Cold Water only during power cut - why??

[Phlize]

Boiler is Worcester Heatslave external 18/25. We were notified of a pending power-cut for today a few days ago. Electricity was due to be off for 12 hours but fortunately came back on after about 6. I assumed that I'd be able to draw the hot water from the 51 litre tank whilst the power was off but when I ran the hot tap, once the warm water had cleared the pipes, the water ran stone cold.
Why didn't it give up its hot water?
Mystified!

Bill.

 

[SJB060685]

When exactly did the power go off? The water stored in the boiler is to produce hot water, not supply hot water. Depending on when the power went out its most likely because the store had cooled down.
I take it everything is normal now?

 

[ShaunCorbs]

Works like a combi so

No elec no heating or hot water
No oil no heating or hot water

 

[SimonG]

The store could be full of steaming hot water, but if theres no power to circulate it through the plate heat exchanger then you wont get anything other than an initial burst of hot water.

 

[DaveyHep]

Depends on your storage tank. Mine is a sealed unit and uses cold water pressure to dIsplace the hot so I can (and have) had hot water during our many power cuts.
if it’s likely to be a regular thing then suggest you have a discussion with a plumber. If it’s a one-off then...

 

[SimonG]

Depends on your storage tank. Mine is a sealed unit and uses cold water pressure to dIsplace the hot so I can (and have) had hot water during our many power cuts.
if it’s likely to be a regular thing then suggest you have a discussion with a plumber. If it’s a one-off then...

On a heatslave combi? Like to see that if you have some pics.

 

[DaveyHep]

Ha.. missed the bit about it being a combi.. DOH !
I'll shut up now....

 

[SimonG]

Ha.. missed the bit about it being a combi.. DOH !
I'll shut up now.***

Thought you had something groundbreaking there. Wish I had a pound for everytime I didn't read something properly on here, I'd be rich.

 

[Big Phil]

The store could be full of steaming hot water, but if theres no power to circulate it through the plate heat exchanger then you wont get anything other than an initial burst of hot water.

Yes, the store was indeed full of steaming hot water - I thought the cold water pressure would just push it out but if it needs the pump to circulate it then that makes sense. The innards of the heat exchanger have always been a bit of a mystery to me - don't suppose you have a photo or a diagram of the water route? How does the cold water bypass the HE to come out of the hot tap - is there a valve in there??
Thanks, Bill.

 

[SimonG]

Cold water into boiler. Cold water into plate heat exchanger comes back out as your domestic hot water. When you open your hot tap, flow switch senses the flow and kicks in pump and diverter. Which then pumps the store water through the other half of the plate heat exchanger and producing hot water. Older models will kick in the burner at same time, newer ones will kick the burner in once the store thermostat calls for heat.
Other manufacturers use thermistors rather than a flow switch.

 

[SJB060685]

Just Google counter flow plate heat exchanger.
As was said above. Store will of beeen hot but electricity was still needed to circulate through the system.

 

[Phlize]

Thanks SBJ, It is now becoming clear!! I didn't appreciate the fact that the heatstore is there merely to provide the hot water for the plate heat exchanger - I thought that water came from the primary HE hence leaving the water in the heatstore available for supply to hot tap. I now realise this is not the case, Very good Google explanation of counter flow plate heat exchangers.
Cheers,
Bill