How to calculate a cold water storage for 144L Hot water cylinder?

[totcambiu]

Hi guys.

Im desperate for a professional advice. I have recently moved to a new flat where I have a problem with my hot water running out very quickly. The plumber boy provided by the landlady made several visits to no avail. First blaming on the scale buildup, then on a faulty thermostat but nothing have helped so far.

Problem: When I try to fill the bath with hot water ONLY. I'm able to fill 60L of hot water and then water start very quickly but gradually running cold. I have no idea how this whole setup works, but if think logically it looks like hot water cylinder is not filled in to its 144L capacity.

Possible problems from my point of view: There is either a air inside the cylinder that blocks the water intake or cold water storage too small for this setup. Ive asked him about air he says that its an open system or something and there is a vent where excess hot water and air goes out back into the cold water storage. It leaves me with the small cold storage tank.

Is there a formula to calculate and adequate cold storage tank capacity for 144L hot water cylinder? or can someone give me a rough estimate? The cold water storage is about 1.60cm high from the floor.


Current cold water storage size (actual water in the box) 39(L)x25(W)x18(H)=17550
17550/1000 =17.5L

Hot water cylinder capacity is 144L (1050x450)

there is also an electric pump that pumps the hot water, because there is not enough pressure otherwise.

any advice will be highly appreciated.

 

[Dannypipe]

So the water goes cold. It doesn't stop.

So it's not drawing the water from the tank faster than it can be replenished?

What temp is the hot water?

 

[Whyme]

you should get the cylinder capacity of hot water if heated properly.

 

[totcambiu]

So the water goes cold. It doesn't stop.

So it's not drawing the water from the tank faster than it can be replenished?

What temp is the hot water?

No it is not. It had this problem before, but I fixed that by adjusting the pumps speed. Before it used to start spiting the water (air and water would come out maikin funny noises, wheres a minute ago it would flow as a constant solid flow) and at the same time it started to become cold. The temperature is set ridiculously high, I haven't tried yet, but I bet I can make my tea right from the tap.

 

[Dannypipe]

OK, well I imagine it's an issue caused by the pump.

I know it's very slow to fill the bath without the pump being switched on (as you have said) but try it. Try filling the bath with the pump electrically switched off, and see if you run out of hot water. If you still do we can discount the pump.

 

[totcambiu]

you should get the cylinder capacity of hot water if heated properly.

That is my problem, i need to find out why its not giving me the full 144L of hot boiling water (it actually should give a bit more considering that cold water is getting in to the cylinder at the same time when the hot water is going out, so it would mix a bit and become a bit warmer at the end but still. )

 

[plumber0808]

Has there been a new immersion heater fitted recently? Might be the wrong size to heat the whole cylinder and is just heating the top half

 

[totcambiu]

Has there been a new immersion heater fitted recently? Might be the wrong size to heat the whole cylinder and is just heating the top half

You see when I set my pump to high roughly at the same mark of 60L it will run out of water and stop(it will suck the water out quicker that it can replenish itself ), then some water will come out again for a second and it will stop again and that will continue for ever until you swithc it off and give it a time to replenish. Thats why I set the pump to a slower pace and now it does not run out of water, but it becomes cold after 60L.

 

[totcambiu]

OK, well I imagine it's an issue caused by the pump.

I know it's very slow to fill the bath without the pump being switched on (as you have said) but try it. Try filling the bath with the pump electrically switched off, and see if you run out of hot water. If you still do we can discount the pump.

We done this before and what happens is that it becomes cold roughly at the same mark of 60L. It takes longer to come to this and water is becoming colder gradually, but not as quick as it would be if the pump was on. Ive done it again just now to make sure that it does this.

 

[Whyme]

if it runs out before you fill a bath you need a bigger fe tank but that doesnt explain why the cylinders not heating fully..where on the cylinder is the immersion if its a top tappin is the immersion long enough..just a thought.

 

[totcambiu]

Lets get back to the cold water storage tank. What is the purpose of it ? Why not to connect cold water pipe directly to the cylinder?

 

[tamz]

Firstly that storage tank is too small to be feeding a cylinder (sounds like a 4/10 header tank) even if it wasn't pumped if that is infact what it is feeding.

You will never get the full capacity of a cylinder. How is it heated?
If it is by electric immersion the amount of water you get will depend on the length of the immersion.
I presume your cylinder is a 1050mm x 450mm? A std 700mm immersion will only heat around 100 litres of that overall capacity in around 2 hrs (longer if it is scaled up).

 

[Dannypipe]

Yeah. It took a minute for the penny to drop but then it is late.

I didn't think about the fact that the cylinder is direct (heated by an immersion heater for the OP).

You will not heat water below the element. So if it's top entry you need to check the length of the immersion heater. Even if it's 27" it still won't give you the full capacity of the cylinder.

Side entry immersions are better, as they're often sited lower in the cylinder.

If you have dual side mounted immersion heaters, are you certain that it is the lower one doing the heating?

 

[totcambiu]

if it runs out before you fill a bath you need a bigger fe tank but that doesnt explain why the cylinders not heating fully..where on the cylinder is the immersion if its a top tappin is the immersion long enough..just a thought.

It might be my explanation, but cylinder not heating fully is not an issue here. It gets real hot, i measured it and my kitchen thermometer shows 85C at the tap end. The problem is that the total amount of hot water stored in the cylinder is about 60L. Its like putting a half empty kettle on instead of full one.

 

[Dotty]

Lets get back to the cold water storage tank. What is the purpose of it ? Why not to connect cold water pipe directly to the cylinder?

Because it's not designed to deal with the pressure.

But that isn't your problem.

 

[Dannypipe]

Beat me to it Tamz.

 

[tamz]

Lets get back to the cold water storage tank. What is the purpose of it ? Why not to connect cold water pipe directly to the cylinder?

This is what would happen.
[video=youtube;68p4ngS-yME]http://www.youtube.com/watch?v=68p4ngS-yME[/video]

You can get unvented mains fed cylinders with all the associated safety controls which are a completely different thing.

 

[totcambiu]

Yeah. It took a minute for the penny to drop but then it is late.

I didn't think about the fact that the cylinder is direct (heated by an immersion heater for the OP).

You will not heat water below the element. So if it's top entry you need to check the length of the immersion heater. Even if it's 27" it still won't give you the full capacity of the cylinder.

Side entry immersions are better, as they're often sited lower in the cylinder.

If you have dual side mounted immersion heaters, are you certain that it is the lower one doing the heating?

Immersion heaters ... is the same thing a thermostat thing that heats the water? That is the next thing he is going to replace next week. There are two of them on long one and one short one.

 

[Whyme]

It might be my explanation, but cylinder not heating fully is not an issue here. It gets real hot, i measured it and my kitchen thermometer shows 85C at the tap end. The problem is that the total amount of hot water stored in the cylinder is about 60L. Its like putting a half empty kettle on instead of full one.

if its a 144litre cylinder and your only getting 60litres hot then its not heating fully...hw drawoffs are at the top of the cylinder and if it wasnt full water wouldnt come out of the pipe at the top..

 

[Dannypipe]

Immersion heaters ... is the same thing a thermostat thing that heats the water? That is the next thing he is going to replace next week. There are two of them on long one and one short one.

I think you need to leave it to the plumber....a different plumber. This should be resolved in one visit. You shouldn't have water at 85 at your outlets as you said you do. (its dangerous). It sounds like you may have a heating feed/expansion tank instead of a proper cold water storage tank...and it sounds like your plumber is now guessing what it might be.

As said, you will not get the full cylinder capacity as the whole cylinder won't get heated by an immersion heater.

Put some photos up. That might help us.

I'm off to bed now, but good luck with it either way, and please keep us posted.

 

[Howsie]

It might be my explanation, but cylinder not heating fully is not an issue here. It gets real hot, i measured it and my kitchen thermometer shows 85C at the tap end. The problem is that the total amount of hot water stored in the cylinder is about 60L. Its like putting a half empty kettle on instead of full one.

85 at the tap?! Get a new thermometer.

 

[totcambiu]

Firstly that storage tank is too small to be feeding a cylinder (sounds like a 4/10 header tank) even if it wasn't pumped if that is infact what it is feeding.

You will never get the full capacity of a cylinder. How is it heated?
If it is by electric immersion the amount of water you get will depend on the length of the immersion.
I presume your cylinder is a 1050mm x 450mm? A std 700mm immersion will only heat around 100 litres of that overall capacity in around 2 hrs (longer if it is scaled up).

Ok but does the FE tank makes a difference? Will the cylinder still be full of water even if the FE tank is small?

 

[totcambiu]

Can you elaborate why? I believe it is an accurate figure, because if is stick it into the boiling water it shows 99c

 

[Whyme]

if it is accurate then its very dangerouse it should only be 60c, if the cylinder isnt full water wont come out of the top pipe. have you got any pics cos i think that will give everybody a better idea of the setup cos its mainly educated guesses if not.

 

[iiplumbing]

Wow mythbusters video

Thats why you need to be certified to deal with unvented tanks!!

Back to OP.

If your hanging a pump off of a hot water cylinder you need a cold water storage capacity of at least 50 gallons (365litres) otherwise you will run out of cold water let alone hot water.


85 degreee hot water wow, what are you trying to do heat your house via the hot water tank, that is soo dangerous almost instant scalding for you and third degree burns for elderly or children

EXPOSURE TIME TO RECEIVE A SEVERE BURN​

Celsius Temperature Fahrenheit Temperature 2[SUP]nd[/SUP]Degree Burn No Irreversible Damage 3[SUP]rd[/SUP]Degree Burn Full Thickness Injury 45° 113° 2 hours 3 hours 47° 116.6° 20 minutes 45 minutes 48° 118.4° 15 minutes 20 minutes *49° *120° 8 minutes 10 minutes 51° 124° 2 minutes 4.2 minutes 55° 131° 17 seconds 30 seconds 60° 140° 3 seconds 5 seconds

The questions you are asking fill me full of dread and foreboding, get yourself a professional on it mate before you kill someone, sorry if thats critical but we are here to help and save people from themselves sometimes

Russ