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Discuss Water cylinder re heat time in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Messages
19
Hello all, i am doing work for a customer who has asked me what the reheat time of a 250 litre cylinder is. To be honest after all the years of doing plumbing this is a question that i have never been asked before because usually if you need a hot water cylinder then it is what it is, you just need to deal with the reheat time regardless of what it is.

I have however experienced that these cylinders tend to heat up relatively quickly (estimating around half an hour). The customer had various questions with regards this system and so i contacted my usual wholesaler to see if they could retrieve these answers from the hot water cylinder supplier, all the questions were answered satisfactorily apart from this one where the rep said that usually they calculate it at a litre per minute which i am not buying to be honest because we are then talking 4 hours to heat a 250 litre cylinder haha.

So...i was wondering if any of you had a definitive or accurate answer to this query or are we all just in agreement that you are talking about half an hour, which i personally would deem to be a reasonable amount of time. The cylinder is going to be run off of 2 immersers simultaneously as we will not be dealing with off peak, so when the customer turns on the hot water it will be running with 2 6kw immersers, there is not going to be a coil in it.

Any help on this would be appreciated...thanks in advance
 

EvilDrPorkChop

Esteemed
Plumber
Gas Engineer
Messages
846
If you go on the manufactures website it'll tell you the reheat times from Cold or 70% Draw off. There will be figures for Indirect and Direct Models.
 
Messages
2,034
. . are we all just in agreement that you are talking about half an hour, which i personally would deem to be a reasonable amount of time. The cylinder is going to be run off of 2 immersers simultaneously as we will not be dealing with off peak, so when the customer turns on the hot water it will be running with 2 6kw immersers, there is not going to be a coil in it.
It takes 5.6 hours for 5.2kW to heat 250 litres by 50 deg C. So, 'about half an hour' is not even close.
 
Messages
19
It takes 5.6 hours for 5.2kW to heat 250 litres by 50 deg C. So, 'about half an hour' is not even close.
Going on this basis, lets say it takes 5.5 hrs roughly to heat 250 litres with a 5.2kw immerser.

We are going to have 2x 6kw immersers going, which will bring it to a total of 12kw heating the water so you would usually get roughly 40 litres of hot water an hour based on 5.2kw heating it.

If this is the case then with 12kw heating the water we should effectively have approximately 80 litres of hot water within half an hour and it should take three hours to heat the entire cylinder, so this should work more effectively than the four hour reheat time given by the manufacturer and lets face it 80 litres would do you for a shower since it will be mixed with cold water, even if you wanted a bath you could give it 40 minutes to ensure that you had enough hot water but again even at that you would be topping the bath up with cold.

Thanks for your help here, i should maybe have phrased my question a bit better to be fair because when i was talking reheat times the customer was really wanting to know how long he would need to wait before being able to use the shower again, so my half an hour comment may not have seemed too outlandish. As we know, when putting this system together there will be a control fitted so that they can activate the hot water on a timer basis so as to hopefully eradicate any issues like this anyway so i don't see it becoming a real issue,

Thanks
 

SJB060685

Plumber
Messages
2,017
There online water reheat calculators that will give you an exact answer. Just Google water reheat calculator. Ultimately its going to take the same power in KWH to reheat the water, regardless of the output of the immersion heater/heaters, its just you will heat water quicker with a 12kw element over a 3kw element.
 

Timmy D

Gas Engineer
Messages
425
Due to insulation, standing losses are now days are quite small. Check the data badge/instructions for the cylinder.

You’ll likely find losses of around 2kw/24h.

I wouldn’t bother with timers/switching it on/off and just leave it on.
 

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