Hot water at half temperature. What's wrong?

[chrishutt]

Last week....my hot water went down to about half its normal heat.

Stakhanov, you've just told us that your DHW temp is 56C (though the flow rate might be low), yet your original post (above) said it was down to half normal heat.

As Tony pointed out, 56C is about as good as it gets from a combi (much hotter would scald!) - a temp sensor stops it getting hotter. Can you explain your original post in light of these figures?

 

[stakhanov]

Wooooahhhh! I never expected that many replies. Thanks everyone for your help.

I also thought the 18 degrees for the cold water feed sounded high so i wondered whether the temperature sensor is naff.

There was one thing i forgot to mention, that according to the temperature meter, i was losing 10 degrees from the boiler to the tap. And that's only through 1 metre of copper, 2 metres of plastic, and then 1 more metre of copper pipe. I couldn't believe it myself but it's true. If you hold the outlet pipe of the DHW heat exchanger, it is too hot to hold for more than a second or two, but by the time the water reaches the tap, it has dropped to about 46 degrees.

When i said the DHW was down to half heat, i was talking qualitatively. It felt that way and i didn't have a temp sensor at the time to check. If the water was as hot as the outlet pipe of the DHW heat exchanger then i'd be happy. But it loses a lot of it before it gets to the tap. So is it that i have a problem with losing heat in the floor space (through joists between ground and first floor - if so, why hasn't it been a problem before) or should the bolier be heating the DHW outflow to higher than needed to take account of the loss in heat from boiler to tap, so although it feels hot enough at the boiler, it should be a lot hotter?

Why 6 litres per minute? Because that was pretty easy to measure roughly. It took 10 seconds to fill a 1 litre jug. I think that for the time i was measuring temps, the burner was at max. As far as i can tell there is a low flame and a high flame (and off). It was at high flame during the temp readings and it was running for a while so i had hoped it would be at some sort of equilibrium.

As for the rest of your advice, i will check later how much gas is being used through the meter (good idea). I may also get a better temp sensor.

 

[chrishutt]

i was losing 10 degrees from the boiler to the tap.

In which case I think the problem is that cold water is entering the hot pipe somewhere downstream of the boiler. I would suspect the shower valve or other mixing valve. Run the shower briefly until hot water reaches the valve. Then open another hot tap and see if the hot inlet pipe to the shower cools.

 

[stakhanov]

You are a complete star.

I tried the mixer shower and thought you might be right because although i was only getting luke warm water out of it, the hot pipe into the shower was plenty hot. So i left the shower running and turned on the hot tap in the sink. The hot pipe to the shower didn't go cooler or anything (not that i noticed) but the water from the hot tap did get hot (hotter than it's been for years).

So i reckon that the cold side of the mixer shower has got lodged open slightly (limescale?) and, as you say, has been flowing back into the hot pipe and out of the tap, mixing with the hot water from the boiler. When i turned the shower on, the pressure of the hot water from the boiler stopped the cold water flowing to the sink (at least partially) and so only hot water from the boiler came out.

Question is, can i fix it. I shall be taking it apart tomorrow.

Thanks for your help.

 

[chrishutt]

can i fix it.

Yes.

The shower valve should contain check valves which prevent flow backing up the supply pipes. You could attempt to repair the check valve on the hot inlet, but it might be easier to fit an external check valve on the hot pipe. They're cheap to buy (£3?) and easy to fit (compression ends).

By the way the test you carried out should have been done with the shower off, but either way it demonstrated what the problem was. Anyway I think you now know what I meant at the beginning when I said there were "several possibilities".