Unvented system and hot water

[Alan]

I've recently moved into a newer house, which has an unvented (not combi) heating system. I've read up about the pros and cons, but one real issue I've got with it is that although I get mains water pressure through the hot water taps, it can take an age (up to 40 seconds at a reasonable flow) for the hot water to get from the tank to the tap.

It doesn't seem to matter which tap I use, the house isn't that big that any of the taps are more than 3-4 metres from the hot water cylinder.

Is this normal behaviour or is something wrong with the system? I had it serviced recently by British Gas as part of my service cover, and the engineer did say the pressure in the system was low and he had adjusted it but it didn't make any difference to my problem.

It's an Ideal Classic boiler with a Megaflow tank.

[marcb]

We have the same issue and we think it will get worse if Thames Water cuts pressure as mooted. On the 2nd floor we're about to take out a pressure driven shower and put in an electric one driven from the cold supply. But like you i don't know why the pressure/flow seems different between cold and hot.

Marc

[Christian McArdle]

It could be convoluted pipework. Sometimes, when these devices are retrofitted, the hot pipework is reused, which in a worst case might have a long leg to the old location of the cold water tank, then another long leg to where the hot water cylinder used to be, followed by a long leg to the tap. This might be done for laziness, or to protect expensive decorated walls that might need to be channeled for new, more direct pipework.

Christian.

[Jim Alexander]

Fill a vessel up from a fully turned on tap over 40 seconds and report back what volume of water you have in the vessel.

Jim A

[Alan]

Flow doesn't seem to be an issue - I get equal flow and probably a slightly higher pressure from the hot tap than from the cold, but it still takes a while for the water to get hot at the tap if it hasn't been run for a while. I reckon I could fill the kitchen sink with cold water from the hot tap before it starts to get reasonably hot.

I did a few tests last night - turned on the furthest away hot tap, took 40-ish seconds to get hot. Went to the nearest other taps, took around 5-8 seconds to get hot.

Upstairs (where the tank is) took around 5 seconds.

Someone at work suggested that an unvented system draws the hot from the bottom of the tank, while the mains cold enters at the top and so thermal flows would end up with a colder volume having to be drawn off to get to the hot water above if the water hadn't been drawn off for a number of hours. Not sure about this but it would make sense.

It's not a retrofitted system, the house is only a few years old, and I don't have a cold water tank anywhere (thankfully!)

[Christian McArdle]

No. Your problem is almost certainly just a very long pipe to the tap.

Christian.

[John]

Your definition of a reasonable flow may not be the same as someone elses Whatever the flow if a pipeful of cold water is in front of the hot water it WILL take a definite time to be pushed out of the tap before hot arrives. This may be affected by the size of the pipe thius a large bore pipe has a greater volume to move than a small bore one. Also heavy gauge metal pipes with a large thermal mass will absorb some of the heat as it passes and slow the response.

Does your 3-4 metres mean as the crow flies or does it include the hidden route which the pipes may take?

The pressure is virtually irrelevant except to say it will provide more "push" to move the water through the pipe. The "rate of flow" is the relevant factor.

If the Megaflow is hot then the boiler has no relevance whatsoever on the situation. "Some" unvented systems utilise a thermostatic mixing valve which just might be affecting the delivery of hot water but I'd be amazed if this accounted for more than a second or two.

[Jim Alexander]

If that's the case then that is the volume of water contained in the pipes, but you still need to be objective in measuring the volume. Perhaps you have poorer flow than you realise. Again you have to measure the mains flow rate before its possible to diagnose much more but from all the evidence so far poor mains flow seems the most likely culprit.

Jim A