Boiler adjusting max useful heat output

[GoodDIYjob]

Ps. You were also right on the Biasi KW output based on NCV being 34.02 MJ/M3.

Although, I found this in a Bosch Technical Bulletin for Greenstar Boilers, TB0105 B, which seems to acknowledge that at least in the past, test gas NCV of 34.02 used for ratings, is a little lower than actual delivered G20 UK gas because it was based on GCV of 37.75. My bill says 39 point something.

Anyway, that is all academic when you look at the discrepancy between the Baxi heat loss calculator of 12.5kw and the rad output we seem to have available inside the house of less than 7.5kw. (The conservatory should be so minimal to not matter, it is on frost setting on TRV and the lockshield only allows a trickle through it.)

If I am not over simplifying, when the boiler is up to flow temp and it cycles the burner 2min 30 on and 2 min 30 seconds off whilst getting the rooms up to temp. That is use of 50% capacity? So if lowest modulation is about 13kw, half that is 6.5kw so it all seems aligned checking back in that way.

 

[D_Hailsham]

Even if the heat loss is as low as you think it is, you are not giving the system any help by having a boiler which has a minimum output 50% more than the total output of the rads. The boiler will be short cycling all the time, which is not efficient. The problem is exacerbated because the boiler starts at max output each time and has to modulate down to minimum, after which it turns off and waits over two minutes before restarting. It might be worth considering an increase in radiator output.

Are you sure that the walls of your house are unfilled cavity? Changing from unfilled cavity to the "best" option saves 3kW; and increasing loft insulation to "best" saves 2.1kW.

Which room thermostat do you have?

 

[D_Hailsham]

On Sat it was -1C outside and took about 2 hours to climb 2.5C. Since it has warmed up a little since then the response is much faster, this morning I think the thermostat was off at 18.5C after about 1 hour in the morning.

Just because your rads can raise the temperature 2.5C (presumably 16C to 18.5C) in two hours, it doesn't mean they can raise the temp of a cold room (below 0C) to 18.5C. I understand that the norm is an increase of 3C per hour.

I have attached a PDF showing the workings of the online calculator.

 

[GoodDIYjob]

Even if the heat loss is as low as you think it is, you are not giving the system any help by having a boiler which has a minimum output 50% more than the total output of the rads. The boiler will be short cycling all the time, which is not efficient. The problem is exacerbated because the boiler starts at max output each time and has to modulate down to minimum, after which it turns off and waits over two minutes before restarting. It might be worth considering an increase in radiator output.

Are you sure that the walls of your house are unfilled cavity? Changing from unfilled cavity to the "best" option saves 3kW; and increasing loft insulation to "best" saves 2.1kW.

Which room thermostat do you have?

Yes, I am 100% with you D_Halisham. I've thought a lot about that. It would be a sensible course of action.

I started writing this thread from the perspective of lowering boiler output, as a stop gap to try to give the boiler a bit less stress, but the other way would be to increase emitter output as after all the discussion, it seems opportunity is limited on the current boiler front.

A new boiler that modulates to 6kw would also help a lot but funds are tightly stretched at present.

In the end, I've just replaced two faulty old imperial sized rads with Myson premier single panels, despite clearly seeing that more output would be useful. It was problematic enough for me dealing with adjusting widths (and pipe depths slightly) and having to cut the old olives off to fit new valves on all pipework for both old and new rads. Dealing with double panels would of needed to start again with the feeds and space in some areas like hallway and bedrooms is limited.

Also for time being, I've stuck with single panels because all the drop pipework on ground floor is beneath a laid laminate floor. And the boards run the wrong way on most rads as well, so removing a lot of laminate and then at least a few boards for each radiator seemed too make things just hard and disruptive. And then, unless doing every radiator in the house, downstairs will probably be significantly out of balance with upstairs... Maybe that wouldn't be so bad, as I prefer it a bit cooler in bedrooms....

We also like the minimal depth of the single panels, and they are used in every room.

But using your pointer, when we come to do some more work, maybe another rad underneath the window in the front lounge and a fan assisted plinth heater in the kitchen would help raise output a bit. There is presently no heat in the kitchen at all and it is pretty cold but there is no room to sit in there apart from a tiny small children's table sitting in the way of some cupboards (so it rarely bother that much)........

The bathroom towel rail does not heat the room sufficiently either, probably needs to be double size or more, so we could up some capacity there by changing it to a vert rad. I could see all that might raise things 2.7kw? So we'd be pushing over 10kw.

All the original walls are unfilled 100%, I've knocked a few holes in them for various things. 75mm cavity. There is a small 2 storey gable end extension to the kitchen and 3rd bedroom. There are bats in that cavity mid 1980s built. It is only about 3m x 1.9m (2 storey) so pretty small part of the overall property.

I use a Honeywell CM927. It does a good job of controlling the room temp, but I have had to change it from the 1min / 6 cycles per hour. It is set to 3 cycles, 4 minutes minimum.

Just for clarity: When I have been quoting burner cycle and rest times in here I have not been confusing it with the TPI on the CM927. All the burner cycling and gas rate measurements are before the actual temp is within 1.5C of the target temp, so before intervention from the CM927.

Thanks again for the pointers.

Ps. even though the boiler has operated like this since presumably being installed. With the set up we have now, TRVs upstairs and the CM927, mostly unfilled cavity walls, no floor insulation. Heat for 17 out of 24 hours most days, temps on 18.5 or 19.0C, our gas use is not horrific. Much, much lower than the SAP energy estimate anticipated when we bought.

Annual Gas use is about 13,500kwh per year. If you took off Hot Water, probably about 10,000 kwh space heating...

Edit: I will measure gas rate at the meter during the 2min 30 on cycles in the morning and see how close the overall rate is to minimum modulation. If it indicates a materially higher gas rate than minimum maybe there could something in getting it wound back to minimum range on CH....

 

[GoodDIYjob]

Just because your rads can raise the temperature 2.5C (presumably 16C to 18.5C) in two hours, it doesn't mean they can raise the temp of a cold room (below 0C) to 18.5C. I understand that the norm is an increase of 3C per hour.

I have attached a PDF showing the workings of the online calculator.

Thanks I'll have a look through and see if I can make an estimate long hand from the methodolgy...

 

[D_Hailsham]

I use a Honeywell CM927. It does a good job of controlling the room temp, but I have had to change it from the 1min / 6 cycles per hour. It is set to 3 cycles, 4 minutes minimum.

Just for clarity: When I have been quoting burner cycle and rest times in here I have not been confusing it with the TPI on the CM927. All the burner cycling and gas rate measurements are before the actual temp is within 1.5C of the target temp, so before intervention from the CM927.

I was wondering if you had a stat with TPI and were getting confused over cycle times.

Why did you have to change 1 min & 6 cycles to 4 min & 3 cycles? I have had a CM927 for 10 years and the only change I found necessary was to increase minimum on time to 2 minutes. This was to reduce the number of times the mid-position valve operated.

With the boiler cycling 2.5 minutes on, 2.5 off, I am wondering if TPI is having any effect.

 

[GoodDIYjob]

I was wondering if you had a stat with TPI and were getting confused over cycle times.

Why did you have to change 1 min & 6 cycles to 4 min & 3 cycles? I have had a CM927 for 10 years and the only change I found necessary was to increase minimum on time to 2 minutes. This was to reduce the number of times the mid-position valve operated.

With the boiler cycling 2.5 minutes on, 2.5 off, I am wondering if TPI is having any effect.

Yes, thought you might be thinking that. I am not confused now [anymore anyway], as I've been so focused on watching for the cycling burner on and off versus the boiler being switched on and off, you can tell when the CM927 starts intervening, the green relay light goes out, and TPI is set for 1.5C, in the CM927, so it should not be able to do anything until 17.5C is reached in the morning. The stat is set point for 19C in the morning.

To 101% prove it, I could bridge the thermostat terminal.

But the only thing it (the CM927), could do, is switch the relay off or keep it on?

For the first 3 years, I did not change any settings on the CM927.

But I started noticing this short momentary cycle with the boiler going off and immediately back on this year with burner firing, it was driving me insane, I assumed and thought I could remember it but it really annoyed me this year, I thought it had always been like that, maybe it got worse this year.

I had a suspicion initially, when posting below thread, it was the CM927 switching the boiler off because the affect was exactly the same (boiler and pump switched completley off and then on but immediately), but it was a boiler fault as Ianmcd posted it might be. And that has just been fixed. (New boiler main control PCB).

https://www.diynot.com/diy/threads/...riod-30-secs-v-3-minutes.492665/#post-4053373

So, to try and stop so much cyclying (before I realised there was definitely a boiler fault), I changed the cycles per hour and minimum on time.

However, the boiler only has a tiny overrun, like 10 seconds after the relay box switches off, so 1 min and 6 times an hour did seem excessive for i) wear and tear and ii) to circulate the heat generated... It was especially noticeable during the day when we used to get a lot of 1 min firing, which for say 7 hours, could be 42 on and offs during daylight winter hours.....

All this said, "if" the PCBs that have been replaced now are the original ones, and I get another 11-13 years!!, I suppose all the starts and stops from the CM927 and the burner on / offs from lack of modulation don't matter that much!!!! The CM927 has only been in place for the last 3 years though before that it was an old traditional stat someone mounted on the external wall on the staircase, it did nothing 90% of the time, may as well not had a thermostat..

Thanks for your reply.

 

[GoodDIYjob]

Just because your rads can raise the temperature 2.5C (presumably 16C to 18.5C) in two hours, it doesn't mean they can raise the temp of a cold room (below 0C) to 18.5C. I understand that the norm is an increase of 3C per hour.

I have attached a PDF showing the workings of the online calculator.

This is a good document, pretty easy to follow!

I'd like to refine my measurements a bit more thoroughly. But first pass heat loss per this worksheet is 7.4kw...

Windows 46.9
Walls 48.1
Roof 24.0
Floor 50.4
Vent 100.00 (305x.33)

269.4 x 27.5 = 7,400

I took a few little shortcuts, like ignoring the kitchen solid floor which may push up a bit but it also is assuming two open chimneys. they are open in the room but capped, so not sure they should be taken into account in the ventilation. Also ignored the cavity wall insulation in the small rear extension at this stage..

But it is remarkably close to the rad outputs previously advised...

 

[D_Hailsham]

Do you have the Service Manual?

If so look on page 16, Section 9.3 and you will read this:

If the power requested is lower than 40% of the maximum power output then control is achieved by switching ON the burner at minimum power, then switching OFF (ON/OFF function). If the power requested is higher, then the burner is switched ON at maximum power and will control by modulating to 40% of the maximum power output.

40% is almost the same as minimum modulation; so it is saying the obvious. What it does not say is how the power requested is calculated, unless it's in the next paragraph, which talks about comparing flow sensor reading with the setting of the CH temp adjuster.

Are you sure the boiler has a pump overrun? There's no mention in the Installation or Service Manuals. In any case pump overrun only comes into operation when there is no longer a call for heat. On some boilers the pump will continue to run if the boiler stat has turned the boiler off (up to flow temp) but the room stat is still calling for heat. (In Germany, and increasingly in USA, the pump stays on permanently if the programmer is in an ON phase irrespective of what the room stat is telling the boiler to do. It's called Constant, or Continuous, Circulation and is normally used in conjunction with Weather Compensation.)

In your other topic you said that you had adjusted the minimum time between burns to 3 minutes instead of 30 seconds. This does allow the water to cool down considerably (if the pump is running), so the "power requested" logic may be saying this needs more than 40%, I'll start at max and modulate down.

I'm thinking aloud.

 

[GoodDIYjob]

Do you have the Service Manual?

If so look on page 16, Section 9.3 and you will read this:

If the power requested is lower than 40% of the maximum power output then control is achieved by switching ON the burner at minimum power, then switching OFF (ON/OFF function). If the power requested is higher, then the burner is switched ON at maximum power and will control by modulating to 40% of the maximum power output.

40% is almost the same as minimum modulation; so it is saying the obvious. What it does not say is how the power requested is calculated, unless it's in the next paragraph, which talks about comparing flow sensor reading with the setting of the CH temp adjuster.

Are you sure the boiler has a pump overrun? There's no mention in the Installation or Service Manuals. In any case pump overrun only comes into operation when there is no longer a call for heat. On some boilers the pump will continue to run if the boiler stat has turned the boiler off (up to flow temp) but the room stat is still calling for heat. (In Germany, and increasingly in USA, the pump stays on permanently if the programmer is in an ON phase irrespective of what the room stat is telling the boiler to do. It's called Constant, or Continuous, Circulation and is normally used in conjunction with Weather Compensation.)

In your other topic you said that you had adjusted the minimum time between burns to 3 minutes instead of 30 seconds. This does allow the water to cool down considerably (if the pump is running), so the "power requested" logic may be saying this needs more than 40%, I'll start at max and modulate down.

I'm thinking aloud.

Yes, I have read that. You can see it operate in that way when very low flow on DHW. We also have terrible cold flow rate, new MDPE supply required, so if you mix cold the boiler often turns the burner on and off to modulate less than 40%. If you use 100% DHW you are usually ok within min modulation.

The manual doesn't mention overrun but if after you close a hot tap for DHW or turn down the stat and the relay clicks off the pump runs for a short period. I'd have to time it but probably 5sec to 10 seconds if you call that over run!

I did ask if I should change the setting (assuming it was on 30 sec) because of the apparent problem (before I realised it was a fault!).

When I looked, it was already set to 3min. But even with 2:30 on and 2:30 off cycles during heat up I now see with the fault resolved, it seems to have no effect because it must be from ignition cycle. So 2:30 plus 2:30 is actually a 5 min delay.

From elsewhere in the manual, maybe the owners manual, they imply the 30 second delay setting is only really fault finding and setting of gas pressures, my read is it should be 3 min as a "normal setting".? I can find the pages if I look later but cycling every 30 seconds would seem to be even worse?

Thanks for spending time in a Biasi manual. Surprisingly (for me) this one at least, seems relatively clearly written, even if the boilers aren't amazing!

 

[D_Hailsham]

Windows 46.9
Walls 48.1
Roof 24.0
Floor 50.4
Vent 100.00 (305x.33)

269.4 x 27.5 = 7,400

I agree on windows - 23.44m² x 2 (post 2002) =46.88; roof (U value 0.4, i.e 4" insulation.) floor and ventilation. However I don't agree with your wall calculation.

Wall area =Total perimeter (2x6 + 10) x total height = 22 metres x 5 metres = 110m² (Party wall is 10m long)

110 - 23.44 = 86.56

U Value of uninsulated cavity wall = 1.6

86.56 x 1.6 = 138.5

This means a revised total of 359.8 X 27.5 = 9895W.

Using the same data, the online calculator gives 10.3kW.

 

[GoodDIYjob]

Ps. I should have said the modulation below 40% evident on DHW in low flow is different from the cycling on CH. On DHW, the pump starts, the fan starts, the burner ignites, then as flow temp approaches 70c, the burner can extinguish and light, the fan remains on all the time so the full ignition sequence is not restarted. I assume this is modulation below 40% the gas valve allows

On CH, when I talk about the 2min 30 burner on and 2 min 30 burner off (rest), the pump is running the whole time, the fan starts and runs only during the burner on time. The ignition process is followed at the start of ignition. I would refer to that as burner cycling not sure if that is the official definition or not.

I am not at all sure the control of the boiler does modulate below 40% on CH. It certainly never keeps the fan running with the burner out that I have seen.

Do you have the Service Manual?

If so look on page 16, Section 9.3 and you will read this:

If the power requested is lower than 40% of the maximum power output then control is achieved by switching ON the burner at minimum power, then switching OFF (ON/OFF function). If the power requested is higher, then the burner is switched ON at maximum power and will control by modulating to 40% of the maximum power output.

I'm thinking aloud.

 

[D_Hailsham]

Your correct: the 3 minutes is from ignition to ignition, i.e on time + of time.

I'm no suggesting that the boiler can modulate below 40%; the Service manual says that it can't (If the power requested is lower than 40% of the maximum power output then control is achieved by switching ON the burner at minimum power, then switching OFF (ON/OFF function).

The good thing is that, if less than minimum is required (how does it know?), the boiler starts at minimum and cycles on/off, rather than starting at maximum and modulating down.

I don't call 10 seconds overrun.

 

[GoodDIYjob]

I agree on windows - 23.44m² x 2 (post 2002) =46.88; roof (U value 0.4, i.e 4" insulation.) floor and ventilation. However I don't agree with your wall calculation.

Wall area =Total perimeter (2x6 + 10) x total height = 22 metres x 5 metres = 110m² (Party wall is 10m long)

110 - 23.44 = 86.56

U Value of uninsulated cavity wall = 1.6

86.56 x 1.6 = 138.5

This means a revised total of 359.8 X 27.5 = 9895W.

Using the same data, the online calculator gives 10.3kW.

Yes, looking at my scribbles I have made a mistake there. Good pickup.

So somewhere in the middle of where we started, and the rads appear a bit undersized than ideal, but only in extreme cold.

In all honesty the house still even warms up when we've had minus 5C, but it obviously is very close to capacity at that and slow to heat up and it depends on other factors solar gains, how many bodies in house, internal gains. etc etc....So practically speaking, based on available rad output I still feel 10kw loss might be a bit high compared to actual.

I could benefit from more heat output from emitters if I upgrade bathroom rad, put a rad in kitchen and another in front room!

But either way, on heatloss or rad output the boiler can't modulate low enough.

I'll try to do the gas rate measure tomorrow morning during the 2:30 on cycles..

Thanks again. I know more than when I started...

 

[GoodDIYjob]

Your correct: the 3 minutes is from ignition to ignition, i.e on time + of time.

I'm no suggesting that the boiler can modulate below 40%;

I don't call 10 seconds overrun.

Yes, sorry bad terminology from me. I should of said deal with less than 40% min demand.

Yes, it's not really over run, but then it doesn't immediately cease the second the burner is out. But that lack of decent over run is one of the reasons I think a longer min on time on the CM927 may help a little.

Thanks