Not getting hot water from kitchen Tap advice needed?

I wonder if any one can help me.

I have a solar hot water system and gas boiler. The gas boiler is in a cupboard in kitchen, next to boiler is a water tank for solar system. I believe water goes up to the loft where it tavels through the solar panel then it comes back down through house to tank and if not quite hot enough gas boiler kicks in and finishes heating water.

My problem is most of time I have really hot water upstairs but when I turn on kitchen tap down stairs I just get warm not hot water. I can leave tap running for 10 or more minutes and still it's just warm and doesn't get hot.

Why would I have hot water upstairs and not downstairs. Also some times the hot water upstairs comes and goes one minute it will be hot next it will go cold. If I turn off taps and wait a couple of seconds and turn back on it goes really hot again. This makes it very hard to have a shower.

We have had problems with the boiler keeping pressure, we have had a engineer out a few times to fix and they have changed parts, this fixes the problem for abit then it comes back.

I don't think my current problems is to do with boiler loosing pressure, as when boiler has lost pressure in past boiler won't kick in and red lights flash on boiler.

I have checked boiler and pressure is absolutely fine and there are no red flashing lights, boiler kicks in fine when you turn any tap on.

Does any one have any suggestions as to what the problem could be.

Any advice much appreciated

Thanks in advance

Clearly only a guess, but boilers often have a minimum output, I had no hot water on one of mothers taps, and it was a kink in the flex pipe, so not quite enough water flowing.

As to solar, father-in-law had solar hot water fitted, and he said how electric bills had dropped, but that was due to gas doing the water heating, when he had a smart meter fitted the hot water stopped. It was a simple cure, relight the pilot flame, but was in summer, and it was clear the solar hot water not working, found one fault, the power to solar came from a switched FCU clearly marked immersion heater which since now no immersion heater fitted he had turned off, but even with power no solar hot water. So we got in some experts to bleed the system, when off it could have over heated, but still no hot water, best it did was remove the chill. It was totally useless, and he had been conned, when he died house sold and new owners ripped it all out, but it did result in a better energy rating for the house.

Now daughter also has solar hot water, and it works A1, there is no other way to heat the water, but one big difference, father-in-law was in North Wales, daughter in Turkey.

You most likely have a passing shower or mixer tap, these rely on cheap plastic non return valves to keep the hot and cold separate.

Which boiler do you have ?

Sorry it has taken me a while to respond not been well. The kitchen tap is a new mixer tap that we have recently installed as old tap broke but is quite a expensive one with touch sensor.
We have a down stairs shower room and have noticed that we are struggling to get hot water from sink tap in there too now.

Just to test I've turned the cold water off to tap in kitchen so only hot water is on and still when turn on we are not getting hot water.

Could it be a faulty tap? Would this effect the downstairs bathroom tap too.

Occasionally we get hot water for a few seconds then it goes straight back to cold. Upstair the hot water is really hot.

Thinking of buying another tap to try just so if nothing else I can rule it out if not the problem.

Any advice much appreciated
Thanks in advance