Mira Mode shower and bath filler tank size

[Ben565]

I've posted on the building section about a cold water tank in the loft ( here ) but I thought it's worth asking more specifically in the plumbing forum about the tank size and whether I need one that big.

It's a Mira Mode digital Shower and Bath filler, low pressure pumped model as I have a gravity fed system. In this setup the cold water for the bath comes from the tank in the loft, the hot water comes from a hot water cylinder in the airing cupboard which is refilled from the same tank in the loft. So effectively all the water in the bath comes from the tank in the loft.

The installation instructions specify minimum 230L cold water tank. Well, the flow rate is 12L a minute, and the bath takes 11min to fill so that's 132L for a start not 230L and the tank is being refilled at the same time from the mains cold water. I'm not sure what the fill rate is at mains pressure but surely it would be enough to prevent the tank running dry ?


I live by myself, there's no other demands on the tank at the same time as the bath or shower is running. And to be honest filling the bath uses up all the hot water anyway.

Refitting the original 25 gallon tank which had been there for decades with no problems, would make me feel better about the weight on the ceiling. Just a question of whether my maths work for the flow rates and tank capacity ? Mira are very specific DO NOT install a tank less than 230L

 

[Harry Bloomfield]

Refitting the original 25 gallon tank which had been there for decades with no problems, would make me feel better about the weight on the ceiling.

25 gallons is around 125 litres. Assuming your hot water cylinder is around 120 litres, it suggests your header tank is a little too small - it is normally suggested that the tank capacity is 150% of the cylinder capacity, so it cannot in use be completely drained. So a header tank of say 170L would be appropriate.

Hot water header tanks, are normally installed over a well supported part of a ceiling, usually directly above the solid airing cupboard structure, or over a structural wall. Where is yours located?

 

[Ben565]

It's not possible to fit it above the airing cupboard, and not possible to site it above a supporting wall either unfortunately. There's a picture of the current 230L tank in the post I linked to, but it's sitting on a platform, on 3x2 joists with a 9ft span. The less weight the better. There is a supporting wall along the long edge of the tank, but but it;s not on it.

 

[Harry Bloomfield]

There's a picture of the current 230L tank in the post I linked to,

having looked at your earlier link - If both hot and cold, are sourced from that header tank, then yes - the tank will need to be larger. You will need to find a way to have that tank properly supported, and over structure, to give it some proper support.

 

[Madrab]

The key with a cistern @ say ~200Kg laden weight is not normally the ceiling joists but how the weight of the cistern is spread. Standard ceiling joists should be able to carry that kind of weight without an issue if it's set correctly. The key is that a cistern @ that size, should be sitting on nice thick bearers that span multiple joists (min 3 I believe) and then sitting on a top that is water resistant and at least 18mm thick floor grade material.

That allows a balanced spread of the weight. From your description it looks like the cistern was just plonked onto the attic floor with most of the weight sitting on a single span.

So chances are it's been an installation issue rather that the cistern being too big that's caused the problem

 

[Ben565]

In this case the ceiling joists are 3X2 with a 9ft span so probably not up to modern code standards as it is. I would imagine standard ceiling joists over that span in a more modern house are a bit beefier than that.

The tank is sitting on a 20mm plywood base, which is supported by three 1.5x2.5 bearers across five 3x2 ceiling joists.

The plywood platform is bowing in the middle as are the bearers. Putting a 6ft spirit level across the five ceiling joists, the middle three aren't touching the level. The gap is about 10mm at it's worst.

It's a 1920's house, it's old, and the joists aren't especially level, and yes probably not a brilliant install either. A 25 gallon tank would be less of a concern, and that's what was there for decades previously without a problem. The new tank has only been fitted when the new bath filler and shower was installed.

 

[Harry Bloomfield]

The new tank has only been fitted when the new bath filler and shower was installed.

Who fitted that new tank?

 

[Ben565]

A plumber who was subcontracted by the builder who did the bathroom.

Guidelines aside, I'm still not sure why I need the 230litre tank. The flow rate for the bath filler is 12L a minute, and the bath takes 11min to fill so that's 132 litres, and the tank is being refilled at the same time from the mains cold water say 5 litres/min.. If I put a 25 gallon tank back in it's not going to run out.

Although I'm not sure how much of that 25 gallon tank is actually usable, usually it's a bit less than maximum I think.

 

[Harry Bloomfield]

A plumber who was subcontracted by the builder who did the bathroom.

Could you not get them back, to do the job properly, it seems that installation is quite dangerous, in fact deadly if it breaks through the ceiling?

If I put a 25 gallon tank back in it's not going to run out.

It likely would, if the water went off, whilst you were filling the bath.

 

[Ben565]

Could you not get them back, to do the job properly, it seems that installation is quite dangerous, in fact deadly if it breaks through the ceiling?

I don't know who it was, I'd have to go through the builder to find out, but would you want him back ? it was 6 years ago too.

It likely would, if the water went off, whilst you were filling the bath

In theory yes, in reality how likely is that to happen though ? I've lived here 48 years and it's not happened yet. For 30 years there were 3 of us with a 25 gallon tank. It's only this new bath filler and shower that's prompted the bigger tank. And I'm here by myself now too.

It's not ideal I agree, but the current install is potentially dangerous, there's no suitable supporting walls or structures I could relocate it to. The only other option would be to fit bigger ceiling joists specced to take the weight, maybe 8x2.

 

[Ben565]

Just an update, I got a response from Mira after I presented my findings, flow rates and measurements for my specific system. They eventually admitted the 230 litre tank is more of a guideline than a strict requirement as the install guide would suggest, but if the tank runs dry and the pump is damaged they will consider it an installation fault.

Fair enough, so, I think they are covering themselves by overspecifying the tank size. I don't think there will be an issue going to a 25 gallon tank.

A 25 gallon coffin tank is all that will fit through the loft hatch now, but that actually benefits me because the reduced height mean it can be shifted across slightly to be more over the supporting wall.

 

[Ben565]

Got quoted £1,000 to swap the 50Gal tank to a 25gal tank. That includes VAT + supply & fit the new tank. Seems a bit high to me, but what do I know. Any thoughts ?

 

[Harry Bloomfield]

Got quoted £1,000 to swap the 50Gal tank to a 25gal tank. That includes VAT + supply & fit the new tank. Seems a bit high to me, but what do I know. Any thoughts ?

DIY

 

[Madrab]

When you can get a suitably sized 25gal coffin Cold water cistern for not much more than £100 then what the feck are they charging £900+ for??

All they would be doing is > isolate current > drain > disconnect > build up new > reconnect >check and test. Not a lot of time or effort in that. If the existing is plastic then once empty it could be left or you could cut it up yourself and remove.

Don't usually discuss costs online as not onsite but in this instance I would say £1000 is pretty excessive.

 

[Ben565]

In this case a 25gal coffin tank is needed to fit through the loft hatch. And they are about £250 (inc vat) but still, thats a lot of labour costs.

Just had a 2nd quote for £950

I was going to leave the old tank up there (empty of course) and then either cut it up myself to get out... or just leave it.

As a stop gap measure I'm tempted to bend the ball cock arm so it doesnt fill the tank as much.