15mm and 22mm flow rate

The hot water pipe coming out of my boiler is 15mm. Logic tells me that having a 22mm pipe further down the line won't magically increase the flow rate. Is this correct?

Just that I noticed my bath taps have 22mm pipes connected to them, but if I follow the pipes, there are reducers to 15mm.

Conventional to combi system upgrade by any chance???

Maybe at some point in the properties life, as it's over 100 years old. Was converted into 4 flats in 1980.

I only just bought it and have just fitted a new combi, but the one I took out was a 10 year old combi too. So maybe the one before that.

What's your point? Do conventional boilers have 22mm hot feeds out of them?

You personally fitted the combi?

If so you should know the answer.

steelmasons said:

You personally fitted the combi?

If so you should know the answer.

Click to expand...

If I fitted the combo, then I'd be a gas safe engineer and plumber, and wouldn't be on here asking this question !

I have just 'had' fitted a new combo.

15mm h/w pipe is fine on a combi, no point having larger. The flowrate you can get out of it is determined by the size of the burner and heat exchanger.

Cool, I have no problem with the 15mm pipe, my question was more theoretical - do you know the answer?

yes

It won't magically increase the flow rate coming through the boiler, though it will marginally decrease the resistance in the pipes,, so marginally as to be unnoticeable.
It will increase the volume of cold water sitting in the pipe between the boiler and the tap though, so you'll have to wait longer for the hot to come through and waste more water with 22mm than you would with 15mm.

Cool, good answer. Didn't think about the volume of water sat there.

Another question I have if you could answer please. Over the years, plumbers seem to have left behind pipes that go nowhere. What's the consequence of this? If I have a hot water pipe spurring off for 5 metres and capped at the end?

It's a dead leg and should be cut out to prevent water stagnation.

That's what I thought. I'll do that.

Thanks

bandejapaisa said:

Cool, good answer. Didn't think about the volume of water sat there.

Click to expand...

By my calculations, one meter of 22mm copper pipe (1mm wall thickness) holds 314.16 ml of water, or almost one third of a liter. This soon adds up, with 10 mtrs of 22mm pipe holding over 3 liters of dead water.

So every time you've been running hot water, you are potentially wasting the energy used to heat that extra quantity of water.

15mm Pipe I believe (0.7mm wall thickness) will hold only 145.27ml of water per meter, or basically half that of 22mm copper pipe.

I ran 10mm (0.7 wall thickness) in from my combi boiler to my kitchen tap, because this only holds 58.09ml per meter, (or 581 ml for a 10 meter pipe run)... just less than one fifth the dead water capacity of the 22mm pipe.

Of course this was OK for my kitchen tap flow-wise, but my bath tap and shower get 15mm feeds to reduce the flow resistance that I would have encountered with the 10mm pipe. anyone used 8mm or even 6mm micro-bore pipe for something like a feed to an en-suite hand-washing basin or similar? I guess they give diminishing returns compared with moving from 15mm to 10mm, but with much worse flow restriction.

You cannot make meaningful scientific comments on this.

The effect on the flow rate of different resistances depends on the supply pressure.

That typically varies between a static pressure of 1.0 bar and 10 bar in domestic properties!

Tony

Agile said:

You cannot make meaningful scientific comments on this.

The effect on the flow rate of different resistances depends on the supply pressure.

That typically varies between a static pressure of 1.0 bar and 10 bar in domestic properties!

Tony

Click to expand...

Why not? If you treat the pressure as a constant, and assume the only variable is the pipe width.

Agile said:

You cannot make meaningful scientific comments on this.

The effect on the flow rate of different resistances depends on the supply pressure.

That typically varies between a static pressure of 1.0 bar and 10 bar in domestic properties!

Tony

Click to expand...

Tony, I think my comments were quite in order for my given scenario (my own dwelling). Please correct the assumptions in my post if you think not.

I was simply indicating that in my house the supply pressure is sufficient to deliver more than enough hot water flow to my kitchen sink through about 8 mtrs of 10mm copper pipe, where in comparison this flow-rate would be less than ideal for my bath, hence I'm using 15mm pipe for that.

My post however, was mainly trying to indicate the relative static volume of pipes of differing diameter, and hence the potentially wasted energy involved in their use. This goes for water wastage too.

Obviously you clearly understand these concepts, but it may not occur to the layman.

For example, if I'd just run an amount of hot water off through a 22mm pipe, which was well lagged, then the water content of that pipe may indicate completely wasted energy if I didn't then draw any more water for say 30 minutes, as that water-mass would have lost too much energy to be at a useful temperature. If however I'd have drawn hot water within only say 5 minutes, then the water in the pipe may still have been at a useful temperature to me. Conversely, a thinner pipe, containing a relatively lesser capacity of water will not hold as much energy, so may be too cold after just those initial 5 minutes, but then much less energy has been wasted if my next demand is not for 30 minutes.

I'm not a plumber, or a heating engineer, but I am a properly qualified 'Engineer' (meaning a degree qualification in a proper engineering discipline, plus 25 years industry experience)

I'm really interested to know about your experiences out there... how often do you see literally 10 bar static mains pressure in domestic properties... isn't this rather exceptional? I remember a whole housing estate in the news because their pressurized hot water cylinders were all developing leaks because the mains was coming in at 10 to 12 atmospheres.

regards