Worcester Bosch 24i control logic

[eliotmansfield]

Hi,
I'm trying to understand the control logic of the boiler to figure out my problem.
System is a loft mounted 24i - a complete new system replacing warm-air. 13 Rads all with TRV's. Kept old hot tank with coil, with a 3 way valve in loft. Timer and 3-way are honeywell. System pressure is 2 bar (is that too high?).

Hot water comes on morning, noon and night. CH is on constant, with a programmable thermostat downstairs which i use to control timing and temp for CH.

Problem:
Quite often in the evening, despite the thermostat being on - the rads don't come on (pump not running). Then after perhapps an hour or so, they come on. This has been happening since install, I assumed that somehow it decided the house was up to temp and didn't need to come on. But last night, the house was freezing, stat was on - but still no heating. So last night I simply switched the hot water switch on and off - and hay presto whole system fires up.

Been thinking about this a little more, and thought it might be connected with when the water tank is being heated - as last night we didn't have a bath or do the washing up(!) - therefore it wouldn't of had to spend much time heating the tank and would of gone off early. (The tank mounted thermostat is set to 50'c)

I need to study it a bit more over the next few days to understand the scenarios when it doesn't come on, but was wondering if theres a problem or installation error.

Thanks,
Eliot.

 

[ChrisR]

24i is a combi, but you have a HW tank as well in addition too?

 

[clf-gas]

24i is a combi, but you have a HW tank as well in addition too?

Not so Chris, 24i is also made in system model.

Eliot, have a look here http://www.worcester-bosch.co.uk/index.php?fuseaction=literature.detail&con_id=125930&from_id=5029

You can download the manual to read and understand working sequence

 

[bengasman]

Hi,
I'm trying to understand the control logic of the boiler to figure out my problem.
System is a loft mounted 24i - a complete new system replacing warm-air. 13 Rads all with TRV's. Kept old hot tank with coil, with a 3 way valve in loft. Timer and 3-way are honeywell. System pressure is 2 bar (is that too high?).

Not all rads should have trvs. From your description the system is wired up wrong as well. Does your benchmark sheet say it is installed compliant with part L?

 

[eliotmansfield]

Its a 24i system. Ive dug out the manuals for it now.
Benchmark doesn't specifically say part L, just the box saying does it meet current regs is ticked. (engineer is registered and checked his # at corgi)

This morning it all came on fine, then about 8.30 all went off. Room stat was still on (remember CH timer is on cont.). The TRV in that room was set to max(5) - all other rooms are on around 3.

Turned DHW on and off, CH and DHW comes on again and then goes of shortly afterwards (<1 min)
Turned CH on and off - nothing comes on.
Turn DHW off - CH comes on and stayed on.

Programmer is a honeywell st6400c

WRT, all TRV's - is that only a problem if you dont have a bypass somewhere - I understand that mine has an integral bypass.

thanks,
Eliot.

 

[simond]

I think you'll find the cylinder thermostat and three port valve have been incorrectly wired up. Try turning the cylinder stat down when your heating fails to respond and I suspect the heating will come back on......

 

[D_Hailsham]

Your have what is called a Y-plan system. The three way valve (actually two way: one in, two out) routes the water from the pump to the radiators or HW cylinder or both.

The wiring for a Y plan is shown below; so it might be worth while checking that this is correct.

It might also be worth checking that you have the correct 3-port valve as Honeywell make two types. The model you should have is the Honeywell V4073. The V4044 is a Diverter valve so you cannot set it to feed both CH and HW; it's CH or HW.

 

[eliotmansfield]

Will keep investigating.

Thanks,
Eliot.