grant combi wiring for honeywell cmt927 and st9400c

[mont1]

I have a grant vtx 26 oil combi boiler and i am looking at using honeywell cmt 927 wirless thermostaat also wiring up st9400c.I dont wish to have the boiler on permanant demand for hot water .How can i wire up to use both.

 

[picasso]

the hot water function is automatic and cannot be over ridden with external controls, you only need the cmt 927 to control the the central heating, why dont you want the hot water on demand , it seems a bit strange when you have a combi installed

 

[mont1]

i know it seems strange but would like to have it on timed rather then the hot what just keep heating itself up during the night in the expansion tank etc.There is a hot water on demand or timed switch on the boiler for that option.

 

[bengasman]

As it is a combi, all you need is the honeywell, nothing you can do with the 9400 that can add anything useful

 

[mont1]

i hear what you are saying as there is several people in the village already with grant combi boilers and they have had all there boilers set up this way by different engineers the only difference is that they are the outside version of mine and i will install the wireless unit instaed of hard wired roomstaat.Why would Grant boilers come with the option of timed function on hot water if it was a waste of time.

 

[picasso]

deleted

 

[bengasman]

Possibly related to the same or similar pcb used on various models. Maybe so they can claim a faster response time with the pre-heat on, whilst at the same time being able to claim lower fuel use with the option off.
Maybe it is because "we have always done it this way". People have always had their dhw timing, and feel that a heating system is not complete without it.

 

[Terrywookfit]

Check out Page 51 of the manual, (fitting a mini programmer).

Download availlable from Grant website.

I always fit a programmer to time heating and hot water.
Just remember to set water 20 mins earlier to allow store to heat.

 

[bengasman]

But the OP specifically stated he does NOT want to use the pre-heat option. Installing the standard timer would be a complete waste of time and money as it will add nothing to the cmt927

 

[Terrywookfit]

i know it seems strange but would like to have it on timed rather then the hot what just keep heating itself up during the night in the expansion tank etc.There is a hot water on demand or timed switch on the boiler for that option.

I took it that he wanted the water timed rather than constant.

There has to be a pre heat time though.

 

[bengasman]

That is odd. Is it not possible to completely disable the preheat on this model?

 

[mont1]

thanks for all your input i here what you are all saying but i don't need constant hot water the 927 only controls the room stat and rather then pulling of the front cover of the grant to switch on the dhw when and as i require it i feel it a total waste of fuel for it to keep switching on to maintain heat in its tank.All i really need to know is how to wire both the 927 and st9400c i am waiting for replys from honeywell and grant verbally over the phone honeywell did'nt see a problem other then being a little ott.

 

[Terrywookfit]

No!
Until the store is heated,The plate heat exchanger has no source of heat.

We are talking about an Oil combi,Not a low water content boiler.

 

[Boilerman2]

Why don't you Phone GRANT TECHNICAL (0870) 7775553

They are very helpfull and will advise you on what can be done,
Generally Internal Boilers maintain HW 24/7, whereas Externals have a Timed function (I think is is to address any potential "noise in the dead of night" issues that may arrise with neighbours?

 

[Terrywookfit]

There is no difference in noise levels between internal and external models, as most of it comes fron the flue.

Page 51 of the instruction manual shows the H/W on wired to T3, replacing P/L.