Can you run out of hot water on an unvented tank?

[Arjay12345]

Hi

Should an unvented tank be able to provide endless hot water or should I expect to have periods when all of the water is used up and wait for water to be heated?

Background: A year ago I installed a new 300ltr unvented Ariston tank powered by a Vaillant 37kW system boiler. There are 2 adults and 3 small kids in the house. Sometimes my parents stay over too. The tank is on a timer so that it's not being heated from say 10pm to 4am. When I turn the tap on in the morning, the water is piping so it's definitely reached temperature by then.

By the time two adults and 3 kids have their showers/baths, we can run out of hot water. We don't take excessively long showers, and the kids sometimes share bathtimes and bath isn't full to the brim. Hence is it normal to run out of hot water by this time, or is something not set correctly where the water isn't continously being heated on entry into the tank?

If it matters, I have 24 rads dotted around the house but they are all zoned off and only being all heated at the same time for brief periods. Does powering rads affect heating of an unvented tank?

Thanks
Arjay

 

[Agile]

You would have to see if the total water consumption equals 300 litres which seems likely. It sounds to me as if they are using too much!

What is the measured flow rate from the shower? 10 min will use about 100 litres so just three like that will empty it !

The boiler will only reheat about 10 litres per minute.

Does the boiler fire up after you have used a little water?

Using heating at the same time will reduce the amount of water which can be reheated.

Tony

 

[scoobydo123]

Hi, are you sure that it is timed not to come on during no draw off? As there seems to be some doubt there. Also as Agile says a lot will depend on the flow rate of your outlets. Also very important is the temperature that your store the water at? This should be confirmed with an accurate thermometer.

I have an installation that has 4 ensuites and "only" has a 170L megaflow this copes without never running out however when I tried to reduce the stored temprature down to 55c within 2days complaints occured.

Also check to see if there is a balance valve on the cylinder as it may not be seeing the full output of the boiler.

 

[Arjay12345]

Thanks for the replies folk.

Agile - Ironically, my bath mixer tap does leak a steady stream so can see wastage there. We're also happen to be about to kick off the family bathroom renovation and have bought a Hangrohe Raindance shower head which has a venturi feature thus uses less water. I'll have to see how I fair when both of these aspects are fixed.

So you're saying that I should be able to heat the whole tank in approx 30 mins?

Thing I still don't get is where in the tank is the temperature measured to instruct the boiler to come on or not and still unclear whether I should run out of hot water. If I assume a 100 litre shower followed by 10 minute stoppage time between successive showers, then I should have endless hot water, shouldn't I? Or does it not work like that?

Scoobydo - That sounds excellent. Is there an ideal temperature I should set at?


Thanks
Arjay

 

[Dan Robinson]

The thermostat is about 18" to 2' up the side of the tank. Look for a wire coming out of the side.

Using the rads at the same time as the hot water will split the heating load of the boiler, but with a 300 litre tank you should still have plenty if the boiler is heating the tank as you use the water.

If you have solved all the issues and are STILL running out of water, consider getting someone in who is able to re wire the system with hot water priority so it holds the heating off whilst the hot water is being charged up.

 

[Agile]

Do you notice a significant improvement in HW when the heating is turned off?

Certainly in normal use your system should give pretty good results but if a lot iof water is taken then it can run out.

Until we see flow rates quoted then we cannot say anythign more.

Its not allowed to set the cylinder aboce 60 C unless you have a blending valve on the outlet.

It would also reduce boiler efficiency too as it would not be condensing.

The boiler flow temp needs to be set about 10 C ABOVE the set water temp!

Tony Glazier

 

[scoobydo123]

OP

You really need to make sure that the HW is stored @60 yes you may think it is hot but it could be someway below which will mean less cold mixed in for showers and baths so more demand on stored HW.

I store mine at 62c (all showers so do not need to comply with legislation mentioned by Agile that came into force in 2010). But as mentioned when I reduced this by just 7c tenants noticed that demand outstripped supply.

Also remember that as you draw off and cold enters the cylinder there is a certain amount of mixing that occurs despite the cold water diffuser.