Water flow rate calculation - not enough water for 2 showers

[SteveBall]

We live in a new build and have a 150l unvented tank attached to an Ideal Logic Heat H15. The house is 4 bed, approx 1500 sqft and has 3 toilets, 1 shower and 1 bath. We don't have excessively long showers, maybe 20 minutes in total for both of us. Sometimes we can get 2 showers but we rush through to make sure we don't run out of water.

We had a plumber come out under warranty and here are his findings - we need to understand if this is acceptable

max flow rate 9.5 lpm @32.5 degrees
Usual flow rate 8.5 lpm @32.5 degrees

I need to understand what the ratio of cold to hot water is to achieve 32.5 degrees - i.e. if the tank is roughly 60 degrees, would that mean we use 55/45 hot and cold, so effectively we are using 4.5 lpm hot and 4 lpm cold, meaning we should empty the tank in 33.5 minutes (150/4.5)

Ideally we would like to have a warmer shower but I can't as we don't have the water capacity.

 

[ShaunCorbs]

Correct ish I would allow 6lpm hot always better to have more

And cylinder wise don’t forget it’s all not hot water would say 60/40 split eg hot to cold

So estimated around 80-90l of hot which puts it at around 15 - 20 mins of hot water

Do you have plenty of height above the cylinder?

 

[SteveBall]

Ahh I am an IT person, this is a whole new world to me.

I have attached 2 pictures - I hope they help. The tank is upstairs, so is the shower

 

[gmartine]

Your house builder would correctly argue that a typical ablution by shower is probably way under 10 minutes and therefore they have more than satisfied the specification they probably sold you. In reality that's the smallest they could've got away with and you like taking longer than normal showers, good news is you have plenty of room to install a larger cylinder.

 

[SteveBall]

That's our issue, there is 2 of us in this house and I don't believe we don't take excessively long showers. I am interested in what peoples opinions are of a typical length of time to take a shower.

Thank you for your reply - what size tank would you suggest?

 

[gmartine]

It doesn't really matter how long we think a normal shower time is, you prefer ten minute showers. I'm not arguing what is normal or not however I don't think you've got a cat in hell's chance of getting it changed FOC or raising a compliant on that basis. The sad thing is there isn't a huge difference in cost between a 150 and a 250 litre pre-plumbed cylinder and that's the minimum I would opt for.

 

[SteveBall]

Yeah we are asking the question as the house next door is 30 sqm larger and they have a 210 - it's just the builder being cheap!

 

[Welder]

Hi Steve. Welcome to the wonderful world of UK housebuilding...

IMHO, if you do not wish to change your bathing habits you can (reasonably) increase the amount of USABLE stored hot water. Currently your cylinder only delivers approx 100litres of usable water, i.e. water warm enough to use in a shower, because the bottom of the cylinder contains cold water. If you have an anti stratification pump fitted, it means you will get another 30 litres of USABLE hot water without having to change your cylinder. That may be all you need.

 

[gmartine]

How does that work, like a secondary pump?

 

[SteveBall]

Hi Dave,

Thank you for the advice, sadly our property is under warranty and I can't make any changes without losing the warranty.

I believe I am quite efficient at showering - I will take some timings over the next few days to use as a benchmark.

 

[Welder]

How does that work, like a secondary pump?

Basically it's placed between the cylinder outlet and the cylinder side of the control block (the cylinder inlet). It must obviously have a non return valve & I always used a mechanical one as they last longer. The pump must be bronze or stainless steel due to the highly oxygenated water. A std htg pump lasts about 10 minutes! Again obviously, the pump needs a timer and be set to start a while after the HW heating kicks off and finish before it finishes in order to minimise wasted heat.

 

[fezster]

Does your cylinder have an immersion heater (possibly more than one - the second being higher up in the cylinder?)
And if so, is it definitely turned off so that your boiler is being used to heat the water? If only the top immersion was being used, for example, you'd only get half the water in the cylinder being heated up.

Once you've confirmed this is the case, can you then check the cylinder stat is satisfied when you have the hot water on (ie. Motorised valve closes and boiler switches off). You can often hear the cylinder stat click once it reaches temperature. If you then put a hot tap on, the water should be too hot to touch if the cylinder stat is set to 60 degrees.

Just some ideas to check.

 

[SteveBall]

Ok I have one switch on the wall that is in the off position, there isn't any label, so I can only assume that's the immersion heater.

I will buy a thermometer to check the temp of the hot water.

 

[Chuck]

Currently your cylinder only delivers approx 100litres of usable water, i.e. water warm enough to use in a shower, because the bottom of the cylinder contains cold water.

The NHBC 2011 standard:

NHBC Standards 2011

requires a tank able to deliver a 145 litre draw at 60°C for a house with one bath and a shower. There are more recent versions of this standard but they require Flash to read them and I've not got that installed on my computer.

One solution to this problem would be to programme the hot water to be heating during the period you normally take showers.