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.