Does a Worcester Bosch 38CDI need wiring centre for 2 zones?

6 Dec 2019
Reaction score
United Kingdom
Hello. Slight breakdown in communication between plumber, who installed a new 38CDI combi to power existing CH and HW, and builder who installed wet UFH and manifold ready to connect to the boiler pipework.

"Old" part of house has 14 rads, new UFH loop in kitchen is about 150m long.

Plumber didn't leave correct pipework in place, and no zone valve on flow, so now we need to sort it out.

I have two questions I'd greatly appreciate your help with please:

1) Can we simply retrofit 2x zone valves and move the UFH flow pipe - as shown - so boiler flow connects correctly to the manifold, which has a build in pump? Or do we need a low loss header as well?

2) Does a new 38CDI allow 2 thermostats (Nests) to be wired directly into it and can it switch the zone valves on and off or do we need a wiring centre?

Thank you very much for your help.
Diverter Quote.jpg
Sponsored Links
I'll leave Q1 to someone else, but regarding Q2 With Zoned installations, the thermostats (in your case Nest) are connected to their associated motorised valve and not the boiler at all. It's the motorised valves that then control to boiler using internal microswitches that operate when the valve opens. These are potential free and connected in parallel, so in theory there's no limit to the number that can be connected to the boiler.

A wiring centre is just a convenient place to bring all of the system wires together and facilitate their connection. It's not essential, the 230V switched supply from the Nest Heat link can be wired straight to the motorised valve, using a simple surface mounting box fitted with a flex outlet for the cable from the motorised valve.


..and another cable run from it for the aforesaid switching wires to the boiler.
Last edited:
I have one more question: is it correct that the 3 core 240 power supply to the boiler (which is currently being switched by the one Nest attached to it) will be replaced by a 4 core mains+switch cable? The builder is telling me this but I don't understand why this is yet. I though the boiler would retain its normal 240v 3-core supply, and the switching wires from the motorised valves would be separate from it...
Sponsored Links
Where you have like me in real terms two houses working from the same oil fired boiler (non modulating) then zone valves to select which area is heated granny flat or main house makes sense.

However in the main house, each room has a different use, we give them names, kitchen, living room, dinning room, office, craft room, bedroom 1, and bedroom 2. Does not really matter what name you call the room, but each room has a different heating requirement, by 8 pm the dinning room is finished with, by 7 pm the kitchen is finished with, craft room and office no pre-set times, and of course internal doors not always closed, so through the house two temperatures, we will call it eco temperature and comfort temperature. Eco set around 17°C and any room not in use is allowed to fall to Eco temperature, not too cold so when room is used, it does not take long to heat to comfort temperature of 20.5°C, either using phone, or simply pressing a button swaps from eco to comfort or comfort to eco. In real terms every room is it's own zone. But no zone valve, it is the thermostatic radiator valve (TRV) with programmable head with controls each zone. The programmable heads start at around £10 each so not super expensive.

In my house still use zone valves to select main house or flat, but in my case the boiler is oil fired, and not modulating, last house boiler was gas, and would modulate, so all room temperature control needs to be done with the TRV setting or the boiler will not modulate correctly, so the old idea of a thermostat working a zone valve which in turn works the boiler does not any longer work with gas modulating boilers as it stops the boiler modulating correctly so wastes energy.

There are two ways to control the boiler modulation, one is with return water temperature, the other is connection to boiler ebus, with some thing like Tado or EvoHome using opentherm OK ebus works well, but with many boilers the wall thermostat using ebus is fitted in just one room, all other rooms need to be set to heat faster than the room with thermostat for it to work, and there is a problem using only water as control, as on a warm day, there is nothing to tell boiler to stop, so if left it would cycle on/off all summer.

So we do still fit wall thermostats, but their job has changed, they no longer set the room temperature, they are there to turn off heating when weather has improved.

I have Nest and 9 electronic TRV heads. It suited in my house which is oil heated, however I really do like the idea of Hive, the wall thermostat with Hive is simple off/on, which at first glance seems wrong for modern modulating boiler, however the clever bit is how they link to the Hive TRV heads, so when any head needs heat, it sends a "demand for heat" to the wall thermostat which will then turn on for ½ hour, so while any Hive programmed TRV head needs heat, boiler runs, and its output is controlled by return water, the wall thermostat is simply a relay, using the Hive method yes it could power a zone valve which powers the boiler, but why bother? May just as well use each TRV head to form a zone.

Problem is cost, £60 a head mounts up, with 13 rooms that's a lot of money, however what I have done, although not using Hive, is to use expensive TRV heads in the main rooms, but cheap eQ-3 programmable heads in the rest, the Terrier i30 is another cheap stand alone programmable TRV head.

To my mind the problem is Hive TRV heads only work with Hive or as stand alone, and it is the same with most other makes, so once to select a make of electronic TRV paired head, your stuck with that make, I know very little about wet underfloor heating, I know they use a head very like a TRV head to control it, but I don't know if you can control the UFH with a Hive, EvoHome, Tado or any other make of head. However I know with electric the limit of floor temperature is some thing like 27°C which is too low to heat most rooms to a comfortable 21°C so the floor heating is supplemented with radiators so control is with the radiator not the under floor heating. Clearly with heat recovery units, and leaving the heating on 24/7 then UFH can work well, but we tend to turn off heating when the room is not used, which in turn means the time to heat room from stand-by temperature to working temperature is rather important, the fan assisted radiator is the fastest to heat up room, and smallest, however these also have a problem, as the water flow is not normally controlled, water always flows, but the fan alters speed to maintain room temperature.

As with any system, it can be thrown in, and then you try to make a silk purse from a sows ear, or it can be designed, unfortunately in the main the former method is used, not only central heating, many things in the home are the same, it seems it's been that way for 100's of years, why would anyone fit a coal fire in a room without ducting to bring in combustion air so as not to cause a draft when used? But finding a home where the combustion air was ducted is rare. There are two methods it seems 1) that's the way we have always done it. 2) that's the best method for this house.

First two houses worked, never thought about heating, then went to live with mother, my thoughts were why does this house not work, where the others did? It was nothing to do with central heating boiler or radiators, it was basic house design, other two houses were open plan, mothers house had doors on every room. So if there is a door on a room, then that room needs to be considered as a zone, cost of 13 zone valves and 13 wall thermostats would be silly, so the TRV is king for room control.
The Greenstar CDi boiler has a set of terminals marked as follows:

Lr = 'Live return' This connects to the orange wire from all 2-port valves. This is the switched live to the boiler from the motorised valve internal switches that tell the boiler to 'run'

Ls = 'Live supply' This connects to the grey wire from all 2-port valves. This is the live supply out to the motorised valve switches.

N = Neutral connection

L = Permanent Live connection

The L and Ls are connected inside the boiler so a three core cable can be used with one wire functioning as L and Ls.

Diagram below added to clarify.

Nest Heatlink 3 Model (1).jpg
Last edited:

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local