Efficient way to run boiler?

[VAG_dude]

Hi there,

Our house has a total 16kw - 17kw of radiators, the boiler is capable of supplying 28kw. On the boilers diagnostic menu you can adjust the maximum kw from 12kw right to 28kw, it came factory set at 28kw but I thought we could use a little less gas by running in exactly at 17kw which is the maximum ever required, however this what I have observed:

When on 28kw the boiler fires at 12kw for a minute, ramps right up to

28kw and quickly achieves the flow temperature of 70=BAc, it then modulates down slowly and happily maintains 70=BAc until the thermostat stops calling for heat. The house warms up pretty quickly.

When on 17kw, again the boiler starts on 12kw, ramps up to 17kw and takes a fair bit longer to get to 70=BAc, once at 70=BAc it will modulate down and maintain 70=BAc as it does when on 28kw. The house takes a bit longer to get warm but still reaches the set temperature in a satisfactory amount of time, just not quite as rapidly as before.

So the question is what is the better way to run the boiler, will it save gas running on 28kw as the boiler modulates down quicker? I have heard it's best to run at 17kw as it stops putting unnecessary strain on the main heat exchanger, how true is this?

The boiler is a Vaillant Ecotec plus 937 (37kw Dhw, 12kw-28kw Ch)

Thanks.

[PeterC]

Similar here, but my boiler is 20kW CH for about 11kW rads. (need the boiler for DHW). The boiler can be reduced to 50% but even that is too much at times. Trouble is, I run the main rooms at 18C and the bedrooms, on TRVs, at 14C, so over half the system is shutting off most of the time.

Currently trying 60% at 70C output to see what happens.

It's difficult to get meaningful figures under real-world conditions as there are so many variables, so I go for an acceptable warm-up time and minimal short-cycling.

[ransley]

Longer cycles are easier on controls, less cycles means longer life and less cycles give longer run times at full efficiency. It takes maybe 5 minutes to get to full efficiency, but I am just guessing. Less cycles you will have fewer cool down periods where the electronocs and pump are running. Longer cycles give more even heating. The only thing I wonder is Is 70c past the boilers peak efficiency temp, is that a factory set temp or is for example 65c or less the boilers peak efficency rating. You need to know at what temp the boiler starts to loose efficiency and not run above that temp if possible except on the coldest days. For a boiler to be sized right it should need to run most of the time on the coldest days, since it isnt cold out yet maybe you are sized right. I would find out from the manufacturer the temp at which it looses efficency it should be online or in their literature you recieved, and set boiler temp at that level, if it heats ok at less overall output then try it, overall output is not water temp.

[newshound]

Are you happy with it otherwise? Although I had almost decided never to buy another combi, the characteristics of this one might well suit my (larger than normal) house. I do like the look of the very comprehensive diagnostics.

[VAG_dude]

Indeed I am, this boiler was installed almost 2 years back and it hasn't missed a beat, it was ideal for our needs as we had no space for an unvented cylinder. DHW is excellent, the boiler runs quietly and it's a very solid and well built piece of kit. We have 2 heating zones in our house so at the time we couldn't use Vaillant controls, I would have preferred the Vaillant controls because if you set them to

2 point analogue operation they automatically adjust the boiler flow temperature according to what the room temperature is. At the time Vaillant controls weren't suitable for 2 zone systems unless you had the VR61 mixer module which was only available in Germany, it is now available here.

[newshound]

That's useful to know as I have two zones as well

[VAG_dude]

Hmm the manual states that it is not necessary to range rate the boiler as it is a modulating appliance however you can range it if need be, there has to be a reason for the setting!

How will it go on and off less frequently? It doesn't cycle on any kw rating unless the Honeywell CM927 tell it to; which it does as it approaches the set temperature.

As it is a combi boiler it is sized on DHW demand. However the installer did not size radiators or pipes correctly but then that's a different issue.

[VAG_dude]

Glad I could help, out of interest do you have space for a cylinder?

[cynic]

Are you or swmbo happy for the house "to take a bit longer to get warm"? You have an automatic system with the capability of warming your house quickly then reducing its ouput to match the maintaining temperature requirement. For the sake of curiosity you could bugger about with it but whats the point of potentially upsetting a working system? IF IT AINT BROKE DON'T TRY TO FIX IT

[Mark]

If the house heats up more quickly with the boiler set at 29kw vs 17kw then you must have more than 17kw of radiators.

[fred]

In article , VAG_dude writes

My own pref is for a long slow burn[1], ie boiler mod'd down to the lowest level that can just maintain the temperature in the property but you then need to tweak the mod level to match the weather or it wont reach temperature during a cold snap. I favour this because it means that the rads stay at a low but consistent temp which in my view gives a greater comfort level.

Good luck finding controls that will support this kind of operation though, Honeywell controls are fixed in proportional mode meaning that they force you to cycle the boiler multiple times per hour (6 on default setting) which they claim gives better temperature accuracy but IMV results in excessive component stressing[2] and inefficiency. There is a quality electronic stat on the market that has selectable proportional control so you can disable it if you think you know better but I can't remember which one it is, poss Danfoss.

The choice, as they say, is yours.

[1] Traffic light racing analogies do apply. [2] Add up the cycles per hour, mult by the hours on per day then factor that into the spec'd cycle life of zone valves and boiler ignition circuits and you'll find that the life is shorter than you think. 3 years I think in my case when calculated.

[VAG_dude]

Makes sense.

I agree with you on this one, the boiler seems to be constantly on and off and on again etc etc And once the house has reached the temperature the boiler cools down a bit but before you know it the thermostat is calling for heat again to keep the temperature steady and the boiler starts all over again on full tilt and usually does not even get a chance to modulate down as the thermostat has stopped calling for heat.

. There is a

Interesting,

What boiler do you have? We have Honeywell 2-port valves and a Vaillant boiler.

[Doctor Drivel]

Our house has a total 16kw - 17kw of radiators, the boiler is capable of supplying 28kw. On the boilers diagnostic menu you can adjust the maximum kw from 12kw right to 28kw, it came factory set at 28kw but I thought we could use a little less gas by running in exactly at 17kw which is the maximum ever required, however this what I have observed:

When on 28kw the boiler fires at 12kw for a minute, ramps right up to

28kw and quickly achieves the flow temperature of 70ºc, it then modulates down slowly and happily maintains 70ºc until the thermostat stops calling for heat. The house warms up pretty quickly.

When on 17kw, again the boiler starts on 12kw, ramps up to 17kw and takes a fair bit longer to get to 70ºc, once at 70ºc it will modulate down and maintain 70ºc as it does when on 28kw. The house takes a bit longer to get warm but still reaches the set temperature in a satisfactory amount of time, just not quite as rapidly as before.

So the question is what is the better way to run the boiler, will it save gas running on 28kw as the boiler modulates down quicker? I have heard it's best to run at 17kw as it stops putting unnecessary strain on the main heat exchanger, how true is this?

The boiler is a Vaillant Ecotec plus 937 (37kw Dhw, 12kw-28kw Ch) ____________

Leave the boiler on maximum as it gets hotter quicker. This means you can reduce the warm up times.

The boiler modulates from 12kW to 28kW. Inefficiencies start when the demand is below 12kW and cycling starts. Lowering to 17kW achieves nothing as the controls modulate from 12KW to 28kW and all between.

The only way you can save is lower the flow temperature, then the return temp reduces promoting condensing efficiencies.

I would consider fitting an outside weather compensator to this model. It can have one.

[fred]

Keston Celsius here with Honeywell 2 port valves. I had the system running off a clunky & crappy Towerstat wired electronic stat and despite it not switching as accurately as I would like, the boiler had karma and pottered away on its low (actually mid) setting, often for hours with the stat just cutting out every now and again to stop the rooms going over temp. In winter I would turn the mod control up and ease it off in warmer times.

It now has a Honeywell CM907 (the wired version of yours I think) and now it is forced to cycle 3 times per hour (reduced from default) so the boiler thrashes for a few mins to get to temp and switches off again almost before it gets a chance to mod down. There are lots of features I like on the Honeywell but it's not controlling the system how I would like it. Also, I have a multi zone setup that I'd like to fully configure but it is impossible to sync the wired Honeywell electronic stats in any meaningful way which is the ideal way to do it.

[dennis]

Not all boilers are efficient when modulating, the efficiency may well be different at different modulation points. You need a graph of efficiency vs. modulation before you can decide whats best. My boiler doesn't modulate at all but runs at the maximum efficiency on and off. The supposed inefficiency of boiler cycling doesn't apply to a boiler that is designed to be cycled.

What is pretty certain is that you want the water temp as low as possible for a condensing boiler even though it means having radiators twice as big.

[VAG_dude]

Seems that you have the same issue as me, I don't particularly like the proportional control on the Honeywell CM927/CM907.

[newshound]

I currently have a direct vented electric cylinder for a shower. I could, as phase 2, replace it with with an unvented one. My original plan was to go for a "system" (non-combi) boiler with a small high output cylinder next to the boiler, as I have limited space there. But I'm definitely coming round to the idea of a 937 instead. My other thought is whether I could run an extra circuit off the 937 to an external plate heat exchanger, and use that to heat the current vented direct cylinder (with an extra pump on the secondary side). Again, that could be a "Phase 2" job.

[Doctor Drivel]

It will not be simple to someone like you.

[Doctor Drivel]

Look at the ATAG 51kW. This is far better.

[Doctor Drivel]

Hmm the manual states that it is not necessary to range rate the boiler as it is a modulating appliance however you can range it if need be, there has to be a reason for the setting! ___________

It can supply the copper coil heat exchanger to a forced air system, where you will need a constant temp and range rating to near as you can get.