There are various possible causes.

One of the actuated valves could be in the wrong position or not operating correctly.

You could have water circulating and hence carrying the heat energy away from your hot water cylinder.

Do you have any form of circulating pump to ensure warm water is readily available at your taps?

If you could post some photo's of your system it may prove useful.

Hello @Curlykatie,

How much hot water do you get through as a family? 300L is decent sized cylinder, charging for 1hr a day should be plenty assuming the ASHP will kick out about 10kWh. As a benchmark 3kWh can raise 65L of water from 10degC to 50degC, so 10kWh a day should be more than enough energy for your hot water needs.

So, baths per day? Showers per day? Washing up? Other? Normal shower head or a high consuming type? rain shower, power shower, etc. Some shower types can draw off huge volumes of hot water. Rule of thumb for sizing cylinders is 35-45l/day/occupant. But lifestyle habits can massively influence this.

We heat a 250L cylinder for an hour a day to 50degC at mid day when then ASHP CoP is usually highest, and it is enough for 3 of us. Sometimes solar PV surplus power tops it up a bit, but not much in the winter months. 

At 50degC store temperature a 300L cylinder will lose about 2kWh a day through standing losses to the surrounding air. The higher the water store temperature compared to the surrounding space temperature, the more these losses will be. You'll see that in the thermostat reducing temperature even if you are not using any hot water.

Moving the thermostat sensor won't really help. A 300 litre mains pressure unvented cylinder does not store 300L of water at thermostat set temperature, say 50degC. Probably about 1/2 of the contents is heated to the thermostat set point. The hot water accumulates at the top of cylinder where the hot water is drawn off, the cold water enters the bottom. The way they heat is using a large coil in the bottom 1/3 which causes warmed water to rise up. Moving the thermostat sensor position might simply mean the cylinder never reaches set point because the probe is always sensing cooler water.

@derek-m thank so much for your reply. I really do appreciate it. I’ve attached some photos- let me know if you need anything else. I think one of them is the circulating pump,

katir 

@allyfish thank for your reply. There are 3 of us- 1 shower in the morning and 2 evening- not more than 5 mins but they are rain shower heads. A bit of washing up etc. but it no way feels like we should be running out of water with the size of the tank!

@curlykatie

The green pump in the pipework near the top of the hot water cylinder, is it running? Is this pump in the pipework that connects to your heat pump or is it part of your DHW?

Confirm that the 3-way valve is operating correctly. The valve is the one about half way up the hot water cylinder, it has one pipe coming in from below, one pipe on the right hand side and one on the left. The grey coloured box on the top of the valve is the actuator which operates the valve. This valve diverts the warm water from your heat pump to either the hot water cylinder or the central heating. If it gets stuck in the middle it could be trying to do both at the same time.

There is a lever on the end of the actuator under where the cable enters. Try moving the lever from one side to the other to ensure that it moves reasonably freely.

@derek-m thank you. I believe the green pump at the top of the cylinder is for the hot water. I m not sure how to check if it’s running but it is making a noise as if it’s doing something!

the grey box that says Honeywell? The lever seems to move ok…

Posted by: @curlykatie

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@derek-m thank you. I believe the green pump at the top of the cylinder is for the hot water. I m not sure how to check if it’s running but it is making a noise as if it’s doing something!

the grey box that says Honeywell? The lever seems to move ok…

 

Where does the cable supplying the green pump connect?

@derek-m see pictures-

@curlykatie

Can you provide a closeup photo of the manufacturer's nameplate on the side of the cylinder.

@derek-m

@curlykatie

I think that the details below relate to your water cylinder.

TELFORD-GROUP.COM "https://www.telford-group.com/uploads/shop/downloads/Tempest+Heat+Pump+Technical+Details+New+Spec.pdf"

What I think may be happening is that the green pump is pumping hot water around your pipework to the taps and shower etc. and is therefore cooling your hot water cylinder.

To check if I am correct then locate the main pipework underneath the sink in the kitchen or the bathroom and see if it is hot. If this is the case then you need to get the green pump switched off. It will mean that you may have to wait a short period of time for hot water to reach your taps, but at least you should have some hot water to make the journey.