In an earlier contribution to this discussion,
wrote:

Hi everyone,

We've just had our house rewired and part of the work was to fit a new
programmer and digital thermostat to our central heating system. It's
just one of the many major jobs on our list since buying our place
last year.

Before the rewiring started, we had a crusty old digital programmer
and an old-style rotary room thermostat. The electricians finished the
rewiring yesterday but failed to connect up the central heating
electrical components properly, so last night we had no heat from our
radiators. When I went into the garage to investigate, the boiler was
running fine but the pump for the central heating wasn't doing
anything.

The electricians came back this morning to sort it out... and their
solution was to remove the room thermostat altogether! As I understand
it, the boiler and pump will now run continuously when the programmer
is set to be on. This just doesn't sound right to me, and certainly
sounds expensive! We don't have thermostatic radiator valves at
present (although plan to have them fitted on all but the bathroom
radiator at some point in the near future).

So here's the first question: does this setup sound wrong to anyone
else, or is it just me? Reading around Google Groups, it seems that
this might actually contravene 2002 building regs.

The electrician's parting shot was to say we should get a plumber in
if we want the room thermostat installed again. Personally, I'd have
thought wiring the boiler and pump to the appropriate gubbins would be
an electrician's job. Assuming the electrician's a dead loss, who
should I call? A plumber? The folks who service our oil boiler?
Another electrician?

Final question (if it can be answered briefly!): how exactly should
the programmer and room thermostat be wired to the boiler and pump?
Before the rewiring started, I could swear that the the room
thermostat connected to the pump (and not the boiler) and the
programmer to the boiler. I guess that means that the boiler was
still trying to heat water when it wasn't needed before the rewiring
work started? It just wasn't get pumped around....

It wouldn't surprise me if it was incorrect before the rewiring, as
much of the work on the house has been done by cowboys prior to us
buying it. We've already found a lot of other things that were done
without any attention to building regulations.

If anyone could offer their opinions, it would be an enormous help!

Many thanks,

Chris Wood

It sounds to me as you have a gravity (convection) HW and pumped CH system.
So, for HW, just the boiler needs to run. For CH, the boiler *and* pump need
to run. The usual,way of achieving this electrically is a follows:

The HW output from the programmer is connected to the boiler. The CH output
from the programmer is connected to the pump via a room thermostat. An
internal link in the programmer forces HW on whenever CH is on - even if the
manual HW switch is off - to ensure that the boiler runs for the CH.

You definitely need a room stat in circuit - otherwise the boiler and pump
will run continuously, and you will fry.

Even when working 'correctly', this system won't meet current building regs
because it doesn't have a boiler interlock. It doesn't actually *need* to
meet them - because they don't apply retrospectively to old systems. But
it's still worthwhile doing a few things to improve efficiency.

The term 'boiler interlock' refers to a method of shutting the system down
completely when both HW and CH demands are satisfied - rather than allowing
the boiler to cycle on its own thermostat when it's not not needed - thereby
wasting energy. In order to achieve this you need thermostats on *both*
circuits plus the logic to shut the boiler down. The easiest way to achieve
this on a gravity system is to install a motorised valve in the gravity
circuit plus a cylinder thermostat on the hot water cylinder, and connect it
all up as per C-Plan shown in
http://content.honeywell.com/uk/homes/systems.htm

The alternative - which is even better - is to convert to a fully pumped
system - either S-Plan or Y-Plan (also shown in the Honeywell link, above) -
but that probably requires extensive plumbing mods whereas the plumbing for
C-Plan is minimal.

Which ever way you go, fitting TRVs on all the radiators except the one in
the room where the room stat is located will improve efficiency even more.
--
Cheers,
Set Square
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