My Viessman boiler journey

[marshian]

22 minutes ago, MrPotts said:

OK, i think its time to get someone in that can investigate how the boiler has been wired.

 

And there is my problem - I got the only Viessmann trained gas engineer for 50 mile radius to do the job - the other installer 50 miles away didn't want to quote because the distance made it not worth his while.................

 

Maybe I should ask Adam from heat geek if he fancies a boiler challenge............

[marshian]

Well time to round this out (Not quite there but damn close)

 

No mater what I did with the Wiser Hub (controller) - All Off, HW On, HW Off, CH On, CH Off or Both CH and HW On the wire from the wiser unit to the wiring centre to trigger the HW was live all the time.

 

I was completely confused by this as it shouldn't be live unless the timer is calling for HW but I checked the wiring instructions and it was all wired correctly 3 for HW and 4 for CH

 

 

I swapped to a different (spare wire) - same result 240V all the time unless the power to everything was killed

 

The CH wire is only live when the timer and the room stat is calling for heat

 

Every time I check wiring in the wiring centre or remove the Wiser Hub I switch off the power to it all at the Fused Spur that feeds it all

 

I don't know why I did what I did next but I took the Wiser hub off the universal back plate without powering down and I heard an audible click and I stopped - a penny (very large one dropped) - selecting CH or HW had always made an audible click but since the boiler install and change of wiring that had been missing for HW the light comes on but no audible click.

 

I switched off the fused spur - re-attached the Wiser hub and if I selected HW I got a audible click, CH again I got an audible click

 

Going up to the wiring centre with nothing selected on the hub I had no power to either HW or CH signal wires

 

Selecting HW gave me 240 volts and same with CH with nothing selected I got no Volts on either HW or CH

 

My only thought is that somehow the relay or contactor for HW had stuck closed and even powering down the unit was not causing it it open.

 

Anyway everything now works and just need the gas engineer to confirm with Viessmann that a switched live back to the boiler is needed (I'm damn sure it is but I'm not about to fit that)

 

I actually need to think about how the boiler gets that in summer or if heating water outside of the CH period because I can't have both CH and HW linked to the same switched live signal to the boiler or CH will drive HW

 

I might have to either

 

A. think about a relay solution

 

or

 

B. think about a switch (Summer / Winter setting) I can only have one SL wire at the boiler so for Summer the SL comes from the HW side but for winter when I only heat water during the CH timed period it can come from the CH side.

[marshian]

So Viessmann Hot water demand box throws 80 deg C at the HW coil and ramps up to 100%

 

Yes water reheat times are super quick but the boiler is quite noisy when it does HW and the tank only really can cope with 3-8 kW without the return temps being 10 deg less than the flow (Old boiler min was 10kW and that gave similar HW times to the viessmann when I ran it at 80 Deg C to get the water done without cycling)

 

So if the boiler modulates based on return temps or flow (or both)

 

 

 

 Hello gate valve - happy to have you join the party - lets shut you down a little and see what happens

 

80 Deg C flow - boiler initially hit 100% but as I closed the valve it  modulated down to 30% - 20 deg difference between flow and return (76 deg flow 56 deg return) - water heated from 20 deg to 52 deg in 22 mins

 

It was a bit shunty to start with till I found a place where the boiler wasn't overshooting ramping down and then as the flow temp dropped too much throwing much higher kW at the circuit again

 

Basically 1 turn open from closed seems to work fine - that'll do for starters

[marshian]

Well my old school plan had a flaw - it wouldn't have been a problem if the gas engineer had used my 3 port valve as I proposed with no mid position in the old airing cupboard arrangement because I only had a bypass fitted for the CH side - HW didn't need one because the flow rate was always far higher (easiest path and all that)

 

However now I have a bypass that will work if pressure in the system is high with CH or HW - closing the gate valve to increase the amount of time the water stays in the coil and reduce the return temp to the boiler is enough pressure to cause the bypass to want to play (By play I mean get involved)

 

So tonight I've been experimenting with pump parameters to see if I can find a pump setting that

 

Provides a min of 0.4 m3/Hr so that the boiler is happy with the level of flow (below 0.4 m3/hr the boiler gets a bit fussy and throws the odd "low flow" fault) when circuit is at it's smallest (ie just one or two rads in circuit)

 

And provides a max of 750 - 800 l/hr (0.8 m3/hr) when all the rads are calling for heat so the boiler can ramp up and get the circuit up to temp

 

Because the alternative plan is fit a NO zone valve in the bypass line so it closes at the same time as the CH NO zone valve...........

 

Anyway no one wants to see a load of tables of what pump results in terms of watts, head and m3/hr is like for 3 different pump operating modes each with 3 different speed settings for both all rads calling for heat and a very tight circuit where I'm left with just two rads still calling (Bypass was closed for this) so I'll just say I've found a setting that meets the requirements but I haven't done HW on it yet.

[marshian]

In other news Gas Engineer has had it confirmed from Viessmann that boiler (in current configuration) needs a switched live from the programmer to the boiler - at bloody last!!!