Hot Water going into cold water tank

[Mark_H]

A few years ago we had an extra hot water cylinder fitted in the loft, chained with the old one. In theory, I am told, it should draw from both at the same time…? Also I have two cold tanks chained together, each one feeding its own hot water cylinder. But one cold tank is considerably hotter than the other, despite there being a pipe at the base that joins the two. Also the feed from that tank to the cylinder is very hot....?

Ever since that installation we have had problems with the cold water in the header tank being hot! So to solve this we turned the hot water temperature down on the old boiler (you could control the heating and hot water temperature separately). This seemed to solve (mask is probably a better word) the problem, until we had a new condensing boiler installed this year , with only one temperature control, that does both heating and hot water. Since then the old problem has re-occurred and the cold water is hot… so much so, that I need to run the power shower on full power for 10 minutes (no exaggeration) before I can even get close to it!!!!

I have had two new cylinder thermostats fitted, and another zone valve so both tanks are controlled separately, yet the cold water is still getting hot!!

Finally I spoke to my plumber again, who is confused, and his last guess is that the vent on the top of the cylinders is too short. Now when the new tank was fitted the previous plumber created an inverted “T from the tanks to the cold water tank. Thinking about it, you now have twice the volume of water using the vent pipe when it is expanding, with not very little extra available volume… make sense? So, his solution is to use a wider pipe and maybe “S”nake the pipe to the cold tank to increase the length as there isn’t enough head room in the loft. Hopefully this will provide sufficient level of expansion, without it going into the cold water tank. Are there recommended guidelines for this?

Does it sound like this is a likely cause of the problem, although i am not totally convinced. If so, is this a viable solution?

Many thanks for your help!

Mark

 

[Nige F]

any chance of a diagram ..............so you have a plastic cistern 50 gallons...in your roof full of hot water DO Whatever is needed to STOP this occuring NOW..........or you could find 50 gallons of Scalding water through the ceiling..............THIS HAS HAPPENED AND KILLED someone before now

 

[Mark_H]

any chance of a diagram ..............so you have a plastic cistern 50 gallons...in your roof full of hot water DO Whatever is needed to STOP this occuring NOW..........or you could find 50 gallons of Scalding water through the ceiling..............THIS HAS HAPPENED AND KILLED someone before now

I will try and draw a diagram - althought there are pipes everywhere!!!

I have turned the temperature down on the bolier, which stops the hot water going into the cold water cistern - thanks for the heads up!! Especially as the tanks are directly above our bed in the loft!!

 

[Mark_H]

Bump

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No use doing that, you haven't provided a picture yet.

Mod Rupert
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[Bamber gaspipe]

The open safety vent terminates over the feed cistern & has to be a certain height over the cistern water level to ensure that it does not overflow into the cistern.
It`s 40mm per metre of head plus 150mm so if the height from the floor to the base of your tank is 3 metres then the calculation would be 40mm multiplied by three & add 150mm. so the height of your vent pipe from the top of your tank would be 270mm ( in this scenario).

 

[dotto]

The open safety vent terminates over the feed cistern & has to be a certain height over the cistern water level to ensure that it does not overflow into the cistern.
It`s 40mm per metre of head plus 150mm so if the height from the floor to the base of your tank is 3 metres then the calculation would be 40mm multiplied by three & add 150mm. so the height of your vent pipe from the top of your tank would be 270mm ( in this scenario).

As far as i know, the distance to take into consideration is that between the water level in the open safety vent ( which is the same as that in the CWSC) and the point where the cold feed enters the cylinder. Bamber Gaspipe's calculation is correct. On average, the extra length of open vent pipe to run from its water level and the centre point of its U bend going over the CWSC is about 250 mm...........so Bamber gaspipe was nearly spot on.