# Understanding hot water cylinder recovery times

#### LeoCh

Hello all

I am buying a flat that has an unvented indirect hot water cylinder (I need to check size but I am guessing 170L or maybe 210L) and a system boiler.

My question is: If the hot water cylinder has a fast recovery time (say ~20 mins or so), and if one tank gives me 20 minutes of showering, does it mean that I can use the shower continuously, because by the time I used the original hot water in the tank it will have already fully reheated?

(assuming that the boiler is on hot water mode and providing the necessary amount of heat of course).

Thanks

Leo

It will depend on the flow rate of your shower which determines how quickly you use up the hot water. Say 10 litres/ min over 20 mins ( you do the maths) I very much doubt that by the end of your excessive shower time that the cylinder will have completely recovered.

I doubt very much that you will use up all 210 litres of hot water in 20 minutes! The cylinder water temperature will be approx 60C so you will need to mix it with cold water to provide a safe shower temperature, which is about 40C. Assuming the cold water is 20C, you will need equal quantities of hot and cold water, so 420 litres in total. This would imply a flow rate though the shower head of 21 litres/min.

Most shower heads have a flow rate of 10 litres/min or less, though there are some power showers which can provide higher flow rates. Changes to Building Regulations mean that power showers and shower heads capable of delivering more than 10 litres/min may soon be unobtainable.

There are two reheat times: (1) from cold, i.e. a tank filled with water at 15C; (2) when 70% of the water has been drawn off, which means a cylinder temperature of 28.5C, (15C cold, 60C hot). Most cylinder thermostats will turn the boiler on when the water temperature has dropped by 10C, which is when about 20% of the hot water has been used.

I'm not sure if it's 210 or 170 I need to go back and check the exact model number.

We like to have strong power showers. Where we live now we have a nice strong shower and we timed it at around 14 litres per minute. That's what we'd like to have.

But if you are saying that the cylinder will give us more than 20 minutes each time, and if the recovery time is 20 minutes, does it mean we can shower continuously?

We are two people, but we often have guests, and they tend to come in two's (my parents, her parents, her sister + husband, my cousin + wife, etc), so assuming we are four in the house and each wants to have a 10 min shower, we need 40 mins of hot water. The cylinder clearly won't provide that in a single go, but if the recovery time is shorter than the time it takes to use all the water, does it mean we could in theory shower continuously for 40 minutes?

Also some hot water is used for the taps during the day.

Thanks

It's no good guessing the size of the cylinder. What capacity is it, and what is the kilowatt rating of the coil, and the boiler output?

I will probably want to replace both boiler and cylinder. They're both from 1999. The boiler more likely because it's on a kitchen wall that I want to remove, so I might as well replace it. The cylinder is in a cupboard I might want to replace it depending on its shape and condition.

So my assumptions are: 170 litres cylinder. It takes more than 20 minutes to use all the hot water in the cylinder. The cylinder's recover time is 20 minutes. Does that mean that I can use the shower for more than 20 minutes? In theory, could it run continuously?

why don't you try it and see before you rip anything out? gotta be a lot cheaper if it actually works.

if your absolutely intent on ensuring you can have 1hr+ worth of continuous shower, (highly unlikely, you'll have at the very least a few mins drying and changing clothes time between people) you could be very wasteful and time it until it run cool?

the reheat time i'd assume is without anything drawing on the cylinder, i.e. a shower.

If you've got a thermostatic mixer shower however, if the boiler can sustain heating the incoming water to over your shower temp (40C?) then the mixer will compensate and let it run as long as the boiler can.

We are two people, but we often have guests, and they tend to come in two's (my parents, her parents, her sister + husband, my cousin + wife, etc), so assuming we are four in the house and each wants to have a 10 min shower, we need 40 mins of hot water. The cylinder clearly won't provide that in a single go, but if the recovery time is shorter than the time it takes to use all the water, does it mean we could in theory shower continuously for 40 minutes?

Will be fine, as said you won't really use more than 10l/min of hot
If you have your hot water on programmer set to allday it will kick in as soon as the temp drops.

So for example if you use 100 litres in 10 mins, the boiler will have already been on for 5+ mins. You get out your mrs gets in 5 mins later and uses 100 litres of hot water and so on. I'd be very surprised there was ever not 100 litres of hot available.
This is assuming it is a modern he boiler with suitable controls and hopefully the 210 ltr cylinder

Sorry I missed your last post about the boiler age etc in theory it would still work with your current system but maybe not quite as well. Storing your water a little hotter cold help but then could be too hot at other taps if not mixers

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