waterflow greatly reduced through combi

[christopher]

My new 38Cdi combi seems to inhibit the flow of water to my bath/shower to a greater degree than I was expecting.

I tested the flow from an outside tap which is directly connected to the mains via its own stop c*ck @ 20 liters a minute, this was inline with a British Gas estimate I received.

The flow coming from the boiler to my bath which is only 5ft away from the boiler is only 9.1 liters a minute. None of the hot water outlets produce more than 10 liters a minute. The cold water supply to the same bath produced 17 liters a minute. The water temperature was excellent.

Flow rates were tested by measuring the time to fill a 4 lit re jug from a running tap

Are these numbers the norm ? or do I have a problem ?

Chris

[GB]

Are you measuring the rate of flow of hot water here? If so, what is the hot water temperature, and what is the temperature of the inflowing water? 38Cdi - what's the heat output? I assume that it gives 100% priority to hot water when there's a demand?

[GB]

Ah, found your WB boiler. The claim is: Central heating output: 30kW Domestic hot water flow rate: 16.4 litres per minute at 35 degree temperature rise (provided adequate mains pressure is available)

So, the question is what is the temperature rise? From your figures, I'm guessing that you have the hot water temperature set at something like

55C, whilst the input will probably be under 5C at this time of year.

[Roger Mills]

Sadly, combi boilers have to obey the laws of Physics. If you work out the how many kilowatts of power are required to raise the temperature of

9.1 litres of water per minute by whatever the temperature difference is between mains input and boiler output, and then compare that with the rated output power of the boiler, you can determine whether or not the boiler is working to spec.

Is the boiler firing continuously while the bath water is running?

[christopher]

My concern is not with the heat output of the boiler but the flow of water. My shower is not powerfull enough i want more water throughput should a combi reduce the flow by 50%

Chris

[Phil]

Maybe not. You ought to be able to get colder water out at higher flows.

Any service valves inline not of the full-bore type? Lots of 15mm elbows/tees/fittings? Small taps?

All can drop flow.

Phil.

[rbel]

As I indicated recently in another thread we have a WB 30Cdi an I have measured the temperature and flow at the shower head at our preferred temperature setting and it was 10 l/min at 40C. I have not measured the bath flow.

Our mains water flow is around 15 - 18 l/min (it fluctuates quite a bit as we are located on the highest point for miles around and the supply has a booster pump which is not really up to the job). The theoretical output of the 30Cdi is 12.6 l/min at a temperature increase of 35C.

-- rbel

[Owain]

turn the hot water temp setting on the boiler down and you should get more water out.

It will be less hot though.

Owain

[Simon Stroud]

On my MAN Micromat EC31S combi boiler (AKA Eco Hometec EC31S) the DHW flow has an adjustable restrictor inside the boiler. When it was first installed (about 10 yrs ago) the hot water flow was initially a "trickle" at first power up. I had to completely open up the flow restrictor to get anything like the expected flow.

Might be worth a look inside to see if there is anything similar?!

Regards, Simon.

[Simon Stroud]

Indeed - 22mm all the way is best! (as per my Man Micromat combi installation)

Regards, Simon.

[Tim+]

Um, no. My mother's kitchen sink is plumbed in 22mm and it takes ages for hot water to get there. Would be far better in 15, or even 10mm. As ever, the answer is "it depends...".

Tim

[Arfa Daily]

I'm not sure that he is correctly figuring the operating principle of a combi. If he did, he would understand that the thing heats DHR on the fly, making heat output and flow rate inextricably linked by the required amount of temperature rise between the input and output water. This is one reason that I don't like them. I have a pumped shower that shifts a lot of water, and if I changed over from stored to combi, then in order to get the same flow rate on a good hot winter shower, I would need a combi that was a lot bigger than was actually needed to run the rest of the DHR and CH. When I finally get round to replacing my old Mexico, I'm going to stick with my stored hot water, 'green' or not ...

Arfa

[christopher]

Anybody know ???

Chris

[Chris Bartram]

It depends. The colder the incoming water, and the hotter you want it, the slower it will flow.

[Roger Mills]

But the two are directly related! If you want the water to come out hot rather than tepid, it can only come at a rate which allows the boiler the heat it sufficiently as it passes through. If you double the boiler capacity, you double the rate at which it can deliver hot water.

I don't think you've answered the question which I asked earlier. When you're running a bath, is the boiler firing continuously? If it *is*, that's your lot - unless the output power isn't adjusted correctly.

[Roger Mills]

The understatement of the year!

[John Rumm]

Without more information we can't say. We need to know the temperature rise as well as the flow rate.

Note also that since with combis the maximum flow rate is inversely proportional to the temperature demanded, there are different approaches used by manufacturers to deal with this.

Some boilers include an automatic flow limiter[1], such that when the heat output capacity of the boiler is reached, the limiter will prevent water being delivered any faster so as to maintain the temperature you have selected.

Other combis don't include this mechanism, and will allow you to draw water at any rate you like, however as you exceed the capability of the boiler to heat it to the desired temperature, the delivery simply gets cooler. Most installers will compensate for this by throttling the maximum flow rate through the boiler using the service valve on the mains cold water inlet. While this approach is ok, it does mean that it is not able to reflect the effects of changing ground water temperature through the seasons, and any fixed setting is hence a compromise. If set to be right for winter, it will be unnecessarily slow for summer etc. Hence you could check the service valve on yours is fully on.

[1] The install manual for yours makes brief reference to one, but provides no indication that it is adjustable.

[christopher]

I retested the water flow to my bath after turning down the water temperature by 80%. There was no increase in the flow , which would indicate that at the time of the original test the flow of water was not being restricted by the boiler to maintain the desired temperature.

Is this a correct interpretation ?

I also repeated the test with the water temperature set to maximum and again there was no change in the flow.

Chris

[geoff]

In message , [e-mail] writes

I think you misunderstood - boilers do not dynamically restrict DHW flow rate, rather the absolute flow rate might be adjusted to ensure that the temperature doesn't fall below that expected

[John Rumm]

Yup sounds plausible.

Which suggests that the boiler has no flow regulation (many (possibly most) don't). You would probably get the same flow rate even with the boiler turned off.

Now you need to go looking for restrictions in the hot supply through the boiler. So the service valve on its input, or any other taps/valves that you can find.