Viessmann boiler question

[mars_man]

Wondering if there are plumbing gurus here who can suggest what could be the issue .Recently changed to gas heating and have a viessmann vitoden 100 , 26 kw condensing boiler .The old oil boiler was a firebird 26 Kw and I have almost already started missing it .

The house is well insulated and has 12 rads /3 zones . I've noticed that the with the thermostats set to 21 deg as before , the rads are lukewarm and dont heat up quickly . The room temp slowly ramps to the set temperature as I can see that the thermostat keeps cutting off frequently .

I've changed the boiler flow temp from the initial setting of 71 deg to 76 and then 78 , but that dosen't seem to have made a big difference.
The only way to heat up to the same temp as before seems to be by setting the thermostat to around 25 , but even with that it seems that the boiler keeps cutting off and when its back on , its not ON for long enough to heat up all the rads .I read the boiler pressure and the boiler pressure is around near the notch near 1 bar .

 

[Stu-B]

Do you have a weather compensation sensor?

 

[mars_man]

Do you have a weather compensation sensor?

No , I dont .
Since posting here ive also tried increasing the flow temp to max /80 keeping the room stats at around 25 and the boiler comes on for a little longer for around 2/3 minutes and then cuts off and then starts after a while again .

 

[ShaunCorbs]

You got a pic further back of the whole boiler and pipe work ?

also pressure wants upping around 1.2 for a cold system

 

[mars_man]

You got a pic further back of the whole boiler and pipe work ?

also pressure wants upping around 1.2 for a cold system

Thanks , attach picture of the installed unit . The extra expansion vessel to the right is missing in this pic

 

[ShaunCorbs]

Is the bypass set below the boiler and not open ? Should be around 3-4 set level

 

[Chuck]

Since posting here ive also tried increasing the flow temp to max /80 keeping the room stats at around 25 and the boiler comes on for a little longer for around 2/3 minutes and then cuts off and then starts after a while again .

From your description, I suspect that your boiler is 'short-cycling' because heat is not being transferred from the boiler to the radiators fast enough. It's a common problem with small zones and/or TRVs and well-insulated houses. IIRC, your boiler's minimum output is 9kW (1:3 turndown) so when the demand from the emitters is low, which it often will be in a well-insulated and zoned house, the boiler will be providing short pulses of 9kW in an attempt to meet the demand.

If my guess is right, and the commissioning was done correctly, the only really satisfactory solution to this problem is to increase the thermal mass of the water in the system, which you're doing to some extent by increasing the water temperature, but which really needs more water. Ideally, you'll have a buffer tank added to the system. If that's a step too far, you'll get a considerable improvement by abandoning separate zones and TRVs so the boiler has all the water in the system to use as a thermal mass. (Use weather compensation and/or a single thermostat controlling the boiler as the main form of control).

The good news is that this is only really a problem in mild weather, once your demand goes up to 9kW the problem will disappear.

 

[mars_man]

Is the bypass set below the boiler and not open ? Should be around 3-4 set level

Thanks will have a closer look tomorrow and post
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From your description, I suspect that your boiler is 'short-cycling' because heat is not being transferred from the boiler to the radiators fast enough. It's a common problem with small zones and/or TRVs and well-insulated houses. IIRC, your boiler's minimum output is 9kW (1:3 turndown) so when the demand from the emitters is low, which it often will be in a well-insulated and zoned house, the boiler will be providing short pulses of 9kW in an attempt to meet the demand.

If my guess is right, and the commissioning was done correctly, the only really satisfactory solution to this problem is to increase the thermal mass of the water in the system, which you're doing to some extent by increasing the water temperature, but which really needs more water. Ideally, you'll have a buffer tank added to the system. If that's a step too far, you'll get a considerable improvement by abandoning separate zones and TRVs so the boiler has all the water in the system to use as a thermal mass. (Use weather compensation and/or a single thermostat controlling the boiler as the main form of control).

The good news is that this is only really a problem in mild weather, once your demand goes up to 9kW the problem will disappear.

Thanks for that .
Shouldn't the boiler do the opposite , that is start at 26 KW and then go down to 9 kw when it perceives the demand from the emitters to be low ? The heating coming on in short cycles seems to be the issue here but would you then say that this is a boiler issue rather than anything else .
after all many other houses with even better insulation levels with the same setup as well .

 

[John.g]

Wondering if there are plumbing gurus here who can suggest what could be the issue .Recently changed to gas heating and have a viessmann vitoden 100 , 26 kw condensing boiler .The old oil boiler was a firebird 26 Kw and I have almost already started missing it .

The house is well insulated and has 12 rads /3 zones . I've noticed that the with the thermostats set to 21 deg as before , the rads are lukewarm and dont heat up quickly . The room temp slowly ramps to the set temperature as I can see that the thermostat keeps cutting off frequently .

I've changed the boiler flow temp from the initial setting of 71 deg to 76 and then 78 , but that dosen't seem to have made a big difference.
The only way to heat up to the same temp as before seems to be by setting the thermostat to around 25 , but even with that it seems that the boiler keeps cutting off and when its back on , its not ON for long enough to heat up all the rads .I read the boiler pressure and the boiler pressure is around near the notch near 1 bar .

I agree re your Firebird boiler, I have one for the past15 years and it has never locked on any fault condition, however, we are where we are.
To rule out constant cycling when the rad(s) output is > the boiler min output you might consider the following or at least run with both CH zones and all l/shield+TRVs opened if the boiler/system is already up to temperature, also roomstats turned up fully:
You have 12 rads, don't know their combined output but assume ~ 18kw and total system water contents of say 80 litres., can you switch in the two heating zones (from cold conditions) with all rad lock shield valves & any TRVs fully open, assuming a boiler SP of 70C then the boiler should fire (100%) continuously for ~ 25 to 30 minutes to reach SP temp and should then continue to run continuously in modulating mode at ~ 58% say 15 kw output. If any big deviations from this then you have a problem but you have to start somewhere. Also as mentioned above, check the setting of the ABV just below the boiler, I would suggest shutting it off (index 0.6 0r 6?) initially and if no problems then set it to ~4.

 

[doitmyself]

Recently changed to gas heating and have a viessmann vitoden 100 , 26 kw condensing boiler .The old oil boiler was a firebird 26 Kw ...

Which type of boiler do you have - combi, system or open vent?

Was a heat-loss calculation carried out before selecting a 26kW boiler (not important if you have a combi)? After all, you do say that the house is well insulated which, presumably, was done after the Firebird was installed. So a smaller boiler would be more likely.

 

[Chuck]

Shouldn't the boiler do the opposite , that is start at 26 KW and then go down to 9 kw when it perceives the demand from the emitters to be low?

The behaviour of the boiler is determined by the manufacturer. A typical boiler will ignite at full (or relatively high) power, then modulate down rapidly to a low power and then gradually ramp up the power while monitoring the temperature of the water flowing through the heat exchanger.

The short cycles occur because the boiler shuts down when a thermometer near the heat exchanger (either on the HX itself or in the exiting water) rises about a limit, typically 5°C above the target setpoint.

would you then say that this is a boiler issue rather than anything else .
after all many other houses with even better insulation levels with the same setup as well .

It might be an problem with your boiler, perhaps a parameter set incorrectly during commissioning.

I think, however, it's just physics. If your house requires 0.9kW average to stay at 21°C, and your boiler has a minimum output of 9kW it has to spend 10% of it's time on and 90% off. The heat capacity of the water in the system will change whether that's a duty cycle of 1 minute on and 9 minutes off, or 5 minutes on and 50 minutes off. If the boiler is connected directly to the radiators occupants may notice the pulsing on/off. Some people seem more sensitive to this than others. The problem can be mitigated by means of a 'buffer tank', which smoothes out the temperature fluctuations arising from the on/off action of the boiler.

 

[John.g]

If the heating zones above are run only on their roomstats, then even just running one zone should require a heating demand > the minimum output of the boiler which I think is ~ 6 kw resulting in the boiler running continuously, that is way I suggest using the above methods to see if there is fundamentally something wrong in the set up or whatever. The problem with cycling gas boilers is that the Hx contents of ~ 3 to 4 litres have really no thermal buffering whereas a oil fired boiler has Hx contents of ~ 25 litres plus they have 4 hefty steel baffles which run practically red hot and will further add to the thermal buffering when the boiler is off, for example with a 4 kw heat demand an oil fired boiler (with 10 deg hysteresis) will stay off for at least 4 to 5 minutes before cutting back in whereas a gas boiler will fall by this 10 deg in only 42 secs which leads to very very rapid cycling and if/with a anti fast cycle time of 3 to 5 minutes will result in a very cold flow temperature to the rads before firing up again.

 

[mars_man]

I agree re your Firebird boiler, I have one for the past15 years and it has never locked on any fault condition, however, we are where we are.
To rule out constant cycling when the rad(s) output is > the boiler min output you might consider the following or at least run with both CH zones and all l/shield+TRVs opened if the boiler/system is already up to temperature, also roomstats turned up fully:
You have 12 rads, don't know their combined output but assume ~ 18kw and total system water contents of say 80 litres., can you switch in the two heating zones (from cold conditions) with all rad lock shield valves & any TRVs fully open, assuming a boiler SP of 70C then the boiler should fire (100%) continuously for ~ 25 to 30 minutes to reach SP temp and should then continue to run continuously in modulating mode at ~ 58% say 15 kw output. If any big deviations from this then you have a problem but you have to start somewhere. Also as mentioned above, check the setting of the ABV just below the boiler, I would suggest shutting it off (index 0.6 0r 6?) initially and if no problems then set it to ~4.

The test with the heating on to max with HW on as well was done by the Viessmann installer after commissioning.With the heating cranked up , the boiler didnt cut off and the radiators .I have only noticed the problem now that we have started using it at normal room temperatures of 21 or so and the short cycling issue mentioned above . I might do the test again to check the length of time it stays on .
Meanwhile on the boiler the ABV seems to be set to 5 .
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Which type of boiler do you have - combi, system or open vent?

Was a heat-loss calculation carried out before selecting a 26kW boiler (not important if you have a combi)? After all, you do say that the house is well insulated which, presumably, was done after the Firebird was installed. So a smaller boiler would be more likely.

Yes house was well insulated after the firebird was installed .Its a 26 KW system boiler .Rads were sized up and replaced with new as I had the house replumbed 3 years back during a major refurbishment and removed all old GB pipes as well
( Only replaced the firebird as we wanted to get rid of the huge oil tank in a small back garden)

 

[gmartine]

Try doing the opposite, open up the TRV's (assuming you have them), set a lower flow temp hopefully that will allow the boiler to fire for a longer periods.

 

[Gasmk1]

was system cleaned i had one a couple of years ago that struggled as wasnt getting the required differential between flow and return this was caused by new alpha pump and also still some sludge in system even though had cleaned

 

[mars_man]

was system cleaned i had one a couple of years ago that struggled as wasnt getting the required differential between flow and return this was caused by new alpha pump and also still some sludge in system even though had cleaned

Yes the system was power flushed and inhibitors added . The system was actually quite clean as they found during the power flush cycle .

 

[Gasmk1]

did they fit a new pump to system

 

[John.g]

If its a systems boiler I think it will have a 7M Grundfos circ pump set on speed 3 which should provide plenty of circulation, obviously if problems then the boiler delaT will be excessive with constant cycling, but that will bescome fairly evident if boiler cycles with both zones on?.
There has to be some starting point for trouble shooting the problem otherwise we will be talking about it until the cows come home.

Just saw post# 17 there now but yes, no harm to repeat those tests. ABV setting should be OK at 5.

 

[siricosm]

... really needs more water. Ideally, you'll have a buffer tank added to the system.

If the boiler is connected directly to the radiators occupants may notice the pulsing on/off. Some people seem more sensitive to this than others. The problem can be mitigated by means of a 'buffer tank', which smoothes out the temperature fluctuations arising from the on/off action of the boiler.

+1 for the suggestion to buffer the heating system.

My heating suffers from this problem. There is just not enough thermal mass in modern rads/piping/boilers to be able to connect the boiler directly to the rads. TRVs make the problem worse. Modulation can definitely help, but IMHO it is a bit of a kludge.

 

[ShaunCorbs]

Weather comp sensor automatically adjusts the rating and temp so you will get it running constantly

 

[doitmyself]

IIRC, your boiler's minimum output is 9kW (1:3 turndown) so when the demand from the emitters is low, which it often will be in a well-insulated and zoned house, the boiler will be providing short pulses of 9kW in an attempt to meet the demand.

The installation manual says that the minimum output is 4.3kW @ 80/60C or 4.7kW @ 50/30C. If the house really needs 26kW for heating then the boiler will only go into on/off mode when the outside temperature is about 17C.

I suspect that the boiler output was based on what was there originally, not on the heat loss.

 

[siricosm]

Weather comp sensor automatically adjusts the rating and temp so you will get it running constantly

That isn't much help on a 16C day when all you have is 3:1 and you need 20:1.

 

[ShaunCorbs]

That isn't much help on a 16C day when all you have is 3:1 and you need 20:1.

install a 200 will modulate down to 17:1 or if you can get an older one 20:1

 

[mars_man]

I agree re your Firebird boiler, I have one for the past15 years and it has never locked on any fault condition, however, we are where we are.
To rule out constant cycling when the rad(s) output is > the boiler min output you might consider the following or at least run with both CH zones and all l/shield+TRVs opened if the boiler/system is already up to temperature, also roomstats turned up fully:
You have 12 rads, don't know their combined output but assume ~ 18kw and total system water contents of say 80 litres., can you switch in the two heating zones (from cold conditions) with all rad lock shield valves & any TRVs fully open, assuming a boiler SP of 70C then the boiler should fire (100%) continuously for ~ 25 to 30 minutes to reach SP temp and should then continue to run continuously in modulating mode at ~ 58% say 15 kw output. If any big deviations from this then you have a problem but you have to start somewhere. Also as mentioned above, check the setting of the ABV just below the boiler, I would suggest shutting it off (index 0.6 0r 6?) initially and if no problems then set it to ~4.

--> (Current) BF 75 , room temp~21.x ,Boiler cuts off at around 2.4 mins and starts after 3 mins
--> BF75, HW ~60, room temp ~18 with rads cold room stats - 25 - Boiler cuts off at around 8 mins .
--> BF 75 ,HW ~60 ( unchanged), room stats - 35 ( max ) , HW ON - Boiler cuts off around 35 minutes ,All rads heat up to uniformly to around 65 . all TRV's fully opened

Also tried with BF reduced to 65 and no change in the short cycling i,e 2 mins .

 

[siricosm]

install a 200 will modulate down to 17:1 or if you can get an older one 20:1

I agree it is possible to fight this problem with technology. Maybe they will develop the "infinitely modulating boiler" that is permanently on at whatever percentage is required for the demand. However, one has to question the expense/complexity/robustness of that solution, when a simple tank of water can solve the problem.