Your opinion on CH Boiler setting

[Justine Time]

Googling brings up different opinions.

I have a Worcester Bosch Condensing boiler and probably like most boilers it has a boiler temperature control.If I have the room thermostat and tvr's on the rads would I put the setting on the bolier (and leave it ) on max. I have read that this would waste gas but, also read the opposite, surely it would heat the room up quicker and therefore shut off the boiler sooner than later, the temp of the rooms governed by the room thermostat/tvr's. Have also read having it on max would (could) have the returning temp above 45 degrees and the boiler would not condense efficiently. I have read definitely not to definitely yes What is your opinion, they seem to vary from what I have looked up as to the best setting.

[Martin Brown]

Entropy essentially determines that all other things being equal running the boiler at a setting where it provides just enough heat to the heating system will be more efficient. It costs slightly more to deliver extra heat to already hot water from the boiler flame.

But don't take my word for it - do the experiment and monitor your gas consumption and min/max external temperatures. Nothing beats experiment!

You may have to wind the wick up a bit in cold weather if you set the boiler thermostat for optimum fuel efficiency.

[fred]

In article , Justine Time writes

My approach is to set the boiler stat to the lowest level that can maintain an acceptable comfort level and gave a reasonably short warmup time. In general this gives a long slow burn which in my view gives more efficient running and a better comfort level.

Compare the highest setting with driving balls out between sets of traffic lights then sitting idling at the next red and a middle setting with moving at a more leisurely pace and (hopefully) arriving at the next light just as it changes to green.

In practice, mine is never set below half, is currently set just over half but it may be set at 3/4 or higher in the depths of winter.

[gremlin_95]

As low as possible really to heat your house, it would need tweaking now and then in very cold weather. I have a Vaillant Ecotec and recently upgraded to weather compensation which does all the work for me :)

[gremlin_95]

By the way, OP is your boiler a combi boiler or do you have an indirect cylinder heated by the boiler?

[John Rumm]

The most efficient will be "just hot enough to get the house to its target temperature"[3].

There are several factors at work here:

First boiler performance, lower return temperatures promote more condensing efficiency. Lower flow temperatures result is smaller overshoots when reaching the set temperature. Longer burns result in less efficiency loss from boiler cycling (although this is much smaller on a modern boiler than an old one). It also means slower warm ups (which is a trade off), but a slower warm up means a lower overall average temperature[1] - and the lower the temperature, the lower the rate of heat loss from the building generally[2].

[1] If the heating runs five hours maintaining a target temp of say 20 degrees but takes 1 hour to reach it the average temperature is slightly lower than with a system that reaches target in 20 mins. [2] Unless taken to an extreme - if cold enough that the walls get damp, then they will lose heat faster than if kept dry. [3] This after all is what weather compensators attempt to do.

[NT]

Just one thing not mentioned yet: max HW cylinder temp is limited by the temp in the primary circuit. So if yuor boiler stat setting is to low, the HW wont be hot enough.

In short, the most energy efficient option is to set the boiler stat low at this time of year, and turn it up when it gets really cold, and the pump's running a lot of the time to try to keep the house warm.

NT

[Scott M]

Excepting that, instead of idling at the red light, the boiler will turn the engine (pilot light) off.

[Justine Time]

Yes it is a combi

[gremlin_95]

Good, you can get away with setting the flow temperature quite low. With a conventional boiler with standard controls (i.e. ones which cannot adjust the flow temperature automatically) you wouldn't be able to get away with much lower than around 65ºc because the water wouldn't get hot.

[Ret.]

I emailed WB tech support on this very issue. This is the reply they sent:

Thank you for your enquiry.

Whilst using your room thermostat it is important to obtain the most efficient flow and return temperatures to extract the highest output from the boiler with the lowest temperature of flow. Please see diagram below from installation instructions:

Central Heating: Max. rated heat input kW 30.9 Max. rated heat output net 40/30 deg C kW 32.1 Max. rated heat output net 50/30 deg C kW 31.8 Max. rated heat output net 80/60 deg C kW 30.0

From 1 to MAX on your CH control knob on the boiler work in 8°C increments starting from 35°C-88°C MAX.

This table shows of the higher kW gains achieved by reducing flow temperature and achieving a more balanced return. Certainly having the boiler short cycling at full output will reduce the economy so by setting your CH control knob to a lower temperature over a longer demand period will improve the efficiency. It may take slightly longer to heat the property but once heating the boiler will be producing more efficient flow/return temperatures therefore increasing the condensing process making the demand more efficient.

We hope this answers your query satisfactorily.

Kind regards

Technical Support Worcester, Bosch Group

[Justine Time]

Interesting.... I can work on that. thanks

[Jonathan]

What is low roughly - do you mean 40C say? I am interpreting you as saying with a condensing boiler I can have water at 55C and heating at

40C and still have hot water.

Jonathan

[gremlin_95]

Basically with a regular condensing boiler (where you have a separate cylinder heated by the boiler and an immersion heater for back up), to get the water in the cylinder hot you would need to have the boiler stat set approximately 10ºc higher than the ideal cylinder temperature which is 55ºc and above to stop legionella, hence mentioning 65ºc as a minimum flow temperature if you don't have a combination boiler or more complex controls. You can't heat water in the cylinder to 55ºc if you have the flow temperature at just 40ºc.

If you had a condensing combi then you wouldn't need to worry about the water in the cylinder.

[cynic]

The logical next step is to incorporate into the boiler pcb a simple controller which will start from a cold house at maximum boiler output temperature then progressively reduce this as the desired set point for room temperature is approached. This would provide for a later start, rapid warm up, and maximum combustion efficiency once warm. Predictive switching on and off depending on exterior conditions can also be employed, together with self-learning. All this has been around in commercial systems for years.

[gremlin_95]

Indeed, my Vaillant VRC430f does all of this. It can even be mild outside but if the temperature has dropped really low inside the house (normally happens when the house is ventilated by leaving windows and doors open for a while) the device will request quite a high flow temperature and reduce it as the room approaches the target temperature.

[fred]

I think some boilers have dual settings, one for CH demand and one for HW demand. My boiler didn't have this feature so I modified it with a relay and a few components to make it run flat out when on HW demand and this works well but would have obvious implications for warranty cover.

[gremlin_95]

Yes you do get some with dual settings, most of the time it involves using the boiler manufacturers accessories/controls

[John Rumm]

Note that some WB condensors support dual temperature operation to get round this problem - running cool for heating, but ramping up the flow temperature for water reheating (or IIRC, actually having a diversion valve and separate heating circuit for the cylinder)

[Ret.]

My WB combi (Greenstar CDi 30) has separate controls for heating and hot water temperature. It also 'condenses' on hot water use which, apparently, many combis do not.