water slow to heat

butterfingers

Hi,

I've a wall mounted boiler (less that 5 years old) which heats the
radiators within about 20mins of being turned on, but which takes
forever (1hr +) to heat the water for the hot taps, and the water never
gets really hot.

The hot water cylinder has 3 pipes going to it. Two on the side, near
the top and bottom and one coming straight out of the top. The two
side mounted pipes get pretty hot (I presume they are to/from the
boiler ?) while the pipe from the very top never gets hot at all.

As the radiators work fine the problem can't be the boiler, but I don't
understand why the cylinder is not heating properly. Any one got any
ideas ?

Thanks a lot !

Butters.

demanufactured

That sounds about right, the rads heat quickly because they dont hold much water , the cylender hold a hell of a lot o water and takes about 2 hours to heatu up to full temp (in most cases , depending on size).

Does your controls have a option for heating water only or rads only or both?
It will heats the water quicker if the rads are off.

butterfingers

Yes, the pipes from the boiler split into two - one going to rads and
other going to cylinder, so I can shut off the rads if I want. I've tried
that and it didn't make a huge difference.

Maybe I just haven't been patient enough but in another house I was
in the water got very hot well within the hour so I was surprised it
was taking so long.

I was wondering might it be something to do with flow into/out of
the cylinder (there are values on the pipes leading to and from it) ?
I guess I'll just try leaving it for a couple of hours and see what happens.

Butters.

Joey the lips

I hope I explain this correctly so if there is a plumber more exp perhaps they can. I think whats wrong is your gate vale at the cylinder is letting the hot water through to slow. Now this is not the gate valve for the hot and cold water this is the gate valve at the bottom of the cylinder. Just close it and count the amount of times it takes to open fully then set it half way..... Does this make sense.

Pete67

The domestic hot water in the cylinder is heated indirectly by water from the boiler flowing through a coil inside the cylinder. If this coil is too small, or the flowrate through the coil is too low, then it will take a long time to heat the water.

If you locate the supply and return pipe leading to the coil (should be one directly above the other) one of them will have a gate valve in it to balance the flowrate. It is primarily installed to prevent the cylinder coil bypassing the radiators. To set the balancing valve, open it fully, turn on the heating with all rads on, and check to see if all radiators are heating properly. Probably they won't all heat up fully. Slowly close the balancing valve a little at a time until all radiators are heating properly.

This should allow the cylinder to heat up in a reasonable time. You can check if the coil is getting enough flow by checking the temperature of the supply and return with the boiler on and the cylinder cold. The return temp should be about 10 degrees lower then the supply. If the difference in temperature is much more than 10C, the coil is not getting enough circulation.

Joey the lips

Pete67 wrote: »

The domestic hot water in the cylinder is heated indirectly by water from the boiler flowing through a coil inside the cylinder. If this coil is too small, or the flowrate through the coil is too low, then it will take a long time to heat the water.

If you locate the supply and return pipe leading to the coil (should be one directly above the other) one of them will have a gate valve in it to balance the flowrate. It is primarily installed to prevent the cylinder coil bypassing the radiators. To set the balancing valve, open it fully, turn on the heating with all rads on, and check to see if all radiators are heating properly. Probably they won't all heat up fully. Slowly close the balancing valve a little at a time until all radiators are heating properly.

This should allow the cylinder to heat up in a reasonable time. You can check if the coil is getting enough flow by checking the temperature of the supply and return with the boiler on and the cylinder cold. The return temp should be about 10 degrees lower then the supply. If the difference in temperature is much more than 10C, the coil is not getting enough circulation.

Thank you....Much better said than me!

butterfingers

Hi Pete67, Joey,

Thanks for the replies. I can remember seeing a value mounted on the
pipe near the bottom of the cylinder. I'll try adjusting that and see if it
makes any difference.

One thing does confuse me though - you said that if the difference
between the supply and return temps was greater than 10 degrees
then I had a problem, but I would have thought that a big difference
was good because that would mean the cold water in the cylinder
was taking the heat wouldn't it ?

And finally...how do I actually measure the temp of the pipes

Many thanks.

Butters.

slavetothegrind

use a thermometer from the chemists!
Hold it in contact with the copper till it stops rising, there ya go!

Pete67

butterfingers wrote: »

Hi Pete67, Joey,

One thing does confuse me though - you said that if the difference
between the supply and return temps was greater than 10 degrees
then I had a problem, but I would have thought that a big difference
was good because that would mean the cold water in the cylinder
was taking the heat wouldn't it ?

Yes, it would. But you have to consider flowrate and temperature drop together. The heat output of the coil is proportional to the flowrate times the difference in temp. If the cylinder is cold and only a trickle of water is flowing through the coil, then it will give up its heat very quickly, but will not heat much water in doing so. In fact most of the coil will be cold.

It there is a torrent of water flowing through the coil then it will not drop in temperature as much as a trickle would, but it will impart much greater heat to the cold water in the cylinder, as the entire coil will be hot. Hope this makes sense.

butterfingers

Thanks Pete, that makes sense. I'll play around with the valve on the
bottom pipe going into the cylinder and see if that makes any difference.
Thanks for the advice.

Butters.