Bath running out of cold water halfway through running.

[Richard1972]

Hi all,

We have a low-pressure gravity system, with the water tank in the loft. We have always had a problem with cold water running out (i.e. if my wife has a bath and then I have a shower straight after (we have a power shower), more often than not the shower runs out of water halfway through - the cold water, not hot).

We had a massive water leak this week, which has resulted in a new water tank being fitted. It is smaller than the previous one, but still guarenteed to be a standard size for a 5 bedroom house (we have a 4 bedroom house).

Now, when we run a bath with both hot and cold running, it runs out of cold about halfway through filling the bath. The only way around it it seems is to run just the hot water first, and then turn that off and run the cold water. The water is not being used for anything else (i.e. this is first thing in the morning, there is only two of us etc).

I don't think the fitting of the new tank is the fundemental issue (i don't think so anyway, but who knows) as we have always had this kind of problem. It seems to be that the tank is not filling up quickly enough.

Can anyone offer any advice how this could be resolved in any way ?

Many thanks,
Richard

 

[SimonG]

Cold water tank undersized or low mains pressure that cant replenish tank quick enough or problem with tank inlet valve.

 

[Ben-gee]

Should be 50 gallon minimum.
Was a new ball valve fitted with the new tank - should have been but you never know.
If the ball valve is new and it’s still slow to fill due to low mains pressure , try swapping the white plastic valve seat for a red one.

 

[snowhead]

Does the water bounce off the bottom of the sink in the kitchen when you turn on the cold tap?

Is there a Stop tap in an airing cupboard below the tank or elsewhere that's part turned off, assuming there is one in the loft that's been used to isolate for the new tank?

Are there any new style Isolation valves used on the feed to the tank?

Is it copper or plastic pipe that feeds the mains cold to the tank?

 

[Richard1972]

Should be 50 gallon minimum.
Was a new ball valve fitted with the new tank - should have been but you never know.
If the ball valve is new and it’s still slow to fill due to low mains pressure , try swapping the white plastic valve seat for a red one.

Thanks very much. I have gone up there today and measured - its 70cm x 50cm x 50cm. Which by my calculations means its a 38 gallon tank. Which is probably the answer.

 

[SJB060685]

It sounds like one of the above. I will also add it sounds like the feed to cylinder is tapped in lower in the header tank than the cold feed to house. This is not desired and should be the other way round to prevent momentary scalding in a case such as this.

 

[Ben-gee]

Thanks very much. I have gone up there today and measured - its 70cm x 50cm x 50cm. Which by my calculations means its a 38 gallon tank. Which is probably the answer.

You might think that, but it is actually only a 25 gallon tank; only approx 2/3 of the tank volume is “useable”.

And yes if you have a pump it is definitely not big enough.

 

[Richard1972]

You might think that, but it is actually only a 25 gallon tank; only approx 2/3 of the tank volume is “useable”.

And yes if you have a pump it is definitely not big enough.

Thanks for that. Ok - so i have gone up today and checked the water level. The water tank itself is 47cm high. But the water only comes up to 23cm. How high would you expect it to come up ? Using 2/3, I would make that 31cm. I pushed the ballcock down, filled it to 33cm and ran the bath. It filled up just fine......just. Any less and I think we would get problems. But if it filled to about 33 I think we would be ok and not need a new tank. Presumably the ball-cock can just be altered somehow to allow more water to fill up ?

 

[Ben-gee]

I would get back to whoever fitted your new water tank and ask them why they fitted a smaller one than the existing, and explain that you’ve had problems ever since.

 

[SimonG]

I would get back to whoever fitted your new water tank and ask them why they fitted a smaller one than the existing, and explain that you’ve had problems ever since.

Probably the only size they could get through the loft access? I know it's a poor excuse with coffin tanks and ability to twin tanks etc.

But doesn't explain why they didnt take the time to set the inlet valve level correctly.

 

[Chuck]

25 gallons is roughly 110 litres, which when mixed with hot water at 60°C should yield about 200 litres of bath-temperature water at this time of year, i.e. two and a half 'average' baths, which are said to take ca 80 litres. And that's if there was no filling occurring while the bath is running.

Do you have a hot water cylinder or a combi boiler? If the former, it's quite possible that when running the bath your combi is starving the tank preventing it from filling but, even so, 25 gallons should be plenty unless you have a huge bath and fill it with cold water. (Apparently cold baths and showers are becoming fashionable again...)

I suspect that a new float valve correctly adjusted would just about fix the problem. I suspect that you are suffering from low flow / pressure so you might want to get your plumber to measure the static and dynamic pressures and maximum flow rate for your supply and check it's within the Offwat specifications.

 

[Richard1972]

25 gallons is roughly 110 litres, which when mixed with hot water at 60°C should yield about 200 litres of bath-temperature water at this time of year, i.e. two and a half 'average' baths, which are said to take ca 80 litres. And that's if there was no filling occurring while the bath is running.

Do you have a hot water cylinder or a combi boiler? If the former, it's quite possible that when running the bath your combi is starving the tank preventing it from filling but, even so, 25 gallons should be plenty unless you have a huge bath and fill it with cold water. (Apparently cold baths and showers are becoming fashionable again...)

I suspect that a new float valve correctly adjusted would just about fix the problem. I suspect that you are suffering from low flow / pressure so you might want to get your plumber to measure the static and dynamic pressures and maximum flow rate for your supply and check it's within the Offwat specifications.

Hi Chuck,
Thanks for the above message. So a 25 gallon tank should be enough then you think. That is effectively what the plumber said when he fitted it. It's quite confusing as a couple of others have said i should have a 50 gallon minimum.

I have a hot water cylinder in the airing cupboard (not a combi). The bath we own is just a standard sized one, nothing fancy

Regarding the float valve, this is probably a really stupid question, but can you adjust the one already fitted so that it doesnt stop the water until the tank is 2/3 full (which I understand is about where the cutoff should be). Or, would a new one have to be purchased, and re-fitted etc.

Many thanks
Richard