Bath On Mains Water

[Onoff]

What are the pitfalls of having the bath cold off the mains?

 

Regular readers might recall I've only a tiny 25gal cold water storage tank. The traditional copper hot water cylinder will just about supply enough hot for a bath but the cold can run out before the bath is cool / deep enough.

 

Yes, I know we've discussed UVC/TS/oil combi before but I'm not in a position to do any of those.

 

So...short term... (my short term ) could I swap the bath cold over to mains? Incoming mains is around 8/9bar but I've a pvr on it.

 

Basin cold is on mains as is the wc. Shower is pumped from the same cylinder & cws tank.

 

Cheers

[dpmiller]

Surely it's the mains into the CW tank that's the problem? Stopcock not fully open/ crap in the float valve?

[Onoff]

1 minute ago, dpmiller said:

Surely it's the mains into the CW tank that's the problem? Stopcock not fully open/ crap in the float valve?

 

Don't know. It's only fed via 15mm from the incoming if that makes a difference? 

 

Empty it and see how long it takes to fill?

[dpmiller]

yup.

[Ed Davies]

Bath's off the mains here; no tank except header for the vented DHW cylinder so all the colds are. No real problems except that the electric shower cuts off if anything else is running, even the cistern refilling, presumable as a result of a low pressure/flow sensor. So that makes me wonder whether running cold into your bath at high speed might drop the pressure for anything else which cares about that sort of thing. Other shower? Washing machine?

[PeterW]

Is the bath tap a mixer tap as if it is then it’s a no-no as the pressure difference will cause problems. 

[Onoff]

9 minutes ago, PeterW said:

Is the bath tap a mixer tap as if it is then it’s a no-no as the pressure difference will cause problems. 

 

Non mixer:

 

 

19 minutes ago, Ed Davies said:

So that makes me wonder whether running cold into your bath at high speed might drop the pressure for anything else which cares about that sort of thing. Other shower? Washing machine?

 

Good shout. Yep, WM & DW off mains.

[Nickfromwales]

9 hours ago, dpmiller said:

Surely it's the mains into the CW tank that's the problem? Stopcock not fully open/ crap in the float valve?

Not really. Its only a 25 gal tank which would have emptied out doing the hot water. Then, with like 4 litres per min std on the ball valve, it will be refilling when you go and whack the cold tap on full so doesn't stand a chance.

8 hours ago, Onoff said:

Non mixer:

Not quite. The two bodies of water meet in the outlet pipework so not a true cold mains / gravity hot tap  If the back pressure from running the cold flat out is greater than the outlet gauze / nozzle doodah on the tap spout then it'll start to push against gravity hot, if both taps are ever fully opened. You'll be fine in the interim if you just run the hot first and then shut the hot tap, and then open the cold tap on its own. Another quick cheat would be a single check NRV on the hot until the UVC fairy gets you a used tank for nowt when one pops up  

[ProDave]

40 minutes ago, Nickfromwales said:

Not quite. The two bodies of water meet in the outlet pipework so not a true cold mains / gravity hot tap  If the back pressure from running the cold flat out is greater than the outlet gauze / nozzle doodah on the tap spout then it'll start to push against gravity hot, if both taps are ever fully opened. You'll be fine in the interim if you just run the hot first and then shut the hot tap, and then open the cold tap on its own. Another quick cheat would be a single check NRV on the hot until the UVC fairy gets you a used tank for nowt when one pops up  

I have never understood that argument.

 

You adjust the hot and cold to get the temperature of water you want out of the tap. If you have got to the point the cold pressure is pushing the hot water back, then you would only have cold water coming out of the tap, and you would have turned the cold tap down long ago to maintain the water temperature that you want.

 

So while it is theoretically possible for the cold to overcome the hot and push it back, in the real world if you are trying to run a hot bath, you would never actually let that happen.

 

Our previous 1930's house was plumbed like this and never gave any problem.

[Nickfromwales]

Totally agree, and it may be that that tap has two separate water paths to the spout and the hot and cold never meet, which is a true unbalanced mixer tap ( eg two separate taps combined into one solid tap body ). I also agree that you shouldn't run a hot bath and then pour cold water over it, and my bath is a thermostatic overflow filler so I can get the bath spot on every time with no 'cooling down'.

Unless its SWMBOS bath where its bloody roasting hot. Women.......................they're just weird  

[Ed Davies]

From a water regulations point of view, isn't flow in the opposite direction the possible problem: from hot to cold rather than cold to hot? I.e., there's a theoretical possibility that mucky hot water could flow back into the cold supply. Very unlikely in practice because the cold's normally at much higher pressure but could happen if there's a limitation to the supply and another cold tap open lower in the house so the cold suctions back. So to be completely compliant with water regulations you ought to have a (double?) check valve on the cold side, too?

[Nickfromwales]

14 minutes ago, Ed Davies said:

From a water regulations point of view, isn't flow in the opposite direction the possible problem: from hot to cold rather than cold to hot? I.e., there's a theoretical possibility that mucky hot water could flow back into the cold supply. Very unlikely in practice because the cold's normally at much higher pressure but could happen if there's a limitation to the supply and another cold tap open lower in the house so the cold suctions back. So to be completely compliant with water regulations you ought to have a (double?) check valve on the cold side, too?

In a nutshell, yes.

The fact is that the tap is currently, and future intention compliant. Its just in the interim when matey-boy want to hook the cold mains up does the issue arise. As theres an air-break, eg the spout of the tap, I doubt syphon would / could ever occur here.