Converting from vented HW cylinder to condensing combi boiler...... temporary HW supplies during changeover?

[Stephen H]

Hi all,

I am doing all the wet plumbing and my Gas Safe engineer is coming afterwards to fit boiler, run new gas pipe and commission the system.

There is likely to be a time period between the wet plumbing and the boiler being fitted. So I need to think about temporary HW supplies during this period.

In the airing cupboard, the back wall is shared with the shower cubicle in the en-suite. There was a shower booster pump there to take hot and cold water from the CW tank and HW cylinder and feed the shower mixer valve.

Now clearly, as part of the conversion, all the CW taps will be converted to mains pressure, while the hot taps will be piped back to the new boiler location for when the combi boiler is fitted.

Now a thought occurred to me. I will obviously be plumbing in the shower mixer into the mains pressure water and the HW pipework. Using washing machine hoses, could I connect the mains pipe (intended for the shower) to the cold water tank in the ceiling, connect the cylinder back in via speedfit/John guest couplings, feed the cylinder hot water output into the shower pump, and then connect the output of the shower pump to the hot water pipes, so that all cold taps have mains pressure water, and all the hot taps have pump boosted water?

Regards

Stephen

[Andrew Gabriel]

For a combi, you'll want a fast acting thermostatic shower mixer, and that almost certainly isn't what you've got for a stored water mixer. If you already have a thermostatic shower mixer that you want to keep and it's a wax pellet (slow) type, then you might get away with adding a pressure equaliser to it, but using the right type of show mixer in the first place will give better results, and doesn't require the extra pressure equaliser.

Conversion of everthing except the shower is usually quite straight forward - cap off the existing feed and have the new one ready to connect up, and you can do that as the boiler is being connected up. For the shower, I would prepare new feeds terminating with isolating valves. Then when everything else is working, you can at your leasure reconnect the shower to the new isolating valves. If you haven't already, empty the header tank via the shower before disconnecting the old shower supply. Never leave the pump in the water circuit, as they have a habit of exploding within a few days when exposed to mains pressure.

[John Rumm]

Not sure about the washing machine hose bit (but a lashup with speedfit etc would be as easy), but in theory yes you could use pumped hot and mains cold... a few things to consider:

If the shower pump is a twin impeller type, then you will need to split your feed such that it goes to both sides of the pump, and then combines both outputs, since its not good to run one side of it dry.

Even assuming you get a plausible match in feed pressures to the shower, you may still find its results a tad unpredictable if it was not designed for high pressure operation. (having said that, if it worked from the pump, and you cold main is not massively higher in pressure than the pump output, you could be ok).

I had a Mira 88 low pressure shower originally tank fed, and moved over to a combi in my last place. It still worked ok, but the temperature adjustment became very sensitive and used only a fraction of the travel of the knobs range of adjustment. It also whistled a bit on anything other than full flail the skin off blast! Fitting a water economising shower head that allowed pressure reduction discs to be inserted in it helped quite a bit. That gave more restriction to the output and made the shower valve much easier to adjust.