Viessmann boiler question

[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.

 

[ShaunCorbs]

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.

surely 1.9kw is enough it’s like two rads ?

ok so why not do this

viessmann 200 combi with a buffer vessel and weather comp

you will be more efficient than your set up and have your rads acceptable to you ?