Another Regular Boiler short-cycling on Hot Water problem...

[Brovagen]

Hope someone can suggest where I should looking to resolve this one...

I have a Bosch 30Cdi regular boiler on an S plan system with a 180L vented, indirect hot water cylinder.
Boiler Temperature is set to 5 (67c) and the stat on the Hot water cylinder is set just below 60c.
A Honeywell DU144 Auto Bypass valve is fitted and set to 0.55 to give a minimum flow of 300l/hr through the boiler at Speed setting 3 on a Grundfos UPS2 15-50/60 circulator.

From cold, when the stat on the cylinder calls for heat, the motorised valve opens and starts the boiler.
The flow temperature on the boiler rises to around 60c within a few minutes, finally getting to 69c before modulating down to 67c.

Within 10 minutes, I measured temperature at the 22mm pipe into the cylinder coil, which was 64c and the pipe out of the coil was 60c.
These temperatures were also reflected at the boiler on the 28mm Flow and Return pipes.

As time went on, the temperature flow out of the coil and return to the boiler got ever closer to the flow temperature. Flow temperature from the boiler was measured at 64c and the Return temperature was measured at 62c.

After half an hour the boiler switched off (while still displaying a Flow temperature of 67c).
The cylinder stat had not been satisfied, the motorised valve was still open and the circulator continued to run.

After a period of time and the Boiler Flow temperature had reduced to approx. 55c, the boiler started up again and continued to heat the cylinder until, once again, the boiler short-cycled.

After a few short-cycling events, the cylinder stat was eventually satisfied, the motorised valve closed, the boiler switched off and the circulator ran on for a period.

Looking back through my Smart Meter records, this has been happening for some time and ends up using around 11KWh of gas, which isn't very efficient or cost effective!

Am I not displacing enough heat through the coil in the cylinder, is the pump running too fast, is temperature setting on the cylinder stat too close to that of the boiler temperature or should I be looking elsewhere?

Any advice appreciated.

 

[ShaunCorbs]

Probably a bit of both as it’s a gravity cylinder chances are it’s a 5-7kw coil so your never going to win unless you upgrade the cylinder to a 20-30kw coil

 

[Brovagen]

Probably a bit of both as it’s a gravity cylinder chances are it’s a 5-7kw coil so your never going to win unless you upgrade the cylinder to a 20-30kw coil

Thanks for the reply.

This is an indirect heated cylinder (ie the coil is fed from the pumped CH pipes via a motorised valve).
The water to be heated is fed from a gravity fed tank in the loft.

Would it be normal to have a coil rating in one of these cylinders, which would be incapable of dissipating the heat from the boiler or is it bad design? When looking at new indirect vented hot cylinders, I can't see any reference to coil ratings?

 

[ShaunCorbs]

Yes will be 5-7kw so no wonder it’s short cycling

Just old cylinder only designed for a 12kw boiler etc

 

[John.g]

Don't know what the minimum boiler output is but assuming 5.5kw and assuming the coil is just about able to absorb this then the coil circulation must be 19.7LPM at a coil dT of 4C which the UPS 2 will deliver at a 4.9M head or 39.4 LPM (coil dT of 2C) at a 2.7M head, either way, the ABV should be closed as its set to 5.5M (0.55).

Is the ABV closed completely??., it should also remain closed on burner cut out and only open when the cylinder stat is satisfied and shuts the motorized valve, pump head should then rise to 6.2M to give the required minimum 300LPH.
Maybe increase Boiler SP to 70/75C to see if the cylinder reheats in one go to 60C without boiler cycling.

It requires 8.4kwh to heat the whole 180L from todays 20c to 60C so 11kwh gas consumption while a bit on the high side (boiler efficiency of 77%) isn't outrageous?.

My own vented 150L cylinder gives a coil output of 7.5kw when cold (15C), falling to 2.5kw when approaching 60C with a oil fired boiler of average 70C temp. The coil area is 0.62M2.

 

[Brovagen]

Don't know what the minimum boiler output is but assuming 5.5kw and assuming the coil is just about able to absorb this then the coil circulation must be 19.7LPM at a coil dT of 4C which the UPS 2 will deliver at a 4.9M head or 39.4 LPM (coil dT of 2C) at a 2.7M head, either way, the ABV should be closed as its set to 5.5M (0.55).

Is the ABV closed completely??., it should also remain closed on burner cut out and only open when the cylinder stat is satisfied and shuts the motorized valve, pump head should then rise to 6.2M to give the required minimum 300LPH.
Maybe increase Boiler SP to 70/75C to see if the cylinder reheats in one go to 60C without boiler cycling.

It requires 8.4kwh to heat the whole 180L from todays 20c to 60C so 11kwh gas consumption while a bit on the high side (boiler efficiency of 77%) isn't outrageous?.

My own vented 150L cylinder gives a coil output of 7.5kw when cold (15C), falling to 2.5kw when approaching 60C with a oil fired boiler of average 70C temp. The coil area is 0.62M2.

Thanks for the feedback...

My understanding is that the Bosch Worcester 30CDi can only regulate down to 10kw for it's low temperature regime, but could be wrong...
Based on that, I'd suggest it's no wonder that the cylinder coil can't absorb enough of the generated heat?

I set the ABV to 0.55 based on the UPS 2's curves on Speed 3 and a the boiler's minimum flow rate of 5Lt/min.
I have to assume the ABV is staying shut during heating, but don't definitely know. (I installed the ABV to replace the old Manual Gate Valve, which was always passing to some degree to allow the pump to circulate through the boiler on valves closing)

Could you explain how you arrive at rating the coil output in a cylinder?

Also, excusing my ignorance, how are you arriving at your calculations for :
"Don't know what the minimum boiler output is but assuming 5.5kw and assuming the coil is just about able to absorb this then the coil circulation must be 19.7LPM at a coil dT of 4C which the UPS 2 will deliver at a 4.9M head or 39.4 LPM (coil dT of 2C) at a 2.7M head, either way, the ABV should be closed as its set to 5.5M (0.55)."

& also for:

"It requires 8.4kwh to heat the whole 180L from todays 20c to 60C so 11kwh gas consumption while a bit on the high side (boiler efficiency of 77%) isn't outrageous?."

As you say, increasing the boiler's setpoint to the next temperature setting may allow for heating of the tank in one firing as the dT would be greater when compared to the stat setting on the cylinder.

I presume that there may be no real solution to this as fitting a cylinder with a higher rated coil (say 10kw) may solve the short-cycling issue, but would dump too much heat when both hot water and radiator demand is requested ending up with slow heating of radiators?

 

[ShaunCorbs]

Just time them separately that’s what I do

 

[John.g]

Thanks for the feedback...

My understanding is that the Bosch Worcester 30CDi can only regulate down to 10kw for it's low temperature regime, but could be wrong...
Based on that, I'd suggest it's no wonder that the cylinder coil can't absorb enough of the generated heat?

Agree, but based on (your) continuous run time of 30 mins then its heating the water to 48/50C based on a 10kw min boiler output.

I set the ABV to 0.55 based on the UPS 2's curves on Speed 3 and a the boiler's minimum flow rate of 5Lt/min.
I have to assume the ABV is staying shut during heating, but don't definitely know. (I installed the ABV to replace the old Manual Gate Valve, which was always passing to some degree to allow the pump to circulate through the boiler on valves closing)

Feel the ABV pipes, especially at the (ABV) return, should be cool/cold after say 15 minutes of boiler firing, right up to the time the cylinder stat is satisfied and boiler shut down, it should then get very hot for the pump overrun period of a few minutes or so.

Could you explain how you arrive at rating the coil output in a cylinder?

Only by calculation below, but see attachment re (very) rapid recovery cylinder, I did the Kw and coil dT calculations.

Also, excusing my ignorance, how are you arriving at your calculations for :
"Don't know what the minimum boiler output is but assuming 5.5kw and assuming the coil is just about able to absorb this then the coil circulation must be 19.7LPM at a coil dT of 4C which the UPS 2 will deliver at a 4.9M head or 39.4 LPM (coil dT of 2C) at a 2.7M head, either way, the ABV should be closed as its set to 5.5M (0.55)."

We now know that the boiler min output is 10kw so LPM = 10X860/60/4 = 35.83 (LPM). Its very unlikely that the circulation rate would be higher than this so assuming that it remains the same then the coil power with a flow/return dT of 2C, in KW = 35.83*60*2/860 = 5 (Kw)

& also for:

"It requires 8.4kwh to heat the whole 180L from todays 20c to 60C so 11kwh gas consumption while a bit on the high side (boiler efficiency of 77%) isn't outrageous?."

Heat required to increase 180L of water from 20C to 60C = 180*(60-20)/860 = 8.37Kw = boiler output, boiler efficiency is (heat output/heat input)X100 = 8.37/11.0X100 = 76.1%.

As you say, increasing the boiler's setpoint to the next temperature setting may allow for heating of the tank in one firing as the dT would be greater when compared to the stat setting on the cylinder.

It would, all these high output coils are designed with a boiler flow temp of at least 80/85C, which really drives/increases the output.
Also, systems with HW Priority increase the boiler flow temp to 80C and then return it to its CH setpoint when cylinder heating satisfied.

I presume that there may be no real solution to this as fitting a cylinder with a higher rated coil (say 10kw) may solve the short-cycling issue, but would dump too much heat when both hot water and radiator demand is requested ending up with slow heating of radiators?

If the cycling is a worry on HW then I would expect the cycling to be far greater on CH as > 10kw heat demand to keep rooms heated once up to temperature seems pretty high?.
Your boiler doesn't have any problems with cycling, unlike some of its modern day replacements, which just don't like cycling possibly due to the tiny volume of water in their heat exchangers. Also cycling apparently doesn't reduce overall boiler efficiencies by much ( a few %) from the bits and pieces I've read.

 

[John.g]

Few Corrections to above calcs.

 

[SheilaF]

The problem is, you are suffering from what I call 'Boiler Pressure Syndrome'. The fact that the flow temperature is rising to 69 degrees, despite your stat being set at 60 degrees, indicates that the boiler is running quite hot. This is quite normal, but the only way to get your cylinder to operate at a lower temperature, is to lower the boiler pressure. Adding a small expansion vessel, will instantly lower the pressure and cool the system down. You can also reduce the circulator output, but that will only work for a little while, because the boiler will keep trying to heat the cylinder up, until the pressure falls below a certain point, and then the boiler will switch off and cool down.

 

[gpbeck]

I have been in exactly the same place with the same boiler. I ended up heating hot water for 30 minutes simultaneously when central heating started, twice a day, which kept return temperatures low enough to prevent cycling, and heated cylinder adequately. Whilst a well made boiler, being only able to modulate down to 30% is a big problem.

 

[John.g]

Is/was your CH demand such that its demand is > 9 or 10kw to keep the boiler from cycling?.

 

[gpbeck]

Is/was your CH demand such that its demand is > 9 or 10kw to keep the boiler from cycling?.

Seemed so, for that first half hour. House had a 20kW+ heat loss.