HW cylinder S-plan heat losses?

[tellme_why]

Hi,

My Hot Water yearly consumption is - 7300kwh/year - which is high for two people. Wish to find out what's the cause of such high consumption.

I manually switch on HW before showering in the evening, and after about 10mins the cylinder reaches 67C from 37C. We start the shower with aqualisa pump 13Litre/mins and two showers last about 18 mins consuming about 230litre/day.

My boiler is oldish Potterton 50e on the ground floor whilst the cylinder is on first floor. Cyl is only 4 years old, pump and valves are also 4 yrs old. The by-pass valve is very old and need changing - not clear whether it's open or close or stuck.

I run a test today, and plotted the temperature rise (after HW switch on) until minute 8 where the boiler reaches its set temp of about 67C.

The strange thing happening is that as boiler stop firing, the cyl supply temp starts dropping like a stone despite there is no water consumption (shower was not running during the test). The pump is still running so water keep circulating from the boiler to the cyl picking up some coldish pipe along the way - but I am surprised that the temperature drops so fast.

You can see the S-Plan P&I .. any suggestion about what could be the cause of such drop? I assume that the drop is an indicator of heat loss somehow..?

 

[Johntheo5]

Why does the boiler circ pump keep running after the boiler stops if no CH demand? Is there a secondary bronze pump recirculating water to give instant hot water at the taps?.
The graph suggests ~ 9C drop in 3 hrs so 3C/hour, assuming a 200L HW cylinder then its loosing lmost 0.7kwh/hour or 16.8kwh/day or 6132kwh/annum!, if only a 100L cylinder, then still loosing 0.35kwh/hour or 8.4kwh/day, 3066kwh/annum. Your daily shower consumption is 230L, add another say 50L for hand washing etc = 280L/day at 40C, assuming a average mains temp of 12C then daily HW usage = 9.1kwh or 3322kwh/annum, sounds OK, your big problem is the cylinder heat loss.

 

[tellme_why]

Why does the boiler circ pump keep running after the boiler stops if no CH demand? Is there a secondary bronze pump recirculating water to give instant hot water at the taps?.
The graph suggests ~ 9C drop in 3 hrs so 3C/hour, assuming a 200L HW cylinder then its loosing lmost 0.7kwh/hour or 16.8kwh/day or 6132kwh/annum!, if only a 100L cylinder, then still loosing 0.35kwh/hour or 8.4kwh/day, 3066kwh/annum. Your daily shower consumption is 230L, add another say 50L for hand washing etc = 280L/day at 40C, assuming a average mains temp of 12C then daily HW usage = 9.1kwh or 3322kwh/annum, sounds OK, your big problem is the cylinder heatit

Why does the boiler circ pump keep running after the boiler stops if no CH demand? Is there a secondary bronze pump recirculating water to give instant hot water at the taps?.
The graph suggests ~ 9C drop in 3 hrs so 3C/hour, assuming a 200L HW cylinder then its loosing lmost 0.7kwh/hour or 16.8kwh/day or 6132kwh/annum!, if only a 100L cylinder, then still loosing 0.35kwh/hour or 8.4kwh/day, 3066kwh/annum. Your daily shower consumption is 230L, add another say 50L for hand washing etc = 280L/day at 40C, assuming a average mains temp of 12C then daily HW usage = 9.1kwh or 3322kwh/annum, sounds OK, your big problem is the cylinder heat loss.

It keeps runnig because hive still demand HW; the cyl stat is only for protection, the boiler own stat start/stops the boiler not the pump..

 

[Johntheo5]

"It keeps runnig because hive still demand HW; the cyl stat is only for protection, the boiler own stat start/stops the boiler not the pump"

Don't know how hive works but it should interact with the HW (&CH) in a S plan the same as a conventional programmer, Basically, the hive HW demand should probably go to terminal 6 on the wiring center, the S plan then works as normal, the cylinder stat when closed opens the HW coil motorized valve, the motorized valve end switch then closes and fires the boiler and starts the pump, when the cylinder reaches its setpoint the stat opens and closes the HW coil motorized valve, the end switch opens and the boiler and pump stop.

Incidentally, what is the HW cylinder capacity?.