Hot water cylinder heating issue

[Mark Batcheldor]

Really hoping you good people can help. My set up is a standard vented HW and CH system. The other evening my HW failed (made known by Daughter moaning how cold the shower was). The cylinder thermostat is a really old Honeywell type so I used a screwdriver to move the dial down to see if it would click in and altho I could hear the switch making and breaking there was still no HW. So used my british gas homecare cover and they identified it as the HW valve (I have separate HW and CH valves). In the meantime I decided to change the temperature switch with a new Honeywell one I had. BG came and changed the old Danfoss HW valve for a new one and system test run OK.

Now here's the drama. Before the valve change I always left my HW on constant. Assuming no one used any HW it would go around 16 hrs before the stat called for more HW and would run for around 3 hours to heat up. What I found with the new stat on the cylinder is it kept clicking in and out every 2 to 3 hours calling for HW, then running for around half hour at a time to top up the temperature. This obviously was a different behaviour pattern to what I was getting with the origonal old stat and it seemed this new one was too sensitive. I changed the stat back to the old one as that was proven to work and it now (again with no one using the HW) there is a gap of only 9 hours between when it calls for heat then it runs for around 4 hours to heat up. So a much shorter time window between cycles and running longer.

This is all so annoying as had I just left the setting of the original stat where it was I probably would have no issue. I can't remember exactly the setting it was on previously but we always had plenty of HW and it used to run fine. The stat is fixed to the side of the cylinder ok too in the correct position.

Is it possible that I need to drop back the stat temperature setting (currently indicating on the very old dial around 55) to compensate for scale build up as we are in a very hard water area and this tank will be at least 25 years old? Working on the theory it's taking ages for the heat to conduct through to the walls of the cylinder. That's my theory anyway?

Thankfully I have smart meter data from before all this haopened showing consistently what level of gas usage was required to heat the tank and at the moment it is higher than previous. I am thinking if I just tweak down the stat in very small increments each day surely I should get back to where it was before?

Does this all sound feasible? My golden rule though is now not to touch stuff if it ain't bloody broke (like the stat as if I didn't move the front dial around on it I wouldn't be in this mess now but at the time I had to do this to fault find the issue).

Thank you in advance for reading all this.

Have attached some smart meter data to demonstrate what I mean.

Mark

 

[Notch7]

Running for 4 hours to heat up a hwc seems crazy, that must be quite expensive


If it used to run for 3hrs every 16 hrs but it now runs for 4hrs every 9 hrs, where is the additional energy going?

 

[Mark Batcheldor]

This is my concern I think that it's full of scale so it will take ages for the heat to transmit through the cylinder wall to the stat. Where i have messed around with the stat setting its maybe thrown it out of calibration. In an ideal world i probably need a new cylinder but this is just what I have always been used to even tho not right. Thank you for replying as it stresses me out all this stuff.

 

[Notch7]

I think that it's full of scale so it will take ages for the heat to transmit through the cylinder wall to the stat

If that was the case, your water would be seriously hot.

It’s possible the scale build up on the coil is seriously slowing the rate of heating the water in the cylinder.

I’ve had a new cylinder fitted and the water heats up very quickly, say 10 to 20 minutes at most.


When I go on holiday in Suffolk the holiday cottages always have water softeners, are they common around your area if you have very hard water?

I don’t know how much the hot water heating is costing you, if it’s a lot maybe you might want to consider a new cylinder.

Hopefully some plumbers / heating engineers will be along to give you professional advice rather than my ramblings.

 

[Johntheo5]

Really hoping you good people can help. My set up is a standard vented HW and CH system. The other evening my HW failed (made known by Daughter moaning how cold the shower was). The cylinder thermostat is a really old Honeywell type so I used a screwdriver to move the dial down to see if it would click in and altho I could hear the switch making and breaking there was still no HW. So used my british gas homecare cover and they identified it as the HW valve (I have separate HW and CH valves). In the meantime I decided to change the temperature switch with a new Honeywell one I had. BG came and changed the old Danfoss HW valve for a new one and system test run OK.

Now here's the drama. Before the valve change I always left my HW on constant. Assuming no one used any HW it would go around 16 hrs before the stat called for more HW and would run for around 3 hours to heat up. What I found with the new stat on the cylinder is it kept clicking in and out every 2 to 3 hours calling for HW, then running for around half hour at a time to top up the temperature. This obviously was a different behaviour pattern to what I was getting with the origonal old stat and it seemed this new one was too sensitive. I changed the stat back to the old one as that was proven to work and it now (again with no one using the HW) there is a gap of only 9 hours between when it calls for heat then it runs for around 4 hours to heat up. So a much shorter time window between cycles and running longer.

This is all so annoying as had I just left the setting of the original stat where it was I probably would have no issue. I can't remember exactly the setting it was on previously but we always had plenty of HW and it used to run fine. The stat is fixed to the side of the cylinder ok too in the correct position.

Is it possible that I need to drop back the stat temperature setting (currently indicating on the very old dial around 55) to compensate for scale build up as we are in a very hard water area and this tank will be at least 25 years old? Working on the theory it's taking ages for the heat to conduct through to the walls of the cylinder. That's my theory anyway?

Thankfully I have smart meter data from before all this haopened showing consistently what level of gas usage was required to heat the tank and at the moment it is higher than previous. I am thinking if I just tweak down the stat in very small increments each day surely I should get back to where it was before?

Does this all sound feasible? My golden rule though is now not to touch stuff if it ain't bloody broke (like the stat as if I didn't move the front dial around on it I wouldn't be in this mess now but at the time I had to do this to fault find the issue).

Thank you in advance for reading all this.

Have attached some smart meter data to demonstrate what I mean.

Mark

Is that not electrical consumption you are showing?

 

[Murdochcat]

I hour heating water during the night and max 1 hour in the early evening. Leaving HW on constant is daft to say the least.

And I suspect your old tank stat was less sensitive than your new one

 

[Mark Batcheldor]

I need to try this too. I do think that the chalk in the cylinder is causing half these issues. I am in Kent and it's renowned for very chalky water so 25 to 30 year old cylinder is going to be an issue. Really I need to get it changed but they cost a fortune but then I need to weigh up the cost of gas I would save.

 

[ajohn]

heating engineers

I'm not but you need to consider the actual water temperature and that thermostats have hysteresis. They turn on and water heats and then they turn off. Then they will turn on again when the temperature has fallen to some level below where they turned off. Old stat say 10C new one 2C. Just a thought. Unlikely or not pass.

I had an odd problem with an older style CH. Newer system divert the boiler heating to either hot water or the radiators both pumped. The older style uses convection through pipes to heat the water and ch is just selected by leaving that as is and turning the pump on. Hot water tank topped up via a header tank. The header emptied due to hot weather and the water being turned off. Air locks prevented the hot water tank from fully filling. Fixed by connecting hot and cold with my hand on a mixer tap. Sort of pulses of cold being forced down the hot water pipes. It took several attempts and the best tap to use from memory was the bath tap. Bungalow though. I tried it having read that this can happen. Tank was hot but no hot water. Might happen if a system is drained.

 

[Justin Passing]

For a few quid you can get something like this https://www.ebay.co.uk/itm/274842104250?
which may settle the mind. Use a pencil to make a tunnel in the insulation to near the copper. Then the probe will slide in and out.
Cylinder stats and their hysteresis are pretty random. Their performance depends on the thermal contact with the copper of the cylinder, and whether the stat is covered or not.

If the heater element is say 2kW, scale can't affect its output, but it may run hotter to dissipate the same power, reducing its life.

 

[JohnD]

Please show us a photo of the thermostat you speak of.

What is the temperature of your tap water?

If the cylinder is being frequently reheated, and is not boiling, there is probably a leak or plumbing fault causing hot water to escape.

Is the cold-water tank hot, or overflowing?

Are radiators warm?

 

[Johntheo5]

If the stat has a temperature scale just turn it up and down and note the temperature difference, this is the hysterisis, normally ~8C max of 10C, min of 6C.
Another test you can do to see what the coil is absorbing is to fire the boiler on HW only, ensure its running continuously and not cycling (assuming gas boiler), take a photo of your gas meter and exactly 2 minutes take it again, multiply the difference X 11.0X60/2X0.85 to give the coil input in kw, (assumes a boiler efficiency of 85%), if you have boiler flow&return temps, note them as well.

 

[Mark Batcheldor]

Thank you for this info. All that has physically changed is the HW valve replaced. I then tinkered with the thermostat setting on the cylinder. I don't appear to have hot water entering the cold water system at least not that I am noticing. Does the stat plate on the back of the sensor physically have to touch the cylinder wall as I notice the plate on the switch won't physically touch the copper just the metal casing of the switch

 

[Mark Batcheldor]

 

[Mark Batcheldor]

Have attached a pic of what stat i am using sorry has posted twice.

 

[JohnD]

A picture of your own, in place, would be more informative.