rads will actually be on and on very low anyway. You need them all fully open to dump huge amounts of heat when heating from fully cold, the valves all soon turn down to like 5%

 

[Uhtred]

What have you done to improve your dT?

Nothing yet but I need to go round the rads and check the individual flow and return temps as there might be scope for some improvement there.

I might see how much it would be to fit a 'system diverter valve kit' for the hot water cylinder so I can change my flow temperature to the rads, but I suspect it will cost too much to be worthwhile.

 

[dirtychinchilla]

Nothing yet but I need to go round the rads and check the individual flow and return temps as there might be scope for some improvement there.

I might see how much it would be to fit a 'system diverter valve kit' for the hot water cylinder so I can change my flow temperature to the rads, but I suspect it will cost too much to be worthwhile.

Makes sense. The alternative is to pasteurise your hot water cylinder, maybe once a week, at 60C. To be honest though, you'll soon find you need to up the temperature on your boiler as it gets cold. I maintained 55C for a while in my house, but by the peak of winter it was up to 70C. I believe the system return was still around 50C at that point. Hoping to maximise the dT this year.

 

[pastymuncher]

I have been running our combi boiler at 55 degrees for the heating since we had gas central heating installed mainly because it was the best setting for economy. The house takes longer to heat up but once it has it's been more than enough even up here in NE Scotland. The installers and our housing association recommended running it at 75 degrees C which is it's maximum temp. I tried it for a couple of days at that setting and while the house warmed up much quicker the cost of heating the house more than tripled!! Needless to say it rapidly got reset to 55 degrees which is something I have to do every year after the boiler service is carried out. Our boiler also has a setting that supposedly keeps the hot water in the pipes hot, something they also keep switching on. I found that it's a massive waste of money so it's always been turned off. I have turned the hot water down to 50 degrees from 55 degrees and have seen a 11.4kw/h drop in August so it seems to have been a worthwhile change.

 

[Richie]

The "E" setting on my Ideal Logic 35 boiler puts the CH temp to 68oC.

I'm unsure if reducing it would be worth it. It has weather comp sensor attached to.

 

[dirtychinchilla]

What sort of external temperature do you have at the moment? That sounds rather high tbh, but it also depends how old it is.

 

[Jimbo Mahoney]

Mine (Worcester Bosch 30Si Compact, which I believe is ~2015) also has its "E" setting at 68'C.

Pretty convinced that this is used as a "higher than this is less economical", rather than "this is the most economical setting". My reasons for this belief are:

• Higher temps = house heats faster (therefore boiler manufacturers don't want to recommend lower temps because people will think the boiler is crap)
• Lower temps = more economical
• Condensing boiler needs return to be < 54 'C, therefore manufacturers spec for a compromise between speed the house heats up and return temps.

The most economical setting would be as low as possible.

I measured the rate at which the living room warms with the boiler set to 68 and when set to 60. It was about 1 degree per hour on 68 and 0.7 per hour on 60.

It will be faster than that now because the radiator is inside a box, which has since had 3 x 218mm fans installed, which come on when the radiator hits 30 'C. I also took a brush to the radiator, which was caked in decades of dust!

The "E" setting on my Ideal Logic 35 boiler puts the CH temp to 68oC.

I'm unsure if reducing it would be worth it. It has weather comp sensor attached to.

Lower is always more economical, but the trade-off is it takes longer to heat the house.

Too low would mean the house would never reach the set temperature, but it would have to be a pretty poorly insulated house and a pretty low setting.

I might experiment with 55 this winter, but that might mean the house takes too long to warm up.

 

[Tinders]

Mine (Worcester Bosch 30Si Compact, which I believe is ~2015) also has its "E" setting at 68'C.

Pretty convinced that this is used as a "higher than this is less economical", rather than "this is the most economical setting". My reasons for this belief are:

• Higher temps = house heats faster (therefore boiler manufacturers don't want to recommend lower temps because people will think the boiler is crap)
• Lower temps = more economical
• Condensing boiler needs return to be < 54 'C, therefore manufacturers spec for a compromise between speed the house heats up and return temps.

The most economical setting would be as low as possible.

I measured the rate at which the living room warms with the boiler set to 68 and when set to 60. It was about 1 degree per hour on 68 and 0.7 per hour on 60.

It will be faster than that now because the radiator is inside a box, which has since had 3 x 218mm fans installed, which come on when the radiator hits 30 'C. I also took a brush to the radiator, which was caked in decades of dust!



Lower is always more economical, but the trade-off is it takes longer to heat the house.

Too low would mean the house would never reach the set temperature, but it would have to be a pretty poorly insulated house and a pretty low setting.

I might experiment with 55 this winter, but that might mean the house takes too long to warm up.

I would guess a vast proportion of the uk housing stock is poorly insulated, Victorian and Edwardian properties are a nightmare to insulate.

 

[Richie]

What sort of external temperature do you have at the moment? That sounds rather high tbh, but it also depends how old it is.

Not got an accurate temp but Google Weather reporting around 15oC during the day and dipping to around 6oC at night.

The 68oC setting is just the default Economy mode for the boiler... I could turn it down to 60 this Winter and see how it goes.

My CH is off though. I have the benefit of a large patio door in the South facing kitchen which adjoins the living room (North Facing). Today I came home to a temp of 22oC (from Nest thermostat readout) simply through leaving the kitchen door open into the Living Room and letting the sun heat the lower floor up.

By contrast, the main bedroom (North facing above Living Room) showed 20oC on the Nest in that room.

It's a modern house and well insulated - I have seen the living room temp increase simply by me being in it and generating heat!!

Lower is always more economical, but the trade-off is it takes longer to heat the house.

Too low would mean the house would never reach the set temperature, but it would have to be a pretty poorly insulated house and a pretty low setting.

I might experiment with 55 this winter, but that might mean the house takes too long to warm up.

I have the benefit of Nest thermostats at home meaning that I can set the temp for when I get out of bed or when I arrive home from work and the thermostat, which has previously learnt how long it takes to heat the room, will turn the heating on for that period beforehand so it reaches the set temp at the specified time... I believe it even takes outside temp into account (uses Google's Weather reporting rather than an accurate actual outside temp).

I've turned the boiler CH down to 60oC so I don't forget.
Hot Water is set to 50oC and is plenty hot enough from taps and shower.

As said above - my house is well insulated.

 

[Converge]

My Veissman Vitodens 050w seems to state that 70c is the ‘most efficient’ setting, yet I read about 55c everywhere for condensing efficiently? Why is this, if 55c is supposedly best?

What should the tap temperature be too, or is that more down to preference than anything?

 

[Jimbo Mahoney]

My Veissman Vitodens 050w seems to state that 70c is the ‘most efficient’ setting, yet I read about 55c everywhere for condensing efficiently? Why is this, if 55c is supposedly best?

What should the tap temperature be too, or is that more down to preference than anything?

Approx. 54 'C is the highest temperature at which condensate can form (in a gas combi boiler), which allows more heat to be extracted from the by-products of the gas being burned.

Nice article, with graphs, here.

However, that is the return temperature (to the boiler after circulating through the radiators), not the output (user setting).

Around 68-70 (output temperature) is set as the "most efficient" on most boilers for the reasons I outlined in my previous post - it's high enough to warm the house up quickly, but low enough (assuming that the radiators are large enough and set up at the correct flow rate) to return water in the region of 50'C back to the boiler, allowing greater efficiency through condensing heat-capture. (Lower would be better, but the house wouldn't heat as fast).

Both CH and tap water temperatures should be set as low as possible if efficiency / costs are your main priority.

 

[Converge]

Snip

Great, thanks for the info! I think I’ll try the main temp at 55c and keep taps at 45c like always. Cheers

 

[MR_Punk]

I've knocked my boiler down a full 10 degrees on the hot water side of things and I cannot tell the difference, the hot water is still extremely hot so probably a nice little saving in the long run.

 

[GeX]

Weather compensation is required now afaik on new installs.
For mine, this is via the thermostat rather than a sensor for the boiler; and the thermostat takes forecast into consideration when controlling the boiler via opentherm

 

[dirtychinchilla]

I've knocked my boiler down a full 10 degrees on the hot water side of things and I cannot tell the difference, the hot water is still extremely hot so probably a nice little saving in the long run.

Your taps will have some sort of anti-scold and thermostatic mixing valve in them, to prevent you getting hurt. So, unless you have a hot water tank running your domestic hot water above 44C is pointless. I turned mine down to 40C IIRC but that wasn't great, so I put it back up to 45C and it's good.

 

[MR_Punk]

Your taps will have some sort of anti-scold and thermostatic mixing valve in them, to prevent you getting hurt. So, unless you have a hot water tank running your domestic hot water above 44C is pointless. I turned mine down to 40C IIRC but that wasn't great, so I put it back up to 45C and it's good.

I have put mine to 45 it was on 55 so a no brainer really.

 

[Jimbo Mahoney]

My water is now set to 46 'C.

As I feared, balancing the radiators is a pain! Sadly, the lockshields I have are terrible! A fraction of a turn (like less than 1/8!) makes about 5 degrees difference on the return temps, and I only have about 1/4 - 1/2 a turn of useful range.

On the plus point, I've got my return temps WAY down (currently 35 'C with an output of 57 'C), but I think the house might take too long to warm up, so I'm setting the output to 65 'C (up from 60 'C).

I'm aiming for about 70% of the radiator input temperature at the output of each radiator and return temperatures to the boiler about 40 'C, which would be about 94% efficiency.

 

[robj20]

My water is now set to 46 'C.

As I feared, balancing the radiators is a pain! Sadly, the lockshields I have are terrible! A fraction of a turn (like less than 1/8!) makes about 5 degrees difference on the return temps, and I only have about 1/4 - 1/2 a turn of useful range.

On the plus point, I've got my return temps WAY down (currently 35 'C with an output of 57 'C), but I think the house might take too long to warm up, so I'm setting the output to 65 'C (up from 60 'C).

I'm aiming for about 70% of the radiator input temperature at the output of each radiator and return temperatures to the boiler about 40 'C, which would be about 94% efficiency.

If they're reasonably modern valves, you don't balance with the lockshield you actually use the side with the TRV on it.

Can't balance mine at all, it just doesn't work with the system I have so there are times when the return can actually be nearly the same as the flow.

 

[Jimbo Mahoney]

If they're reasonably modern valves, you don't balance with the lockshield you actually use the side with the TRV on it.

I don't believe that's correct.

The TRVs are thermostatically controlled, and will shut off the flow once the room temperature reaches the point corresponding to the TRV setting (granted, there may be some element of slowly reducing the flow, rather than on/off).

It's better to balance the lockshields with the TRVs fully open to achieve a decent temperature drop across the rad and then set the TRVs to the desired room temperature.