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Lets look at 4 pipe vs 2 pipe

The boilers referred to in this article all have the functionality to control the diverter valve from the software in the boiler, ie there are some terminals to attach the DV valve cables to.  Not all boilers have this (Ideal and Vokera being a good example) however these  boilers can still be  easily set up for PDHW with a touch of jiggery pokery . More on those  and how to retro upgrade in a later article.

Some boilers (Valliant are a good example) leave the S plan in place & using the appropriate ‘V’ box will control the two NC valves only opening the one needed at the appropriate time.  Only one valve is ever open at one time. Using the EPH or Resideo Open Therm controls can also achieve this on any Open Therm boiler. More on this later as well.

The 4 pipe has to have the DV inside the boiler and sometimes even uses the same combi diverter, as it does the same job, by diverting the flow to either Heating or DHW. In the case of PDHW the DV switches the flow to the coil in the cylinder rather than the plate heat exchanger.  Note however that both returns join inside the boiler as there is still only one return port on at the heat exchanger.

I don’t see many (if any) advantages to a 4 pipe from a hot water/heating point of view , but with a 4 pipe kit all the components & controls (so I believe) are covered by the warranty.  If the boiler is next to the cylinder it will work fine but on retro fit you need to get the fourth pipe to the cylinder, which can be endless upheaval for no real gain.   Most (all I think) 4 pipes can be used as a standard 2 pipe with a relay box. The DV will not be inside the boiler so the boiler will be a standard type 2 pipe system boiler. The relay box allows a DV to be used remotely when needed.  Both systems will work fine without any loss of functionality. Both offer high flow temperature to the cylinder & much lower flow temperatures to the radiators. As far as I am aware the only 4 pipe boilers are the WB range and the Viessman range.

The two pipe is by far the most common. Most boilers out there (including Combi’s) can control a 2 pipe set up in one format or another.  The two pipe always uses an external DV however this DV can be a normal spring return (Honeywell) type or the MoMo (Motor On Motor Off) valve (6 wires) that consume less energy, as the motor only consumes power when opening or closing.  The spring return (Honeywell) type use a  ‘stall motor’ that consumes about 11Watts of energy during the entire period it’s activated for heating. There is also the X plan – a bit more on that later.

Intergas & ATAG use MoMo valves however the Intergas can easily be ‘tweaked’ to use the more common spring return valve. Boilers like Alpha and Navien come ready for external spring return valves.

The 2 pipe setup is much easier to use. In a standard S/Y plan layout the flow pipe is all ready in the airing cupboard or remote position.  For a Y plan the pipework is all ready perfect for a DV (the brassware is identical) or you can easily rewire a Y plan Mid position Valve to a Diverter Valve by simply joining the grey and white wires together (this forms the new ‘brown or live’ wire) isolating the orange (as it isn’t needed but can become live at times) and of course an earth if required.


So should the DV be B to bath or A to bath?

If B goes to bath then for 6-10 hours a day, the valve needs to be activated during heating to hold it open (through port A) , so consuming 11 Watts for for those 6-10 hours every heating day.

Common sense says it needs to be B to heating.   This means that for the 90% of the boilers time in heating mode  the valve does not need to be activated so not consuming 11Watts. the only time the valve is consuming energy is on a call for DHW as now the valve is only activated for a few minutes each day.

The other advantage for having A to bath is that the G3 safety valve for un-vented cylinder’s will not need to be fitted as in this configuration the valve spring closes on fault.  With B to bath it still works fine but needs a further safety valve to be installed.

The MoMo valves do not have this issue. It only consumes power when opening or closing.

A  ‘D’ or Diverter plan  is an  easy upgrade from a  Y plan  without modifying any pipework and needs less wiring, simply reverse the 3 port valve.


In this example the boiler is using WC to lower the flow temperature for heating -it could just as easily use load compensation (Open Therm). There are pro’s and con’s to each method but both are fine and save energy. Its all about getting the return temperature down on heating to force the boiler into condensing mode.




The S plan is all piped up ready for a X plan so how does an X plan work?

A X plan is a NO and a NC zone valve working simultaneously. Both browns (live) and blues (neutral) are connected to ensure they both work together. The NO valve goes on the heating. This ensures for the 90% of time the boiler is on heating the NO valve does not need to be energised.  When there is a call for hot water both valves instantly react  closing the NO heating valve and opening the NC DHW valve. The boiler at this moment will ramp up the flow temperature to ensure fast recovery of the coil.

Both methods (D plan or X plan) only need 2 wires from valves to boiler. Add to this the two wires from the cylinder stat (hopefully a ntc rather than an old fashioned inaccurate on/off stat but it will work with either method)  you only need FOUR wires from the remote location to the boiler. This setup is MUCH easier to wire than a S or Y plan. Even with a HLS on a unvented cylinder, only the same 4 wires are needed.

This is the 10 way box for a PDHW setup. the two thin conductors are from the cylinder NTC .

The grey & white can be seen joined (t form the live conductor) the blue is the Neutral the orange is just parked up out the way & of course an earth if needed.  Everything about PDHW is better & simpler – Everything.


More on wiring later.



Author Kimbo

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Join the discussion One Comment

  • John Ewart says:

    I am looking to fit an unvented cyl to a combi using heating flow to supply P DHWto cyl then switching over to 2 separate circuits to proide WC control to the heating circuit and separate circuit to bathroom rads for summer use. I would prefer to fit combi as I live in L shaped bungalow with boiler and cyl at either end of L shape and a hot water req at boiler end (trying to avoid a secondary DHW pump.)Not sure if this is poss with either WB or intergas boiler.I am an ex Gas Safe retired eng brought up on origanal diverter valve systems but eager to now comply with new Part l regs for priority hot water system.Your thoughts would be interesting before a contact a local Gas Safe eng and accredited spark to carry out work
    Regards John Ewart

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