15mm and 22mm flow rate

[sadanorakman]

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

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

I'm with you on this one... for domestic flow rates (and pressures), I would think that the relationship between pressure and flow through any given length of static plumbing is relatively linear. Given that the viscosity of potable water is likely about the same in lands end as it is in john o-groats!

 

[footprints]

I have just had the boiler change to a combi and I am considering replacing the 22mm in 15mm to reduce the amount of water I have to run off before the hot gets through, because of the long runs involved.

The main reason bath taps are 3/4" is to give a good flow rate from a tank supply as soon as you go to mains pressure 15mm is fine.

 

[Agile]

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

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.

regards

You are quite correct in relation to YOUR own property.

Unfortunately the way forums work is that every comment is meant to be addressed to the OP ( starting posting ).

Therefore what applies to YOUR property will not be very relevant to the OP.

Domestic supply pressures are usually between 1.0 bar and 3.5 bar. But they can increase greatly during the night when no water is being used.

A few areas do have pressures of around 10 bar.

At the Vaillant training centre at Dartford they told us the pressure there was 12 bar.

In any practical case there are three variables, supply pipework resistance from the street, boiler internal resistance and the resistance of pipework to the taps. In an ideal case the tap is controlling the flow up to the maximum required from that outlet.

Furthermore, if you are addressing plumbing according to the textbook approach then you need to limit the flow rate to what is appropriate for each type of tap. That's saves water wastage and avoids splashing.

Few bother to do that but to get best performance from a combi boiler its necessary to limit the flow rate at each hot water outlet. There are small flow limiters that you can buy. They use a rubber band!

Tony

 

[sadanorakman]

I have just had the boiler change to a combi and I am considering replacing the 22mm in 15mm to reduce the amount of water I have to run off before the hot gets through, because of the long runs involved.

The main reason bath taps are 3/4" is to give a good flow rate from a tank supply as soon as you go to mains pressure 15mm is fine.

Well then the 15mm pipe will certainly only hold half the amount of water, and I don't personally think that it's flow resistance would be a problem, unless (just maybe) your dynamic pressure is so low as to make the use of a combi boiler a marginal choice in the first place.

Just another thought you might be interested in... I have fitted a combi-save to my newest boiler, but this boiler doesn't get commissioned yet until Monday, so I'm not too sure how genuinely beneficial this device will be just yet.

Basically it works in a similar manner to an old-fashioned car's diaphragm thermostat... in this instance it sits directly at the DHW output of the boiler, and forcefully restricts the flow of water through the combi whilst the water is coming through cold, which allows the boiler to more rapidly heat up the water that is within it. When the water starts to emerge hot, it progressively opens up to increase the flow through the boiler. It means less wastage of water, and more immediate delivery of water at a useful temperature.

The very same could be achieved manually, by disciplined tap use... only opening a tap enough to allow the boiler to fire, and then gradually opening the flow more over the following 30 seconds, but how many people would consciously do that? (OK, I used to do this in my old house, but couldn't expect the kid and missus to have the mechanical empathy do it!)

Anyway, I thought I'd try one out for only about 45 quid.

 

[sadanorakman]

Few bother to do that but to get best performance from a combi boiler its necessary to limit the flow rate at each hot water outlet. There are small flow limiters that you can buy. They use a rubber band!

Tony

I attach turn-offs before every one of my taps, so I can manually throttle the flow back to stop splashing, and also balance the flows.

The Atag boiler I just bought came with a little nylon insert to limit the DHW flow. The translation into English from the Dutch manual would make you believe it has space-age properties like a flux capacitor, but all it really does is manually restrict the water flow! I left it out!

Tony, do you have any experience of using combi-saves? will I be disappointed with the results? I hope not.

 

[Agile]

You should not have removed the boiler's flow limiter.

Its purpose is to limit the flow through the boiler to what the boiler has the power to heat effectively.

The combisave does no more than a careful user can do by limiting the flow until the boiler output has warmed up.

The Potterton Puma boiler had one built in. Oddly the Combisave man was not aware of that.

Many boilers have a hot water preheat which significantly reduces the warm up time, albeit at the expense of higher gas consumption.

But a more simple solution is to put lagging on all hot water pipes and particularly where 22 mm has been left in place as there is more heat to be lost. Rarely done though.

Tony

 

[sadanorakman]

You should not have removed the boiler's flow limiter.

Its purpose is to limit the flow through the boiler to what the boiler has the power to heat effectively.

The combisave does no more than a careful user can do by limiting the flow until the boiler output has warmed up.

The Potterton Puma boiler had one built in. Oddly the Combisave man was not aware of that.

Many boilers have a hot water preheat which significantly reduces the warm up time, albeit at the expense of higher gas consumption.


Tony

I didn't remove the flow limiter Tony, there isn't one fitted to the boiler by default. It comes shipped in a zip-top plastic bag and they give you the option to fit it if you want to.

The Atag A325ECX has a gas saver module built in (Flue Gas Heat Recovery Unit), and is supposedly capable of 15 ltrs DHW per minute at 35 degree delta, burning only 34 kW of gas. alternatively it's meant to deliver 18.8 ltrs per minute at a 28 degree delta... my dynamic pressure won't achieve an 18.8 flow rate in the first place!

The manual states:
'In the cold water pipe in the boiler a dosing valve (supplied loose) can be installed if required. The dosing valve ensures that a quantity of water supplied has a guaranteed temperature of 60 °C (assuming a cold water temperature of 10°C). The amount of water is virtually unaffected by the water pressure.'

I had downloaded the manual and read it before buying the boiler, and was hoping that this was indeed a built-in combi-save type device like the Puma has built in, but alas, it is just a fancy nylon collet!

Atag tech support told me it just limited the flow to something low like 6 litres per minute to absolutely guarantee an output of 60 degrees, and that they had argued to have the paragraph removed from the UK manual, as it was misleading.


I already stated that the combi-save does nothing that careful tap control wont do, but good luck to anyone trying to teach my missus careful tap control!


As for the pre-heat, I'm going to start with it disabled to see how long it takes to receive hot water from a cold-start with the combi-save fitted. If it's a ridiculous wait, then I will enable the feature and take the extra gas-use on the chin.

regards