Short Cycling, any advice?

[Johnnyt]

During the recent cold spell my 25kW Viessman w200 system boiler returned 5 firings over a burner run time of 62 hours modulating down to 8.7%. Mind you it was minus 8.5C outside. I wish I could repeat as a general rule.No weather comp just setting flow temp of 43C and ufh 5 zone with room stats at 22C and 4 rads on 1st floor.

[Super_Paulie]

9 minutes ago, John Carroll said:

So, what is the cycle time?, ie, burner ON time & burner Off time.

 

Can you post a link to your Boiler MIs.

 

I don't have that exact figure at the moment, but setting the flow at 60, and the mixer at 45 it took an age burning while the buffer gets to temp, at least 30 minutes. At that point it goes over the target temp +5 then the flame out (anti cycle?) kicks in for exactly 3 minutes. In that time the flow temp has dropped to match the buffer return temp then it takes another 30 minutes to get it back up again before repeating. So it's pretty much always firing, except for the 3 minutes it isn't.

 

I have the manual if that's what you mean? The figures start from page 14 and I can't find anything on there or the internet about being able to adjust the cycle timer.

Baxi 800 Combi Installation and Service Manual.pdf

[marshian]

Thinking about it that's a heck of a circuit temp drop in 3 mins

 

65 deg down to 50 (ish)

 

Have you got an ABV in the circuit?

[Super_Paulie]

10 minutes ago, marshian said:

Thinking about it that's a heck of a circuit temp drop in 3 mins

 

65 deg down to 50 (ish)

 

Have you got an ABV in the circuit?

I've got a rad in the bathroom, not sure if it's better to have that thing fully open or throttled back.

 

As soon as you get flame out the pump is still running so I guess it cools quickly to the temperature of the return from the buffer.

[John Carroll]

29 minutes ago, Super_Paulie said:

 

I don't have that exact figure at the moment, but setting the flow at 60, and the mixer at 45 it took an age burning while the buffer gets to temp, at least 30 minutes. At that point it goes over the target temp +5 then the flame out (anti cycle?) kicks in for exactly 3 minutes. In that time the flow temp has dropped to match the buffer return temp then it takes another 30 minutes to get it back up again before repeating. So it's pretty much always firing, except for the 3 minutes it isn't.

 

I have the manual if that's what you mean? The figures start from page 14 and I can't find anything on there or the internet about being able to adjust the cycle timer.

Baxi 800 Combi Installation and Service Manual.pdf 3.09 MB · 0 downloads

(say) 30 minutes on & 3 minutes Off is a cycle time of 33 minutes, not even 2 cycles/hour, nothing to complain about now?, and assuming 6kw min output means a UFH demand of 6*30/33, 5.45kw?.

[John Carroll]

9 hours ago, Super_Paulie said:

 

I've had to call it a night up there, but before I did I set the boiler at 60, the mixer at 45. This takes a while to fill the buffer and the boiler creeps to 65.1 then flames out for 3 minutes, the buffer is at 51 at this point. After the 3 minutes the boiler is now at 54 and it starts up again. Runs back up to 65.1 in around 30 minutes. 

 

Seems to be a lot of bypassing , if the UFH return temp is say 38C then ~ 3.5LPM at ~ 38C must be mixing with ~ 5.0LPM at 60C of bypass (assuming no temp drop through the rad), to give a buffer temperature of 51C, the manifold return should be well down to its return temperature after 30 minutes, your UFH system with 400M of 16MM (12mm ID) piping should contain around a total of 95L, (45L in the pipework + the 50L buffer) will/should have circulated through the system after ~ 95/3.5, 27 minutes, check those readings again sometime after say 3 cycles. Its no wonder the boiler was short cycling with (If) that sort of bypassing.

[JohnMo]

8 hours ago, John Carroll said:

What make/ output Boiler do you have

Atag A325ECX, rated modulation is about 7 to 31kW, at lower flow temperatures.

[Super_Paulie]

really appreciate all this help guys, its helping me understand this all a bit better. The numbers give me brain struggles so its very much appreciated. I have re-drawn my system to maybe show a bit more of the reality.

 

Looking at it as you say, 60 (65) down to 50 once the UFH is away is a big old drop for what should just be a bypass. I do get some, what i thought was, reverse circulation in the rads marked Rad1 and Rad2 in the attached image. Would that combined with the bypass rad in the bathroom account for the loss? To test this should i lock off the 2 rads and see if the return temperature increases? As it is my boiler burner is essentially "never off", and is only off when it flames out at the temp +5 for the anticycling time of 3minutes. I kinda expected the return to get so high that the boiler stops the flame but then continues to pump while being fed from the buffer for however long it takes, not just being off for a pre-determined flame off due to anti-cycle. Probably totally miss-read the situation there. 

 

 

Edited 9 hours ago by Super_Paulie

[marshian]

Oh that's odd - I would have expected the volumiser to be on the hot side of the boiler not the cold side but I guess it doesn't matter

[John Carroll]

47 minutes ago, marshian said:

Oh that's odd - I would have expected the volumiser to be on the hot side of the boiler not the cold side but I guess it doesn't matter

Maybe not quite what you are looking for.

https://blog.heatspring.com/2-pipe-versus-4-pipe-buffer-tank-configurations/

Edited 8 hours ago by John Carroll

[marshian]

1 hour ago, John Carroll said:

Maybe not quite what you are looking for.

https://blog.heatspring.com/2-pipe-versus-4-pipe-buffer-tank-configurations/

But it’s not being used as a buffer - it’s being used as a volumiser - useful link tho will read it properly later

 

I considered fitting a volumiser with my old boiler to aleviate some of the short cycling issues I had (resolved instead by replacing the boiler with one that had a far better turndown/modulation)

 

My intention was to fit it to the hot side so after the boiler hit the overheat limit the volume of water that needed to pass thro the rads before the restart temp was reached would be higher than the circuit without a volumiser but I had a flow rate of 10 lpm so With a few rads in circuit and the volumiser the total volume before the return temp dropped down to the flow temp (aka boiler restart time) I would have had maybe 6 or 7 mins of coast rather than 3 mins.

 

However once I worked out the valliant/glow worm anticycle settings I could force the boiler to wait so no more need of a volumiser

 

 

[Super_Paulie]

I've likely misunderstood what I've been trying to achieve here. In fact, I'm not even sure what that is at this stage. I guess my tank just creates volume and not a lot else.

 

So tonight I ran a few tests. I locked off both upstairs rads which get a slight reverse circulation I think as they warm slightly when the UF is running. Ive been using a probe on the return under the boiler to read that but instead I'm going off what it shows on screen when it's not firing and that is surely the return temp.

 

I went with: Boiler 60, mixer 45. Bypass rad (tiny thing, shown above) 3/4 open.

 

So it now fires for 9 minutes as it creeps 60+5. Flames out with a 3 minute anti cycle and the return temp drops to 52 at the tank but shows 56 on screen.

 

It then fires back up and repeat, which I guess is a cycle time of 12 minutes.

 

So guys, ultimately what is my goal here, I'm not entirely sure anymore. Keep the boiler firing for as long as possible until it reaches overtemp and flames out?

 

For what it's worth, it's toasty warm in here at 22/23° and it's cost me £2.50.

Edited 3 hours ago by Super_Paulie

[marshian]

That's not a lot of difference between flow and return for the boiler to re-fire??

[Super_Paulie]

6 minutes ago, marshian said:

That's not a lot of difference between flow and return for the boiler to re-fire??

 

It's always around -10° no matter what I set the flow at it seems. 65 drops to 55. 55 drops to 45 etc. It just drops to whatever temp is in the tank in those 3 minutes.

 

I have absolutely no "off time" except when it's in that anti-cycle mode.

Edited 2 hours ago by Super_Paulie

[marshian]

4 minutes ago, Super_Paulie said:

 

It's always around -10° no matter what I set the flow at it seems. 65 drops to 55. 55 drops to 45 etc. It just drops to whatever temp is in the tank in those 3 minutes.

 

I have absolutely no "off time" except when it's in that anti-cycle mode.

 

But you said you set it to 60 flow and at 56 return it fired up?

 

I realise that 60 = 65 when boiler coasts but I would expect a 60 deg set point to relight at 50 deg not 56

[John Carroll]

29 minutes ago, Super_Paulie said:

I've likely misunderstood what I've been trying to achieve here. In fact, I'm not even sure what that is at this stage. I guess my tank just creates volume and not a lot else.

 

So tonight I ran a few tests. I locked off both upstairs rads which get a slight reverse circulation I think as they warm slightly when the UF is running. Ive been using a probe on the return under the boiler to read that but instead I'm going off what it shows on screen when it's not firing and that is surely the return temp.

 

I went with: Boiler 60, mixer 45. Bypass rad (tiny thing, shown above) 3/4 open.

 

So it now fires for 9 minutes. Flames out with a 3 minute anti cycle and the return temp drops to 52 at the tank but shows 56 on screen.

 

It then fires back up and repeat, which I guess is a cycle time of 12 minutes.

 

So guys, ultimately what is my goal here, I'm not entirely sure anymore. Keep the boiler firing for as long as possible until it reaches overtemp and flames out?

 

For what it's worth, it's toasty warm in here at 22/23° and it's cost me £2.50.

 

Can you go to the UFH manifold and read off the flowrates from each loop as accurately as possible from the flow meters ("glass tubes") and also the manifold flow & return temperatures when the boiler has been firing for say 5/7 minutes, also note the boiler flow & return temperatures.

Your boiler may have a internal bypass which would account for the difference in displayed return temp & the measured return temp, you could alos shut that bypass after say 5 minutes, note the boiler flow&return temps before shutting it and after another say 1 minute recheck those temperatures, I would then recommend returning the by pass to its original setting.

You are doing very well with that cycle time IMO, your cycle time suggests a UFH/boiler demand of at least, 6*9/12, 4.5kw but will probably be more as the boiler will fire at 20kw (its max on CH) for a few minutes to get the flow temp up to its target temp of 60C and then ramp down to its minimum of 6kw, your manifold data will give a fair idea of what the demand actually is (roughly).

 

Edit: whats this €2.50 cost? is it per day or what.

Edited 2 hours ago by John Carroll

[John Carroll]

5 minutes ago, marshian said:

 

But you said you set it to 60 flow and at 56 return it fired up?

 

I realise that 60 = 65 when boiler coasts but I would expect a 60 deg set point to relight at 50 deg not 56

The (any) boiler  should not refire until the flow temp is target temp - 5C, 60-5, 55C in this case, any modulating boiler cannot work if either the cut in/cut out are  at  target temp.

[marshian]

2 minutes ago, John Carroll said:

The (any) boiler  should not refire until the flow temp is target temp - 5C, 60-5, 55C in this case, any modulating boiler cannot work if either the cut in/cut out are  at  target temp.

 

Hmm OK I'll defer to your knowledge but I think 5 deg delta from flow temp is really bloody narrow at 60 deg flow temp - especially as on that flow setting the boiler is getting no benefit from the condensing process with a return of 55

 

The glow worm I had would only re-fire when return temp = flow temp (typically with a 50 deg flow temp return needed to get down to 31 (ish) before the boiler fired

 

Same with the Viessmann I have now (trouble is I'm running much lower flow temps but a 30 deg C flow temp it re-fires when it sees no difference between flow and return temps which is around 24 deg C (I thought this was because most of the heat in the circuit had been extracted)

[Super_Paulie]

14 minutes ago, marshian said:

 

But you said you set it to 60 flow and at 56 return it fired up?

Ah, gotcha. Ok so maybe my probe is a better indication rather than what is on screen.

 

10 minutes ago, John Carroll said:

Can you go to the UFH manifold and read off the flowrates from each loop as accurately as possible from the flow meters ("glass tubes") and also the manifold flow & return temperatures when the boiler has been firing for say 5/7 minutes, also note the boiler flow & return temperatures.

I'll have to do that tomorrow as the missus is back in, boiler in loft, manifold ground floor. I've sat looking at the manifold the last 10 minutes and to be honest the needles barely move at all, maybe a degree or do. All flow rates are 2.5, ish, balanced with Salus.

 

Yeah the smart meter was £2.50 so it's not going to be much higher by the time it goes to it's set back at 11, and 30p of that is the standing charge. It'll stay 20+ in here all evening no doubt.

[John Carroll]

13 minutes ago, marshian said:

 

Hmm OK I'll defer to your knowledge but I think 5 deg delta from flow temp is really bloody narrow at 60 deg flow temp - especially as on that flow setting the boiler is getting no benefit from the condensing process with a return of 55

 

The glow worm I had would only re-fire when return temp = flow temp (typically with a 50 deg flow temp return needed to get down to 31 (ish) before the boiler fired

 

Same with the Viessmann I have now (trouble is I'm running much lower flow temps but a 30 deg C flow temp it re-fires when it sees no difference between flow and return temps which is around 24 deg C (I thought this was because most of the heat in the circuit had been extracted)

The cut in temp on the majority of (gas fired) boilers is when the anticycle time has elapsed once  the flow temp is target temp - 5C, I have seen posts re some boilers which have a settable (negative) hysteresis where the cut in is either at this target-hysteresis OR the anticycle time.

[JohnMo]

A couple of images. The first is the boiler firing in a small loop and not getting to the heating system.  When in normal running the flow temp is 38.5. at about 1.15 a pump stopped, flow temp increases 5 degs, then refires at 30.

 

No setting changes but normal running, against a full open heating system.

 

Second image a few hours previous to above.

 

 

Restart is 26 degs.

 

So I suspect your engaged heating system is just to small to move the heat away. The mixer is just pushing most of the water straight back into the boiler.

 

[marshian]

2 minutes ago, Super_Paulie said:

Ah, gotcha. Ok so maybe my probe is a better indication rather than what is on screen.

 

Yeah it may well be but the boiler is reacting to the temperature it gets from the internal sensor.........

 

[Super_Paulie]

8 minutes ago, JohnMo said:

A couple of images. The first is the boiler firing in a small loop and not getting to the heating system.  When in normal running the flow temp is 38.5. at about 1.15 a pump stopped, flow temp increases 5 degs, then refires at 30.

 

No setting changes but normal running, against a full open heating system.

 

Second image a few hours previous to above.

 

 

Restart is 26 degs.

 

So I suspect your engaged heating system is just to small to move the heat away. The mixer is just pushing most of the water straight back into the boiler.

 

That's interesting, thanks for that. So volume is the thing, I could probably increase that with an additional rad say in the hall plumbed in before the 2 ports.

 

But what advantage would I gain from this increased volume, longer cycle time?

[John Carroll]

25 minutes ago, Super_Paulie said:

Ah, gotcha. Ok so maybe my probe is a better indication rather than what is on screen.

 

I'll have to do that tomorrow as the missus is back in, boiler in loft, manifold ground floor. I've sat looking at the manifold the last 10 minutes and to be honest the needles barely move at all, maybe a degree or do. All flow rates are 2.5, ish, balanced with Salus.

 

Yeah the smart meter was £2.50 so it's not going to be much higher by the time it goes to it's set back at 11, and 30p of that is the standing charge. It'll stay 20+ in here all evening no doubt.

Cant see the bottom (return) gauge properly but if its the same as the top one (scale wise) then maybe reading 40/42C ish? so ~ 7C dT, fairly normal, so UFH output 2.5*4*60*(48-41)/860, 4.88kw?, or there abouts.

[Super_Paulie]

15 minutes ago, John Carroll said:

Cant see the bottom (return) gauge properly but if its the same as the top one (scale wise) then maybe reading 40/42C ish? so ~ 7C dT, fairly normal, so UFH output 2.5*4*60*(48-41)/860, 4.88kw?, or there abouts.

Yeah it's dead on 7 lower, the actuators see to that and seemingly work a treat.