Why am I getting hot water out of the cold tap?

[mckenzie]

Downstairs it's all fine but upstairs, when I turn on the tap at the sink in the ensuite, it comes out tepid and if I leave it running to get really cold, it actually gets hot!
Ditto the sink in the main bathroom.
The bath has a bath and shower set up and if I put the water to come our of the bath tap and turn the dial to the coldest setting it comes out luke warm.

The system was put in 14 years ago when we bought the house have had new en suite and new main bathroom in the last 5 years.
We have a mega flow system with boiler downstairs in the utility room and tank on the landing in the airing cupboard.

The cold water comes in from the mains apparently.

How can we be getting hot water from the cold tap?
I had a plumber visit today and he left scratching his head.

I'm menopausal and really need freezing cold water in the night to put on my wrists to cool me down. Any ideas what can be wrong very gratefully received.

TIA

We quite often get hot water from our cold top. It's because the hot and cold pipes are next too each other (not lagged) and heat transfer warms the cold pipes.

if you can get at the pipes, you can slide lagging over them (it comes in tubes with a slit seam that springs shut)

Pigletjohn, is that post for Notsoaccidentproneanymore?
Or both of us?

Both

Also, a Megaflow is a brand of unvented cylinder so it has to be inspected and repaired by a plumber who is qualified to work on it. Not all are. You can usually find a heating engineer who is qualified on gas boilers and also on unvented cylinders, and have both annual services done in the same visit. It is possible for a fault to occur, though not very likely.

"G3 ticket"
www.niceiccertification.com/our-services/water/unvented-hot-water

Thank you so much pigletjohn. You are a font of knowledge.