Domestic hot water supply problem

[Graeme]

This is a message I posted in November 2013 :

Just had a new hot water tank fitted (not brave enough to DIY) and it is fine. Hot water is hot etc., but hot water now takes much longer to reach the taps than it did with the old tank. The pipe work all looks the same, so is there any logical reason? Tank and boiler stats have not been moved. Not a problem as such - there is plenty of hot water, and at the right temperature, it just takes a long time to arrive. The old tank would have been at least thirty years old. Have tank designs changed? System is entirely conventional. Oil fired boiler feeding hot water and central heating. Not a combi boiler.

Since then, nothing has changed in spite of plumber coming back saying nothing wrong, me forcing cold water back through the hot water etc.

Yesterday, a couple of guys came and changed four radiators, fitted TRVs and removed an old bath, basin and toilet, capping the pipes. Much turning off of hot and cold water, partial draining of heating etc. Today, the hot water is back to how it was before the new tank was fitted in 2013. What have they done? Whatever the problem, they have fixed it. All I can think is a valve somewhere was only partially open (although flow was fine), and yesterday it was fully opened.

Delighted, but flummoxed.

[Tricky Dicky]

Used to get a similar problem at our old house after draining down the DHW due to partial air locks when refilling The simplest solution was to connec t the washing machine hose cold feed to hot feed and give it a quick blast at mains pressure until water came out of the main tank overflow, problem s olved.

Richard

[Muddymike]

When we had the cottage in Langthwaite we had that problem every time the system was re filled after the winter drain down.

Simple cure was to block the outlet of the kitchen mixer tap with a thumb whilst both taps were turned on thus forcing cold back up the hot pipe. Wait for gurgly noise, turn off.

Mike

[newshound]

My guess too, classic airlock problem with vented gravity systems.

[Graeme]

In message , newshound writes

That is my only conclusion too, although I tried the suggestions by Richard and Mike, holding my thumb over a mixer tap, and running a short length of pipe from one tap to another. The second time I must have left the cold forcing itself through the hot system for several minutes, yet these guys did something yesterday which cleared it immediately.

[Brian Gaff]

He did that already, but if an airlock and a partly closed valve was present the pressure would not shift the aire in a pipe with a sag in it. It happened to me some years ago. Brian

[therustyone]

Most of the lag is just the time it takes to heat all the copper fittings en route. The cooled water comes through fast. Have they taken some fittings off?

[Graeme]

No, the only change to the hot water was removal of a bath and basin, the supplies to each being capped off. No changes to run length or anything else, except turning the hot water supply off at the tank for perhaps half an hour.

Tried again this morning, and first use of a hot tap results in hot water arriving much quicker.

[RobertL]

Is there a small hot-water leak somewhere? i.e. is hot water being leaking at some place between the boiler and the tap? This would dleave a shorter pipe run for the hot water to travel before reaching the tap

Robert

[Graeme]

That made me run upstairs to look! No, no leaks. All pipes neatly capped and not leaking. There were no other changes to any pipes, other than in the bathroom.

[GB]

I can think of a possible but highly unlikely explanation: There are two possible routes through the pipework for the hot water to reach your tap. The shorter route got air locked when they drained down the system to cap off pipes in the bathroom. That left the water to find its way round the longer route. The air lock sorted itself out overnight, and now the water is taking the shorter route.

[Harry Bloomfield]

GB presented the following explanation :

That is the only explanation I would agree with.

[Graeme]

In message , GB writes

Brilliant. That sounds entirely plausible. Really. Tracing cables and pipes in this house is a nightmare. It is quite a large house, built

1880, and, many years ago converted for guest house/B&B use with en suite bathrooms. Then a small room on one side was extended to form a Post Office and, at some point, central heating was added, but only downstairs. We extended the CH 15 years ago. Oh, and a rewire probably 20 years ago. Add to that multiple telephone systems over the years with two lines, extender bells and all sorts, and trying to find what goes where is almost impossible. I could mention an alarm system and dedicated wiring for the PO, and an extra bathroom built on one side of the house. There are blanked off electrical fittings everywhere.

Getting back to the water, including what was the PO, there are nine toilets, two baths (one of which was removed earlier this week), six showers, ten hand basins, four sinks plus the usual supplies for washing machines and dish washers, in two separate rooms. The chances of someone, at some time, creating a loop in the hot water pipe run sounds quite likely or, at least, not unlikely.