Hot water cylinder heating issue

[Harry Bloomfield]

I suspect the hysteresis (difference between switch on and switch off points), of the old stat, is wider than the new one, plus you are perhaps setting it a bit higher in temperature.

A separate issue, might be the boiler stat setting. The boiler needs to output it's heated water, around 10C higher than the setting of the cylinder stat - otherwise the boiler will cycle on and off forever, trying to heat the cylinder, and trip it's stat to off. For instance - cylinder stat set at 55C, the boilers own stat, needs to be at least 65C, then tested to ensure the cylinder stat does turn off.

 

[Johntheo5]

I suspect the hysteresis (difference between switch on and switch off points), of the old stat, is wider than the new one, plus you are perhaps setting it a bit higher in temperature.

A separate issue, might be the boiler stat setting. The boiler needs to output it's heated water, around 10C higher than the setting of the cylinder stat - otherwise the boiler will cycle on and off forever, trying to heat the cylinder, and trip it's stat to off. For instance - cylinder stat set at 55C, the boilers own stat, needs to be at least 65C, then tested to ensure the cylinder stat does turn off.

Each cylinder is different even with a perfectly calibrated stat, the cylinder top can be 10C or higher than where the stat is attached depending on stat location and coil circulation flowrate, they should really be set up by monitoring the HW temp even with a household thermometer with a hot tap just trickling hot water, the stat should then be turned down until it clicks off at a measured temp 60 to 65C, otherwise, with the stat set to 60C, the cylinder top may be 70C and the boiler may never cut out even with a setpoint temp of 70C.

 

[Harry Bloomfield]

Each cylinder is different even with a perfectly calibrated stat, the cylinder top can be 10C or higher than where the stat is attached depending on stat location and coil circulation flowrate, they should really be set up by monitoring the HW temp even with a household thermometer with a hot tap just trickling hot water, the stat should then be turned down until it clicks off at a measured temp 60 to 65C, otherwise, with the stat set to 60C, the cylinder top may be 70C and the boiler may never cut out even with a setpoint temp of 70C.

Exactly, which is why I suggested double checking the cylinder stat, actually clicks off.

 

[Johntheo5]

Its surpising that all pumped coils dont circulate from the bottom up, no problems like above and a full cylinder of HW at exactly the temperature you require.

 

[fixitflav]

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?

Taking 3 or 4 hours to heat the cylinder doesn't mean the boiler is using gas at design rate all that time. It either modulates down, or more likely as this sounds like an old system, cycles on/off.

 

[fixitflav]

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.

Those figures are unusally high, no doubt caused by a very high cylinder stat differential and badly furred up coil. But presumably you've been happy with it till recent events.

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.

The ratio of 2-3 hours to the old 16 hours is about the same as the 1/2 hour to 3 hour heating time. Caused by a much lower differential on the new stat. In either case the boiler isn't running flat out the whole time.
Maybe the differential is a bit too low, but unlikely it's adjustable. Why not just use the new stat?

 

[Mark Batcheldor]

Thank you yes this is the case the boiler (which is a Worcester bosch) does cycle when the HW is on. I.e. the green burner light is on for around 20 mins then off for 10 mins. As mine is taking ages to reach set point obviously this cycling gets increased I guess as the stat temperature setting is close to or exceeds the boiler stat? What i will try is reducing the cylinder stat setting down very slightly and see if this improves the situation failing that I know a heating engineer guy who will no doubt be able to look at this all and see what can be done. You have all been really helpful on here. Am glad I posted

 

[Agile]

I would strongly suggest you fit the new stat and follow Harry's advice about boiler flow temperature. The shorter the time the boiler is actually heating the water the lower the likely gas consumption.

And then get a time clock set to ensure it only heats the cylinder before the water is to be used.

It would be good if you had a thermometer to actually check what the water and the boiler temperature really are.

 

[Harry Bloomfield]

Taking 3 or 4 hours to heat the cylinder doesn't mean the boiler is using gas at design rate all that time. It either modulates down, or more likely as this sounds like an old system, cycles on/off.

Presumably, the boiler is also serving the central heating too? So not all of those peaks will be to heat the HW?

My own system - open-vented, set to bring the cylinder to temperature, does so, in three of four runs

 

[Mark Batcheldor]

Thank you Harry no this is just HW alone. The CH isn't on yet. Eeeek!!!

 

[fixitflav]

What i will try is reducing the cylinder stat setting down very slightly

It must already be below the boiler control-stat setting, or (as Harry says) you wouldn't get any CH. Lowering it a toch (or raising the boiler-stat) would reduce the heat-up time.

failing that I know a heating engineer guy who will no doubt be able to look at this all and see what can be done.

Why do you think something needs to be done?

I doubt you would notice any difference in gas usage if you change HW to timed periods rather than 24/7.

 

[fixitflav]

Presumably, the boiler is also serving the central heating too? So not all of those peaks will be to heat the HW?

Not clear what point you're making there. What peaks?

My own system - open-vented, set to bring the cylinder to temperature, does so, in three of four runs

Sounds much like mine. I suspect my coil is well furred up, but it it doesn't cause a problem.

 

[Harry Bloomfield]

Not clear what point you're making there. What peaks?

The gas consumption, in usage graphs.

 

[Harry Bloomfield]

Sounds much like mine. I suspect my coil is well furred up, but it it doesn't cause a problem.

I wouldn't expect it to satisfy the demand in a single run, due to mixing and thermal currents in the water of the cylinder. It will also cycle more, if the boiler cannot effectively modulate, to a low output.

 

[fixitflav]

The gas consumption, in usage graphs.

OK with you