Boiler firing constantly for hot watere

[pwater]

Hi,

I have one coming for the flow today, but it’s definitely the cylinder firing it, the heating is off and it’s the hot water circuit coming on.

It’s a greenstar8000 35kw system boiler, set to 65.

Ta

Paul

 

[MNW67]

I have one coming for the flow today

Are you able to get one for boiler modulation % as well for the same time period?

 

[pwater]

Are you able to get one for boiler modulation % as well for the same time period?

I have been checking it manually, usually around 25-30% modulation when the hot water is on

 

[Johntheo5]

It’s a greenstar8000 35kw system boiler, set to 65.

I have been checking it manually, usually around 25-30% modulation when the hot water is on

That gives a boiler output and coil rating of ~ 35*25%, 8.75kw, where the mystery still is though is the 15 minutes running to increase the HW temperature by a max of 1.5C, it only requires, 300*1.5/860, 0.523kwh which should, theoretically, only take, 0.523/8.75*60, 3.6 minutes say 5 minutes max even allowing for reheating ~ 20 litres of system water. Of course the whole thing is distorted by this ridiculous tiny switching differential, you should look seriously at getting that stat changed, can you identify the manufacturer?, they can then be contacted to find what the actual switching differential is/should be.

 

[pwater]

That gives a boiler output and coil rating of ~ 35*25%, 8.75kw, where the mystery still is though is the 15 minutes running to increase the HW temperature by a max of 1.5C, it only requires, 300*1.5/860, 0.523kwh which should, theoretically, only take, 0.523/8.75*60, 3.6 minutes say 5 minutes max even allowing for reheating ~ 20 litres of system water. Of course the whole thing is distorted by this ridiculous tiny switching differential, you should look seriously at getting that stat changed, can you identify the manufacturer?, they can then be contacted to find what the actual switching differential is/should be.

Yea, so I removed their temp probe today, turned the power off to the tank once it was up to temp and left it while I was out. Over almost 6hrs it lost 3.5 degrees, which given the thermostat pocket seems to be at the bottom of the tank, doesn’t seem too bad, so I am wondering if instead of the system leaking hot somewhere, it’s just the thermostat is too sensitive, as during that time it would normally have fired 4/5 times, which would roughly equate to every time I dropped a single degree….this is the thermostat - https://www.unventedcomponentseurop...GzOJeNeve9awYUd_6Cxb0CfOhD6Dtt7xoCOJMQAvD_BwE

It it normal for most tanks stats to have a 3/4 degree differential before calling for heat?

The hot water circuit return is around 40 degrees usually when it fires every hour, so the boiler is taking whatever is in the system (it’s prob 20m of pipe to the boiler) and the coil at roughly that temp up to 65, then heating the water. Most of the time is spent getting it up to temp, the actual cylinder reheat seems to just take a few mins.

 

[Echo the husky]

I assume that is some sort of electronic cylinder stat? Not seen one like that before in the many unvented water heaters I've wired up. I wonder if it is programmed to cycle that way??

 

[pwater]

The graph is just from my own temp probe I popped in to the pocket to see what was happening, the actual stat is https://www.unventedcomponentseurop...GzOJeNeve9awYUd_6Cxb0CfOhD6Dtt7xoCOJMQAvD_BwE which seems to have come with the unit originally.

Sorry if I have misunderstood!

 

[Johntheo5]

Yea, so I removed their temp probe today, turned the power off to the tank once it was up to temp and left it while I was out. Over almost 6hrs it lost 3.5 degrees, which given the thermostat pocket seems to be at the bottom of the tank, doesn’t seem too bad, so I am wondering if instead of the system leaking hot somewhere, it’s just the thermostat is too sensitive, as during that time it would normally have fired 4/5 times, which would roughly equate to every time I dropped a single degree….this is the thermostat - https://www.unventedcomponentseurope.com/heatrae-sadia-megaflo-megaflow-indirect-thermostat-95612697.html/?utm_source=Google Shopping&utm_campaign=Copy Unvented Components Europe&utm_medium=cpc&utm_term=577425&gad_source=1&gclid=CjwKCAiA8sauBhB3EiwAruTRJunL3TvwpdvWi7iRnKz5KhGzOJeNeve9awYUd_6Cxb0CfOhD6Dtt7xoCOJMQAvD_BwE

It it normal for most tanks stats to have a 3/4 degree differential before calling for heat?

The hot water circuit return is around 40 degrees usually when it fires every hour, so the boiler is taking whatever is in the system (it’s prob 20m of pipe to the boiler) and the coil at roughly that temp up to 65, then heating the water. Most of the time is spent getting it up to temp, the actual cylinder reheat seems to just take a few mins.

Paul
Just a query or two, can you confirm that the stat is a (Cotherm) GTLH3161? and how did you arrive at the 40.71C average?
Will do a few calcs then.

but "is it normal for most tanks stats to have a 3/4 degree differential before calling for heat?"
Most certainly NOT IMO.

 

[MNW67]

Yea, so I removed their temp probe today, turned the power off to the tank once it was up to temp and left it while I was out. Over almost 6hrs it lost 3.5 degrees, which given the thermostat pocket seems to be at the bottom of the tank, doesn’t seem too bad,

Loving your data! Just to check, you hadn't turned off the flow of hot water from the cylinder this time? Why does the temperature of the cylinder shoot up so quickly from 20C to 60C right at the start?

How might this explain why the boiler didn't come on at all when previously you turned the flow from the cylinder off?

If you are right, though, and the fall is just 0.5C an hour, then it does sound to me like a dodgy thermostat. Which would be a great outcome!

 

[pwater]

Paul
Just a query or two, can you confirm that the stat is a (Cotherm) GTLH3161? and how did you arrive at the 40.71C average?
Will do a few calcs then.

but "is it normal for most tanks stats to have a 3/4 degree differential before calling for heat?"
Most certainly NOT IMO.

Hi, yes it does appear to be that model! The average is just what the app the sensor is connected to shows, it’s not correct though as thats over 24hrs and the probe was only in for 6 hrs, so best ignored!

 

[pwater]

Loving your data! Just to check, you hadn't turned off the flow of hot water from the cylinder this time? Why does the temperature of the cylinder shoot up so quickly from 20C to 60C right at the start?

How might this explain why the boiler didn't come on at all when previously you turned the flow from the cylinder off?

If you are right, though, and the fall is just 0.5C an hour, then it does sound to me like a dodgy thermostat. Which would be a great outcome!

Ah the probe wasn’t inserted into anything before that, so that’s just the amident air temp. The flow of hot/cold was on during this time, so all was normal apart from the fact the cylinder had no power so couldn’t call for heat.

I am going to do the same test today but with the hot flow turned off, see what it loses then.

 

[pwater]

but "is it normal for most tanks stats to have a 3/4 degree differential before calling for heat?"
Most certainly NOT IMO.

Would they generally not have a differential or have it higher than that?

 

[Johntheo5]

I read that incorrectly as a dT of 0.75C (3/4), yes definitely a minimum dT of 5C which is what I have set my digital controller to but most mechanical/electrical cylinder stats are designed to switch at ~ 6C to 10C, even if the whole cylinder cooled naturally overnight by 10C then you still have 300L at 50C when the boiler kicks in so no problem, if the boiler had just cut out and you start using HW, then once you use up say 20/30L of HW, the boiler will again start but you still have 280L of stored HW at 60C.
Will do those calcs later.
I saw a UK number of a COTHERM supplier so I might give them buzz later.

I took a few readings off my cylinder yesterday which may be of interest, will post later.

 

[Johntheo5]

I rang the UK re the Cotherm GTLH3161 and am awaiting a call back from some technical person.

Below are the calcs on my 150L cylinder and the calcs based on your info above. Your losses, if not due to leakage, are quite high.

150L HW Cylinder
elapsed time, hrs 4.49
loss. Deg C 2.5
loss/hr. Deg C 0.56
loss/24 hr. Deg C 13.36
loss/24 hr. kw 2.16
Heated Vol Ltrs 139
Loss Rating watts 90

300L HW Cylinder
elapsed time, hrs 5.266
loss. Deg C 6.00
loss/hr. Deg C 1.14
loss/24 hr. Deg C 27.35
loss/24 hr. kw 9.54
Heated Vol Ltrs 300
Loss Rating watts 397

 

[pwater]

I rang the UK re the Cotherm GTLH3161 and am awaiting a call back from some technical person.

Below are the calcs on my 150L cylinder and the calcs based on your info above. Your losses, if not due to leakage, are quite high.

150L HW Cylinder
elapsed time, hrs 4.49
loss. Deg C 2.5
loss/hr. Deg C 0.56
loss/24 hr. Deg C 13.36
loss/24 hr. kw 2.16
Heated Vol Ltrs 139
Loss Rating watts 90

300L HW Cylinder
elapsed time, hrs 5.266
loss. Deg C 6.00
loss/hr. Deg C 1.14
loss/24 hr. Deg C 27.35
loss/24 hr. kw 9.54
Heated Vol Ltrs 300
Loss Rating watts 397

Thats great info, thankyou. So I am basically losing around triple what your 150L tank is losing in kWh and double in temp. The manual for the tank says that the standing heat loss for the 300L tank is 2.72kWh per day, so I am losing around 3x that based on the above.

Just waiting for the tank to come up to temp, then will see what I lose with the hot flow closed, try and rule in/out a leak.

Thanks!

Paul