Optimal boiler temperature setting

[asalcedo]

I have installed two new Worcester 30 CDi system boilers at home. (They work in tandem, at the same time, as if they were one unit)

According to the user's manual, under "Tips for Energy Conservation", if the property has TRVs and room thermostats (which is my case), the boilers should be set at the maximum temperature setting and use the thermostats and TRVs to regulate the temperature as needed in each room.

I certainly agree that that arrangement gives the most flexibility. However, based on my reading of condensing boilers, the most condensing and thus, the highest efficiency is achieved at 55 degrees Celsius.

If I set the boilers to the maximum the boiler's water reaches about 92 degrees.

If I use a 5 setting, 7 being the maximum, the boilers' water is about

62 degrees and still gets the house warm enough (at least now that it is not too cold outside)

What is then the most energy efficient setting?

Thanks,

Antonio

[YAPH]

I think the user's manual is wrong in this respect. As you percieve, the lowest possible flow temperatures which actually keep the house warm will result in /marginally/ greater efficiency. Note that condensing boilers are more efficient than non-condensing types, even when they aren't condensing: because they /can/ safely condense the designers can get more heat out of the combustion than in non-condensing designs which have to allow a margin of safety (and inefficiency) to avoid any possibility of prolonged condensing in normal operation.

So in practice I'd say set the boiler temperature control about halfway to start with and experiment. If it doesn't get the house up to temperature in very cold weather turn it up until it does. Not ideal in this age of programmable, automatic controls, I know.

There are boilers which wring the last drops of efficiency out of their designs by modulating their flow temperature to achieve the necessary heat output to the building, using controls which "tell" the boiler the actual room temperatures rather than giving a simple stop/go signal as a normal thermostat (programmable or otherwise) does. Unfortunately such boilers cost half as much again, or more, than an ordinary good quality condensing boiler costs[1], and the extra money can usually be better spent on improving draught-proofing, insulation etc rather than on very marginal improvements in boiler efficiency.

[1] there doesn't seem any good reason why this should be, except "because they can".

[Andrew Gabriel]

Most efficient is to set the boiler temperature low enough such that the room stat never quite switches off. That's the coolest water setting you can get away with, which equates to highest boiler efficiency. The setting will vary according to outside temperature (which is where a temperature compensation system wins through), and it will need boosting when the house is being heated up from a temperature below the thermostat set point. A good control system will do this all automatically for you.

[Bruce]

There is another issue here, which is the central heating pump.

A higher water temperature will mean that the pump runs for less time each day. Lowering the water temperature to the point where the pump runs almost all the time might be more energy efficient at the boiler, but it definitely uses more energy to run the pump for longer.

How the additional energy used running the pump almost all the time compares with the energy saved by using a lower water temperature, I don't know.

Apart from the additional energy, there is also the question of the reduced pump life resulting from extended running.

[dennis]

That may not be true.. my boiler is most efficient at full output and is a few % less if the output is reduced. Don't ask me why, that is what the manufacturer says.

[Andrew Gabriel]

I suspect full output is referring to burner power, not the water temperature.

[dennis]

True but at full output it will heat the water rather quickly, So it will only fire for a few tens of seconds if the water temp is set low.

[Doctor Drivel]

Condensing boiler heat exchangers are bigger so even when not condensing they are still extracting more heat.

That is wrong. The Glow Worm xi range can have a proprietary controller and integrated weather compensation as standard. Look at the Broag boilers with OpenTherm controllers and weather compensation integrated too. They will modulate the burner to the room and outside temperatures. Broag Remeda are cheap are excellent.

[Dave Liquorice]

I'd add full output with the return temperature low enough such that maximum condensing occurs.

[Andrew Gabriel]

Well, yes. Ky Keston is most efficient when it switches on initially at full power with the system cold -- 26kW+ and flue gases less than

25C. Keeping it operating like that is not possible though, as it will by definition start warming up the water.

[Malcolm]

We have a tandem system at our church (Keston 40s) and I was told by the installer that the temperatures should be set so that one ts the 'lead' boiler, coming on first with the other set a degree or so higher. From cold both come on, but when maintaining temperature, only the lead boiler comes on. I was also advised to switch lead boilers at intervals (presumably to even out the wear)

Malcolm

[asalcedo]

Hello Malcolm,

That is exactly what I did.

I came to that exact same conclusion when observing how both boilers work.

I had both of them at the maximum temperature setting and noticed that when maintaining temperature only boiler one would come on. Boiler two would not fire.

Perhaps because boiler two's thermostat is not calibrated the same as boiler one's or because of the physical water pipes layout.

Anyway, it seemed like too much work for one boiler.

I changed boiler's one temperature setting to slightly lower than boiler's two and now boiler two is the one that maintains temperature.

I agree, I will change the temperature settings so that I even out the wear of each boiler.

I will also try to contact Worcester Bosch to confirm the optimal temperature setting.

Thanks,

Antonio

[Doctor Drivel]

need boier sequencing. Look around there are many about.

[Doctor Drivel]

£190 + VAT