New Boiler: What to do about Pump Overrun when there aren't enough wires?

[Razor900]

is there any 3 wire system boilers ? would still need 4th wire for perm live to power pump afterdemand

Yes you would have to use a relay to switch the boiler but at least it would take care of the pump.

Must admit I've only ever seen jobs once or twice where you can't pull a new cable to the boiler

 

[trojanhawrs]

Yes you would have to use a relay to switch the boiler but at least it would take care of the pump.

Must admit I've only ever seen jobs once or twice where you can't pull a new cable to the boiler

You could do that regardless of boiler style, if you use the boilers 24v contact though you could switch it through a relay without worrying about live wires still present when isolating the boiler locally (from a plug socket etc)

 

[Johntheo5]

is there any 3 wire system boilers ? would still need 4th wire for perm live to power pump afterdemand

Havn't thought this through but if a system boiler doesn't require pump overrun why cant the switched live go to the boiler permanent live as well as long as 230V required by both?

 

[ericmark]

On the fairly large (53MW) Gas fired steam Boilers that I was very familiar with, a post purge, (fan overrun) always occured, but to clear any unburnt gases from the furnace after a normal shut down.

OK, the system I worked on was slightly larger at 600 MW combined power, but was looking at around 24 hours to wind down, this is the problem using gas fired power stations, the time it takes to fire up and cool down.

I have not seen many domestic boilers over 36 kW at even 1 MW you are well out of the domestic boiler class, for one thing, in spite of the name boiler, if they do boil, some thing is considered to be wrong.

So some old boilers, one simply supplied power and they run and remove power and they stop. In the main old non condensing, oil or gas, once we moved to the condensing boiler, in the main it is supplied 24/7 and it is a completely independent cable which tells it to run or stop, or at what rate to run.

Low voltage and extra low voltage can interfere with each other, so some boilers have twin junction boxes, one side for low voltage and other side for extra low voltage.

But the question is, do you really NEED to have a control system? Most boilers measure the return temperature of the water, and adjust output to suit, it is desirable to stop the boiler cycling to have some external control, but it does not NEED it.

As boilers increase in size, we see pumps and valves being moved outside of the main casing, but most homes need between a 10 kW and 20 kW boiler, so all is within the casing. A shower may use 18 kW for the time taken to have a shower, turning off central heating while supplying a shower does not really matter, so only with a bath is there a problem, I had a Main 7 around 30 years ago, 18 kW from memory, and it took an age to fill a bath, fitted because wanted to make airing cupboard into a bedroom, net result was although we had a bath, hardly ever used.

I have seen 35 kW boilers fitted so the bath can fill in a reasonable time, but most homes only need around 6 to 10 kW to run the central heating, so it means central heating running inefficiently 4 months of the year, so one can run a fast bath, is it really worth it?