Hot Water Cylinder Thermostat

P1Fanatic

Like most we have been tweaking our hot water settings to try and minimise our monthly gas costs, especially as we are now in the warmer months. We have a gravity fed cold water system (tank in the loft) and a hot water cylinder in the airing cupboard. The thermostat on the cylinder is always left on 60c. Our Hot Water timer was set for 30 mins in the morning and evening as its just me and the wife (previously it was 1hr each). The wife says she is running out of hot water when showering in the mornings (so a volume issue not a temp issue). So presuming the cylinder is not getting fed enough hot water I have turned the timer up from 30 to 40 mins in the morning but I want to make sure I understand fully how our system works.

My understanding is that the hot water cylinder (not an immersion as no element in the top of it) thermostat is to determine the required temp inside. So if the hot water being on for 30mins provides hot water to the cylinder that hits 60c then the thermostat should tell the timer no more needed even if the timer is set for another 10 mins (40 mins total), timer tells boiler and boiler turns off. Does that sound right or am I totally missing something?

Gerry1

You'll probably find that the thermostat turns off a motorised valve so that hot water from the boiler no longer flows through the coil in the tank, but the boiler can still heat the radiators if the CH is switched on.

Reed_Richards

It's more likely the thermostat signal is connected directly to the boiler and the boiler is connected directly to the motorised valve.  So if there is no demand for central heating the boiler should turn off when the temperature set on the thermostat is reached.  If both the central heating and the thermostat on the hot water tank are calling for heat then the normal set-up give priority to the hot water.  However different variations are possible so we cannot give you a hard and fast answer.

Shedman

Even power showers don't generally deliver more than 9/10 litres a minute and, given that you would generally mix the water temp down from 60° to around 38/40° for a hot shower, then maybe would use say 5/6 litres/min of HW.  So unless she is staying under the shower for a very long time and/or your HW cylinder is very tiny (or poorly lagged) then I would have thought 30 mins both morning and evening would have been more than sufficient especially at this time of year.  Our cylinder is 160 litres, we heat to 60° for 30 mins in morning only and there is sufficient for two of us to have a good shower of 5-6 mins each using pumped power showers

What temp setting do you have on the boiler?  If boiler temp is too low (ie below 60° or towards the min setting if no temp scale) then it won't heat HWC to 60°.  

As Gerry says once the HWC stat hits the required temp then it will close the flow to HWC and if CH is off or CH stat set to low setting will stop boiler firing so might be worth adjusting timing to 40-45mins (at least in morning) and see how it goes.  Also how old is your HWC stat as it might not be working properly...if you turn the temp setting dial up and then down can you hear a click where it switches on and off and how close is that to 60°.  I replaced ours a couple of years ago as it was way out and although it was pointed at 60 was switching off at nearer to 45 (you'd know if it was around 60 as just running hot water you'd just about tolerate holding your hand under it although caution advised with that test clearly)

[Deleted User]

Most cylinders have a very low heat loss per day. I have a 210L cylinder with a thermostat located towards the bottom of the cylinder. It is set to 55C.

My hot water programmer/timer is set to come on for 3 hours each day from 3.45 to 6.45AM. When 3.45AM is reached, the HW zone valve opens and the boiler comes on to heat the coil inside the HW cylinder. This, in turn, heats the water in the cylinder. As you can see from my smart meter usage graph, my cylinder reaches the set temperature within an hour using just over 4kWh of gas.



We never run out of hot water.

grn99

I suspect 60 degrees is not being met with only 30 minutes heating time, but the boiler flow temp is also a factor. Bear in mind when hot water is used, cold water refills the cylinder, On our system with 2 of us using it, I need 45 minutes heating to reach a temp between 52 and 58 degrees measured on the tank. To get a tank up to 60 takes disproportionately longer as the cyclinder temp gets closer to the boiler flow temp. My boiler flow is set to 72.

P1Fanatic

Shedman said:

Even power showers don't generally deliver more than 9/10 litres a minute and, given that you would generally mix the water temp down from 60° to around 38/40° for a hot shower, then maybe would use say 5/6 litres/min of HW.  So unless she is staying under the shower for a very long time and/or your HW cylinder is very tiny (or poorly lagged) then I would have thought 30 mins both morning and evening would have been more than sufficient especially at this time of year.  Our cylinder is 160 litres, we heat to 60° for 30 mins in morning only and there is sufficient for two of us to have a good shower of 5-6 mins each using pumped power showers

What temp setting do you have on the boiler?  If boiler temp is too low (ie below 60° or towards the min setting if no temp scale) then it won't heat HWC to 60°.  

As Gerry says once the HWC stat hits the required temp then it will close the flow to HWC and if CH is off or CH stat set to low setting will stop boiler firing so might be worth adjusting timing to 40-45mins (at least in morning) and see how it goes.  Also how old is your HWC stat as it might not be working properly...if you turn the temp setting dial up and then down can you hear a click where it switches on and off and how close is that to 60°.  I replaced ours a couple of years ago as it was way out and although it was pointed at 60 was switching off at nearer to 45 (you'd know if it was around 60 as just running hot water you'd just about tolerate holding your hand under it although caution advised with that test clearly)

Thank you. Its a pumped shower by Aqualisa (seperate pump in loft) with a large 30cm circular spray head so it pushes out a fair bit I reckon. I am not sure on the cylinder capacity but it takes up most of the airing cupboard. Will see if any marking on it to say capacity. It doesn't have a jacket just the foam coating / lagging. She definitely takes more than 6 mins in the shower although stops and starts when lathering. Tbh it used to run out on the odd occasion even when HW was on for 50-60mins. But now she says its happens almost daily.

HWC stat must be as old as the house - 22yrs. Just turned it and it clicked at 35c but obviously its been drained of hot water due to her shower earlier and not been on since. I'll check it again tonight once its been on for 30 mins.

CH is off completely and has been for weeks.