Multizone boiler wiring
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Multizone boiler wiring

by swidders » Wed Jan 14, 2009 11:50 pm

Am shortly to replace my heating system with a combi (worcester greenstar junior 28).

This will run 3 heating zones - 2 underfloor zones (one in each reception room), each of which will come with it's own timer/stat to run its respective pump on the manifold, the final zone will be for the rest of the house (kitchen, hall, bathroom, bedrooms).

Obviously I will get a professional to do this work, but I am curious as to how the wiring will work - will the third zone operate the boiler via a 2-port valve, and can this all be run from a wireless stat/timer.

In particular, when ordering the boiler, what control box do i need to order to enable my sparky to, er, facilitate this three separate and autonomous zone arrangement?


by ericmark » Thu Jan 15, 2009 12:18 pm

There are many ways it can be done but I was asked before to supply a diagram and you can view it here ... ilerC2.bmp

With central heating boilers there are three options which are independent of each other.
Combi - Standard
Open vented - Closed vented
Condensing - non condensing
Often people think a combi boiler is also condensing and closed vented which is not always the case. Today it would be unusual not to use a condensing.

Where there is a lot of expansion of the water, possible like with underfloor heating open vented is still often used. Also where solar panels are used often open vented.

The closed system will limit the amount of water that can be lost in the case of a burst and increases the temperature you can heat water to. But if you use open radiators (normal type) you can't use the increased temperature only with Myson type radiators where you can't touch the hot surface can the higher temperature be used so in most homes only advantage is leak limitation.

One other huge difference with modern boilers is unlike the old type that just switched on and off the power is variable between two limits. This variable output is controlled by the temperature of the return water.

When they just turned on and off if the check valves were not set it made little difference but with new system if a radiator is left wide open then because it will allow hot water to return too quickly it will reduce the output of the boiler.

This is a further problem with zone valves as if a new zone is switched on the return water from existing zones will prevent the new zone from being heated as quickly as it should.

The only way to redress this problem is to use a management unit of some type which will switch off zones which are already hot to allow new zone to catch up.

The simple diagram I have shown would not do this and some form of programmable logic control would be required.

This is really beyond the remit of most electricians including myself without some very heavy reading. And to do this sort of work you will need a specialist in heating and ventilating they seem to call themselves "Engineers" although many have not got degrees to their name.

The whole idea of a combi boiler was so that the people fitting the unit did not have to be so skilled with all the complicated bits being pre-designed and set within the boiler. Big problem is when you want to add extras the main being solar panels. As a result in the last few years highly skilled heating engineers have tended to move away from these and fit either an open vented or closed vented cistern with a supper duper hot coil that can give instant hot water at mains pressure. Well they can give it at over mains pressure the power shower. Some of these special tanks can have three coils one to heat water, one to supply mains hot water and one to use heat from solar panels.

The whole world of central heating has been turned on it's head and except for small flats the combi boiler is now old hat, although of course there are a load of stick in the muds who have never up-dated their skills and this has all happened in last few years.

Unlike the electrical trade where we have a set of requirements for electrical installations which mean we have to up-date our skills many in the plumbing trade have not upgraded their skills in years. So you will need to be careful in who you select to do the work but since a plumber is by definition a worker in lead even the name now is old hat and only organ builders still work in lead so look at heating and ventilating engineers to find someone who is up to date. In most cases they will also do whole job pipe work and electrics so any problems you are dealing with one firm. Non of the pipework guy blaming the electrician and electrician blaming pipe work guy when it does not work as expected.

I will also point out you have posted this in the wrong section really.


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