Advice for hot water cylinder connection with a pump

Joined
30 Sep 2004
Messages
126
Reaction score
1
Country
United Kingdom
Advice for hot water cylinder connection with a pump

We are having a complete new hot water system installed upstairs with loft tanks and a large capacity indirectly heated cylinder. Because the bath is quite large we are going to supply it with pumped hot and cold water, which will also feed to a separate shower. This means in effect all the hot water is pumped.

The plumbers connected the pump to the cylinder on Friday via a side tapping about 1/4 of the way down from the top. The boiler has not yet been commissioned and so at the moment the cylinder is heated via a top mounted immersion heater.

This morning we tried to have a bath in the existing normal bath, but the hot water ran out very quickly. This is obviously because the draw off point is only just below the immersion heater. The plumber has advised that when the boiler is working, the whole cylinder will be hot and so it won’t be a problem.

However, I am concerned that we are going to lose out on the hot water at the top of the cylinder. In effect because our draw off point is 3/4 of the way up, we can’t access 25% of the hot water.

The plumber says this is not a problem because the boiler coils will cause the hot at the top to be circulated round. I am a bit sceptical – I think that as the hot is drawn off, cold will come in at the bottom until it reaches the draw off point. At that moment, the hot will have run out, yet there is still plenty left above it. The cold will start to circulate and mix with the hot at the top, but that just means it will take less time for the cylinder to reheat, but that in terms of how much hot you can use before it runs out and you have to wait, we are losing out.

Is my understanding correct and would it be better in this scenario to take the draw off from the top ?

The pump is a Stuart Turner Monsoon 3 bar.
 
Sponsored Links
He's right so go easy on him. The reheat time depends on the coil in the cylinder, which you can usually find out from the label.

If you need more electrically heated water for backups, just get him to install a longer immersion heater.

So, why didn't you go for an unvented cylinder so you could high pressure to all the house and do away with a pump?
 
To reinforce the point:
when the boiler heats the coil in the cylinder, convection currents in the hot water part of the tank will tend to stir it up, so the whole tank contents will get up to roughly the same temperature. The immersion will only heat the top bit that it actually reaches (as you point out). If you've got a bottom tapping for an immersion heater, use that instead (but be prepared for bigger electricity bills!).
 
Thanks guys for your replies. I have already fitted an immersion at the bottom of the cylinder today, but actually that is not my real concern as normally the boiler will be heating it.

My point is thus:
Reason for having a cylinder is to store hot water so that you can use it faster than you can heat it (otherwise go for a combi!). What is important therefore is "how much hot water can I run off before it goes cold", at that point you have to wait for the cylinder to recover and that depends on how good the coil is and the power of the boiler. If the draw off point is 25% down the cylinder then we can never use the top bit - it's like putting bricks in the thing !

So I think that as long as we don't have problems with aeration or the pump scavenging from the vent pipe, then that is where we should connect it.

The installation instructions to me favour the side option because most people would be using a pump just for a shower. In that case you wouldn't be critical in terms of hot water volume and the advantages for the pump in terms of aeration and gravtity looping outweigh the fact that you can't use the water above. In our case ALL the hot is coming out through this side valve.


Also, to htgeng "why didn't you go for unvented cylinder" - basically plumber is old school and everything we are doing is over specced, I am not sure if he is fully up to date with such things, but also we have a very large cylinder due to having a large bath - cylinder is 72" by 27 ", around 500 litres (bath is a whirpool bath without the whirlpool stuff, 500 litres to overflow) - I am not sure if
a) they make such large unvented cyls
b) the mains pressure could supply a flow rate as good as a pump from a tank fed set up ( I changed our old bath to use pumped hot and main cold and the limiting factor was the cold, we had to turn the hot down when filling it )

{We have 2 100 gall tanks in the loft, in case worried about replenishment rate of cold tank with our mains pressure}

Basically we want to be able to fill the 500 litre bath and let someone else have a shower immediately after without running out of hot.
 
Sponsored Links
If you took water from the top of the cylinder, you'd suck air down the vent pipe into your pump.
I daresay your pump will be capable of taking water out of the cylinder at 25 litres a minute. Have you sized the cylinder correctly? At that rate a "standard" 36" x 18" will last 3 minutes or so...

If your feed pipe to the cylinder is 22mm, it might need to be replaced with a 28, to avoid the sucking down of the air even with an Essex flange which you have.

But the height of the flange doesn't determine the height the water's taken from anyway. You have to extend the pipe into the cylinder to avoid bubbles which rise, clinging to the sides. So if you want to bend the intrusive pipe up, or down, nae problem.

"Power" showers are only about gravity and pressure and pipe resistance and how much water and simple stuff like that, but seem to be installed all too often by folk who would rather not work it through.
 
we need to know the rating of the coil in the cylinder and the specification of the boiler. I'd be more concerned with how quickly the boiler can react to you drawing off cold water and get to full output!

You'll have to time this when its all commissioned but my estimate is that it will be a couple of minutes from when you turn the bath tap on.

Yes, you can get 500 litre unvented cylinders that size.
 
The boiler is an oil fired Worcester Bosch 15/19; it will only be heating the hot water as our heating is warm air via a separate boiler. I am not sure on the coil, but we specified a high efficiency one rather than the standard. Cylinder is about as big as we can fit - it's taller than me and fatter ! It holds about 500 litres. I'm hoping for a combined hot/cold fill rate of about 60 litres/minute, so it would probably be 40 hot, 20 cold.

All pipework is 22 mm although the cyldinder itself has 1 1/2 " fittings.

We inherited a similar set up on our old cylinder that was a normal size and the pump attached via the top. However, the plumber says someone had fitted a 1 way valve to the top of the vent pipe and at some point the cylinder had had itself sucked to death so I guess we don't want to go down that road.

Sounds like we may be OK with a vertical pipe inside the cyldinder and experiment with length to see how far up we can go without getting air problems in the pump.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Back
Top