How much energy does a condensing boiler save? is it worth worrying about?

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27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
I seen on this forum many times where people use on/off wall thermostats to control the home temperature, and in my mothers old house we had a Honeywell Y6630D thermostat that had built in anti hysteresis software which started to cycle the boiler off/on as it approached the target temperature which to be fair worked very well, however not so sure if it saved any money by not over shooting, as each time boiler switched on it was at max output, so in theory a very simply wall thermostat and using the TRV's to control room temperature would be better, the wall thermostat wants a high slewing range around 1°C so once it does switch off it stays off for a reasonable time.

However using TRV control with wall thermostat and setting up so wall thermostat only turns off when whole home is warm is hard enough, but add some zone valves and in essence you have zone valves controlling zone valves as each TRV is a zone valve really and it becomes really complex to get a boiler to run with a OpenTherm thermostat with double zone valves and no area where it is always heated to actually put the Opentherm thermostat.

So we end up with an unworkable system, looked at EvoHome, Wiser, Tado etc, non seem to interconnect with old zone valves, they will all interconnect with TRV heads, but the old zone valve is still found even in new homes.

So this is the question, if some one ripped out the silly zone valves and fitted all programmable TRV heads instead connected to an OpenTherm thermostat so the boiler modulated as it should, what difference would it make to the heating bill?

In other words although we all know zone valves are stupid and only work with oil non modulating boilers is it ever worth the cost to correct, we can make zone valves work but not in theory efficiently, so nothing to do with comfort and having each room at correct temperature, that is not the question, the question is having a boiler modulate and gain the latent heat how much money will it really save?
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the square root of not a lot £. ;) Serious answers will be read with interest(y)
Thank you @muggles that account seems to say 8% is gained if correctly used and installed. So with a £500 annual bill, it could mean £40 per year, the temperatures quoted 54°C or less for the return water temperature are much lower than I expected, but to get the temperatures that low with maximum output would likely need larger radiators, but in the main it's not cold enough to need the maximum output from radiators, so flow can be reduced, however I seem to remember my radiator would not turn on until the water is around 32°C and clearly it has to stay above that figure for the fan to continue to turn, and once the room was warm it switched off the fan, but did not switch off or reduce water flow, does this mean we can no longer use fan assisted radiators, or is there some new device to reduce flow when the fan stops?

My expensive TRV heads have a sensor for water temperature, which compensates for heat from radiator, but the cheaper ones do not have this, so I would assume one could design a TRV head to be used on the return side, which would close if water went over 54°C? But not seen one, it of course could not be used on the supply side it would need to be on return.

But in a home with programmable TRV heads where rooms are maintained at say 16°C and only heated to 20°C when in use, then the radiators need to be a lot larger one to heat up the room fast, and two to keep the return temperature down, also the boiler would need to be a lot smaller, it states:-
24kW to space heating is four times too much for the average domestic property at -3°C.
so if all rooms not heated then this is reduced further so looking at less than 12 kW if the boiler is able to modulated to 4 kW or less, so if it can't be achieved, is there any reason to try?

The reason for the thread is the amount of people who seem to think the wall thermostat is to control the home temperature, to my mind the wall thermostat simply stops the boiler cycling when warm weather arrives, but then we get products like Nest, a very expensive thermostat that does not link to any TRV head, with all sorts of bells and whistles which are not required to simply stop cycling in warm weather.
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In theory, it should save quite a lot, but it does require correct installation. For a sensible answer, you can't go far wrong with Richard Burrows' blog on his website...
Good article. With Load compensation, when the setpoint is wide of the rooms temp, as he states the boiler may ramp up to the set maximum boiler temperature before modulation down when approaching the room setpoint. Many boilers maintain a delta T across the flow and return, so will not run up to maximum flow temp. That is if the return is 35C with delta T maintained at 25C, the flow will initially be 60C. Rising as the return temperature rises. Also if weather compensation and load compensation is used, the outside temperature dictates the maximum flow temperature. So, maintaining the delta T and the outside temperature the flow temperature is not allowed to run away wild.
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No, I don't think it saves much, it can't although I guess it's all relative, if £5 is needed and you haven't got it it's a fortune.

Depending on contract you'll pay around 50p/m³

The costed examples that follow use gross figures as that is what a consumer will actually pay for:

Boiler continually condensing; 11.66KW @ 50p/m3 = 4.3p/KW

Boiler operating at non-condensing temperature;10.73KW @ 50p/m3 = 4.6p/KW

Both those outputs are potential, not actual.

It's perfectly possible to construct an example and operate a boiler ON/OFF and non-condensing and be more fuel efficient than running continuously condensing never cycling. That's fuel efficient not combustion efficient e.g cheaper. So much depends on property, household, expectations, are doors left open, coming and going, how many baths.

Cycling isn't necessarily bad if you understand things. If a room requires 1.4kw that's what it requires, how you get it there whether continual burn or ON/OFF with slight over and undershoot is up to you.
I see part of @vulcancontinental point 6.5% saving is not much, and seems hardly worth worrying about, I also see that cycling is not necessarily bad, if the cycling is done by the boiler, it will switch back on at minimum output one would hope. But not so sure when a wall thermostat causes the on/off (mark/space ratio) as likely each time it switches on it will be at full output.

I felt the Honeywell Y6630D thermostat was about the worst thermostat one could use economic wise, very good at control, and it had a built in fail safe, but either very simple or one which works out when to switch off to stop over shoot like Nest is better than the mark/space used with Honeywell Y6630D thermostat. My problem was the thermostat cables had been ripped out by the new boiler installer, so had to be wireless, cheap wireless had not fail safe, medium priced use mark/space anti-hysteresis, so you have to use an expensive thermostat which seems rather pointless when all it does is turn off boiler in warm weather to stop boiler cycling.

So a cheap hard wired thermostat around £35 and cheap electronic heads around £10 so 8 rooms £115 reasonable outlay to save £40 a year, 3 year pay back. But Honeywell EvoHome I think and only have 5 zones, and it costs over that total bill before you start adding the TRV heads, but I did find an advert for 8 heads and a controller for £600 that's a 15 year pay back time, so it would seem pointless money wise.
Cycling is a bad thing. Each time the burner comes back on it dumps hot air outside in the ign/proving sequence. It wears out controls faster.

If the wireless stat is a problem cycling, and the boiler has opentherm then try the EPH Combipack which is wireless with opentherm and about £50.
It was Bosch so no OpenTherm, but more resent installs often have zones, so unless you use one zone not controlled by zone valve where a single Opentherm thermostat can be located, it would require alteration to plumbing to use OpenTherm even if the boiler is Opentherm enabled, plumbing is costly.
The air dumped out during cycling is cooler than the flue gases.

Cycling consists of a short post then pre purge, combined typically less than 10 seconds.

For the rest of the time there is little movement particularly in a powered flue boiler as the fan does not run.

The SHC of air is very low also so it cannot remove much heat in 10 seconds.

Residual heat in the exchanger is kept moving through the heating system where it is needed by the pump.

Yes, I have heard the arguement about wear and tear on components but have trouble equating that with the older boilers that would cycle far more often and last 20 years plus.
The air dumped out during cycling is cooler than the flue gases.
It is still heat that is wasted that you bought (called purchased heat). Take into account the number of needless cycles per year and that is a lot of purchased heat lost. And a lot of wear and tear on controls, fan, etc. The needless cycles could be thousand per year. So controls will suffer in longevity and wear. Fans will sound rougher, etc.

Manufacturers in the past 25 years have tried to eliminate cycling, hence Opentherm controls, etc, or get to a point it so low it is meaningless.
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So, a guide:
  1. Have weather compensation to ensure the flow temperature is as low as possible.
  2. Have load compensation to give room temperature accuracy (superior comfort conditions) and near eliminate boiler cycling. And also keeping the flow temperature as low as possible. Opentherm, etc.
  3. Have a balanced rad system. The Intergas shop article mentions the combined thermostat/flow IMI rad valves which have a the lockshield function (the flow) integrated. This is handy as it comes with a chart stating how to set the flow side.
The above three will ensure a condensing boiler works more efficiently for sure.
ericmark, one way for you and your set up, is have a low loss header. This will give independent temperature control of all rooms.
  • The boiler on weather compensation, with no load compensation (centralised stat).
  • The boiler pumps around the header to the the flow temperature the weather compensation
  • dictates.
  • Off the header two Smart pumps, one to each CH zone.
  • Rad valves on all rads, so independent control for each rad.
  • No master control like a central stat.
  • If DHW, a pump off the header. A relay that switches out CH pumps and is also in the weather compensation wire that sends the boiler temp to the set maximum, by-passing weather compensation. In effect a two temperature boiler. High temp for DHW and variable for CH.
If all rad valves close up the Smart pump winds down the speed. If one rad valve just opens its Smart pump will wind up taking heat from the header.

You could have a simple tamper-proof room stat as a central high limit control, set to higher than any room temperature would be.

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