Combi Bolier with Hot Water Cylinder - Any help appreciated

Postby learningallthetime » Sat Jan 07, 2012 11:43 am

We have an interesting setup in our house that we recently purchased and some help/advice would be much appreciated.

We have a Worcester Bosch 24i Combi boiler that is currently used for the central heating system. The boiler has the programmer fitted to allow heating to be preset for time/days. We have a wireless thermostat for controlling the room temperature.

On a separate system we have a solar panel connected to a vented twin coil hot water cylinder. The solar panel is connected to the bottom coil which leaves one coil spare. We have the longest immersion heater possible but it does not heat enough water for a bath or power shower.

I would like to connect the combi boiler to the second indirect coil on the cylinder to provide more hot water. One idea is to zone the central heating system into 2 zones, 1 for the radiators and another zone for the hot water cylinder.

I think I would need to fit 2 zone valves (something like a Honeywell 22mm 2 port valve) and a hot water cylinder thermostat with timer (Maybe the wireless hot water cylinder thermostats I have found via Google)

My plan was to fit the zone valves near the boiler (and before the first radiator). I would need pipe work (22mm I guess) from the hot water cylinder to the area where the zone valves were installed. The only thing is I'm not quite sure if this plan works?

If the cylinder thermostat and timer is controlling one of the zone valves, the other zone valve is then controlled by the wireless thermostat. As both of these are then linked to the boiler programmer, I get a bit stuck.

Is it possible to program the boiler so that the hot water heating zone can be timed to come on independent from the central heating zone. If so, do I need a new dual zone controller or something?

Ideally, I would like to heat the hot water early in the morning and then have the central heating come on a little later. in the summer months, I would also like to switch off the hot water cylinder zone completely as the solar will do the job.

Any ideas or suggestions or wiring/pipe diagrams would be a really big help.
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Postby plumbbob » Sat Jan 07, 2012 3:29 pm

Really you could configure the boiler as if it were a system boiler in a traditional heating system.

For information do a bit of Googling and search for Sundial "Y" Plan system. The Honeywell site will give details of both the wiring and plumbing configurations.
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Postby sparx » Sun Jan 08, 2012 9:29 pm

You can do as you say with zone valves each one calling the boiler as required, you can get multi channel programmers for this so each can be on or off as you want.
N.B. I would strongly advise against using 3 port 'Yplan' type valves for several reasons which I will state if requested.
Each prog output goes via a thermostat to its respective valve, all the valve switch lines (Orange) go to boiler 'call'.
As said elsewhere the diagrams are available on line, but would recommend getting a good electrician involved to connect it all together to ensure things like pump overun are included.
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Postby learningallthetime » Thu Jan 12, 2012 8:41 pm

Thanks for the information sparx. A bit more Googling and I was thinking a S Plan for 2, 2 port valves might work. Would this be better than a Y plan option? If so, could you explain why just so I understand. Also would they need to be the "normally closed" option rather than "normally open"? ... ux-switch/

The boiler already has a Worcester Bosch DT20 Digital Twin Channel Programmer installed. ... ideo_id=15

I have gone through the settings on the boiler and I can set timers for both the hot water and central heating on separate programs. Presumably this would be able to control the required operation. Is this correct?

I presume the pipework to carry the hot water around the indirect coil would be ok in 22mm? If so, I think it is a case of the following:

1) 2, 2 port valves coming from the flow pipe before the first radiator. The pipe work would tee off before the first radiator and then 1 valve on the pipe onto the cylinder coil and 1 on the pipe work the goes onwards to the first radiator.

2) A cylinder thermostat with electric cable running back to the boiler. I will get an electrician to wire up as suggested. I could run the cable though. What cable would I require?

3) Cables running from the 2 valves back to the boiler. Would I require a separate wiring box for everything to connect into?

Can anyone think of anything I'm missing?

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Postby sparx » Thu Jan 12, 2012 9:40 pm

Hi again,
slightly confused now as the prog. you show is not used for a combi boiler as with a combi the hot water is on-demand not timed at all??
That prog I am pretty sure is only usable with a condensing system boiler with inbuilt divertor valve.
Your idea to split off using 2-2port spring closed valves (H) is ok but it means you can only heat the water when the CH time is on, presuming the lack of hot water is mainly in winter any way that shouldn't be a problem!
Wiring would need some sorting out but would be quite easy, if valve wiring needs extending to prog. then from local to v/v's J box would need 2X 3core and earths or a 6 core control cable.
Please confirm boiler set up and will happily carry on with this exchange,
regards Sparx
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Postby learningallthetime » Thu Jan 12, 2012 10:17 pm

Hi sparx,

Thanks for the feedback. Since my last reply I was researching the DT20 a little more and came across the same thing that the programmer can only be used with a diverter valve (although I don't understand what it does).

The programmer physically installed matches the picture in the manual and on the internet (previous owner had boiler installed and then later on the solar system). It states DT20 - Twin Channel Programmer for Greenstar i models.

The boiler manual and the label on the boiler is stating Greenstar 24i Junior (under the header on the manual it says " Wall Hung Gas Fired Condesing Combination Boiler).

I'm confused now, do you have any ideas?

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Postby learningallthetime » Thu Jan 12, 2012 10:39 pm

Just tried testing something...

There is a tap fitted to the boiler where you can draw hot water. Its pretty pointless as its just a tap in the boiler cupboard. However, I turned it on (with the hot water turned off) and initially the water was warm i.e. first litre or so (presumably just a result of the central heating been on) .However it soon went cold. I turned the hot water on, via the DT20, ran it for a few seconds and then the water was hot.

Does this suggest it is definitely a combi as the water is on demand?

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Postby LUFC » Tue Dec 18, 2012 10:25 am

Hi All,

I have just fitted a WB 34CDi Combi & have used a WB DT20RF Prog & Stat. With ref to the issue around timing the HW on the combi using this prog........... the combi provides HW on demand. The prog can be used to set pre heat HW on time as you would normally. As this suggests, it pre heats a small amount of HW & stores it so that supply times / temps are better than just drawing on demand. This is a benefit but the downside is it uses more gas! If the HW times are unset on channel 2 then the boiler facia should show the ECO light on. If preheat is on & timed, this Eco light is unlit = off.

Basically, this 2 channel programmer is fine but you have the option of not using channel 2.

Hope this helps.
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