New Glow Worm Combi Low Flow Rate

[rideforever]

@Gasguru
@ScottishGasMan
@denso13
@lightning

On the Xtramax (my old boiler) just checked it has a flow limiter of 16 l/min.
Even though it has a delta 35c of 11.4l/min ... and is 28kw > both less than the Ultimate.
Which is why we are noticing a significant reduction in shower pressure - although it is actually still reasonable.

So it seems that the flow limiter may actually be the way to compare expected shower pressures ??

I am not sure if removing the flow limiter is going to improve the situation ... how can I know that. At max hot it is doing 10l/min from the kitchen tap.
Is the question then, when I am in a shower if the temp handle is under max hot ... does that mean there is unused heating capacity that would be available if the limiter was removed or perhaps enlarged ??
If I reduce the boiler's hot water temp from 60 to 55, then the shower pressure noticeably drops ... does any of this indicate anything?

 

[Gasguru]

The Xtramax obviously has a higher flow liter due to it's 42 litre storage facility.

If the boiler temperature is set to 60 degrees the 10 litre/min flow limiter will not cause the boiler to modulate its kW power down so you will be using it at full capacity.
By my calcs setting the temp less than 55 degrees will cause the burner to modulate since the summers incomming cold temp is high say 15 degrees.

Feeding the shower valve with 60 ish degree water would rewuire perhaps 35 to 40% cold water to give a suitable showing temperature...and therefore your flowrate should increase.

 

[rideforever]

@Gasguru
So you reckon 60C x 10l/min is the max output?
What's the point of the 10l/min limiter ?
If it's removed will it drop to 55C x 12l/min ??? Is that really how it works, you have to have a plastic disk squeezing the outlet pipe to make the boiler work right?


In the specs it says 12.2l/min continuous flow ... but not the temperature? Does that not mean 12.2l/min at 60C ?


How about this basic question : if a customer wants super powerful showers how can an installer accurately estimate that from the boiler specs ...? How can that be done?

 

[Gasguru]

No I said with a target temperature setting on the front panel of 60 degrees and given a maximum flow rate of 10 litres/min the boiler should not modulate down ie. the boiler will run flat out at 30kW
I've explained before the purpose of the flow limiter.

If you remove the flow limiter the flowrate will be limited by your water pressure, friction within the pipework/boiler and how much the tap is opened...ie it requires the user to be sensible (and since many aren't a flow limiter is fitted).

An installer should carefully check the boiler specification.

If you set the boiler DHW temp to 60 degrees what flowrate (at the desired shower temperature) do you get from the showerhead, you will manually blend in cold (or the thermostatic element will do so)...measure using a measuring jug. Most Weir gauges are useless.

These are flow limiters....an o ring restricts the water through the perforated disc. As the flowrate increases the o ring closes more of the channels until the specified rate is acheived.

https://www.croydex.com/products/sh...-saving-products/shower-flow-limiter-pack/289

 

[bernardgreen]

How do flow limiters react to change in the pressure of the incoming mains water supply ?

Does the flow rate remain the same for a range of pressures. ?

 

[rideforever]

If you set the boiler DHW temp to 60 degrees what flowrate (at the desired shower temperature) do you get from the showerhead, you will manually blend in cold (or the thermostatic element will do so)...measure using a measuring jug. Most Weir gauges are useless.

In the shower at the desired temp it's
14l/min or so.

 

[ianmcd]

just remove the flow limiter and try it, you can always put it back in again, takes literally 10 mins

 

[Gasguru]

.

 

[Gasguru]

The forum formating is hopeless...

A little maths to try and explain...(ignoring pipe heat losses etc)


(Equation 1) flowrate in litres/sec = required kW boiler power / (4.18 X delta T) (delta T being the difference in temperature between cold water in and boiler hot water out)


and...

(Equation 2) kW boiler power required = (cc/sec X 4.18 X delta T) / 1000


Assume the cold water inlet temperature coming into the boiler is around 14 degrees at the moment (obviously it's much colder in the Winter)
The boiler flow limiter limits the hot outlet to 10 litres/min so that's 0.166 litres/sec or 166.7 cc/sec
We want a desired showering temperature of 41 degrees

By re-arranging equation 1 we can calculate the inlet to outlet temperature difference at a flowrate of 10 litres/min with the boiler running flat out.....


(Equation 3) delta T = (kW boiler power / flowrate in litres/sec) / 4.18


Plugging the numbers into equation 3 we get a delta of 43 degrees.
Therefore we must set the boiler temperature to at least 57 degrees (that's a 43 rise + 14 degree cold inlet temp) in order to exploit the maximum boiler power
output (without it modulating down). Any lower a temperature setting and the boiler will modulate down to prevent a temperature overshoot.

So we feed the shower valve with 57 degree water at 10 litres/min

Now to calculate the required cold water flowrate to blend the temp down to provide a 41 degree shower temp.


(Equation 4) cold water flowrate = hot water flowrate X ( [temp(hot inlet) - temp(shower outlet)] / [temp(shower outlet) - temp(cold inlet)] )


Plugging the numbers into equation 4 we get a cold water flowrate of 5.9 litres/min

Therefore we should achieve a shower at 41 degrees with a flowrate of 10 hot + 5.9 cold = 15.9 litres/min ......not far off your measured rate (of course we have pipe losses and you
may have the shower temp higher than 41 degrees).



Now with the boiler DHW temp as before you could remove the boiler flow limiter and only open the shower hot valve, setting the flowrate to 15.9 litres/min and you would achieve exactly the same shower temperature
as before without the need to blend in any cold water...this time the boiler would be operating with a delta of 27 degrees with an outlet temperature of just 41 degrees

15.9 litres/min converts to 0.265 litres/sec or 265 cc/sec

Using equation 1


0.265 litres/sec = 30 kW / (4.18 X (41 - 14))


Using equation 2


30 kW = (265 X 4.18 X (41 - 14)) / 1000


Without limiting the flowrate problems will arise in the winter time when users will open bath taps/shower valves too much and the desired outlet temperature will not be achieved.
Some traditional 3/4 bath taps only require opening an 1/8 turn...any more and the flow is excessive and low bath temps will result...of course users don't realise this.
Therefore to mitigate this situation boiler manufacturers will fit a flow limiter, however I believe the 10 litre/min is too low but it wouldn't surprise me if the approvals required such a low flowrate
in the guise of energy efficiency. You must also bear in mind than most thermostatic mixing valves require the hot inlet temperature to be perhaps 10 to 15 degrees hotter than the mixed outlet
temperature...if not their performance (and possible safety shutoffs) may not work. Again the flow limiter helps to ensure correct operation.

 

[rideforever]

And the exciting conclusion to this silly story :

I have a main shower and an ensuite with a twin shower. The twin shower has been converted to a single head and is better, the main shower has gone from being an enjoyable experience to be a fast and practical shower.
I have to adjust the temp handle maybe 3 or 4 times in an average shower and I am not convinced that a new flow limiter will help; Glowworm also once again reiterated a 2nd time that they were not interested in it being changed.

The 4 installers who gave me a quote all use the advertised 35cRise rate to chose a boiler ... and it is completely meaningless wrt the shower experience. You are better off digging deep into the bowels of the manual and look for what the size of the flow limiter is -> that tells you what you will get.
On the manufacturers/vendors websites they all quote the 35cRise rate and also the "efficiency" as if 0.2 of a percent will make any difference to your bills.
Well it's all total rubbish, installers/manufacturers/vendors none of them will help you choose a good boiler. The only good thing is that it does indeed have a 10 year warranty on it, and is a straightforward machine and not a space age boiler which will be expensive to repair; 10 year warranty as long as the annual service engineer fills in x number of forms every time he visits of course. And also I paid £1900 for the installation in total including a bosch mag filter, so I wasn't ripped off ... that was at least £500 less than any other quote.

And just goes to show you can't trust a single person on this planet for even small things, and everything that people say as advertising is likely to be a bold faced lie. And if the worst thing that happens to you is that you have practical showers for 15 years - well you got off likely. Move along.

 

[bernardgreen]

any more and the flow is excessive and low bath temps will result...of course users don't realise this.

In defence of "users who do not realise"......

Many "users" of a new combi boiler will have previously experienced filling a bath with stored hot water where the flow rate has practically no effect on the temperature of the hot water while there is still hot water in the top of the cylinder.

 

[rideforever]

Anyone who wishes to select a combi based on flow limiter might try typing into Google "boiler spares 14 l/min flow regulator" (or "flow limiter") and varying the number 11,12,13,14,15,16 etc... in order to find a boiler that has a larger one. Of course perhaps someone here knows.

Also I replaced the awful Climapro1 control with a Horstmann single channel standard control. I can only imagine that your energy bills fly through the rafters with one of those thermostatic controls switching the boiler on to heat up empty rooms depending on the weather and what the EU's agendas of the day are - if you have a large wallet ... let alone the fact that one comes as a gigantic empty plastic box with minuscule readout on it.
Just one more idiotic example of progress. So boring !

 

[Gasguru]

Are you a remainer or brexiteer...if the former you have yourself to blame.
EU regs (in the heating industry) are largely driven by the German systems and they tend to be different than over here.

 

[rideforever]

..

The EU is not really German, it does not reflect real German culture it's just a mindless political grave that serves its own purposes, it doesn't really produce regulations in the sense of sane meaningful rule... it just endlessly nags people until they submit !! It belongs to no democracy and it's organisation is isolated behind a wall and functions without any of its subjects knowing how it functions; it presides of the greatest economic and cultural destruction imaginable on the continent.