Water flow rate

[DerbyBorn]

I am getting 12 litres / min from my cold tap and only 7.5 litres/ min from my hot. It seems to have changed whilst we were away for 2 weeks. My combi is a Worcester-Bosch 30CDi in the loft. Any ideas? I have asked the waterboard if they have reduced the pressure - they said not - (but a few days later dug up the road around the corner!!) Stop tap is fully open. Show is quite poor - it used to be quite stinging when on the small jet setting. Now it is similar to how it would sometimes be on a Sunday Morning when people were out cleaning their cars!

[The Natural Philosopher]

scale?

[Jonathan]

A recent posting suggested that a combi will slow the hot flow down in colder weather to maintain the temperature.

Jonathan

[Roger Mills]

Is the burner in the boiler running flat out and continuously when using the shower? If so, that's the best you're going to get when the input water temperature is low.

[DerbyBorn]

But it has no mechanism for doing this.

[ARW]

There are the laws of physics!

[Tim Watts]

Some do.

[newshound]

You would normally expect a lower flow rate for hot than cold because of resistance in the heat exchanger. That feels like the sort of ratio which I had when I had combi's.

Do you think the cold flow rate has dropped too? In that case I think I would be challenging the water board again. There's normally a DHW temperature control which you may need to adjust to maintain the temperature in cold weather. This works by reducing the hot flow (just like an electric shower). Has someone else adjusted it?

[DerbyBorn]

The boiler is in the loft and has no apparent means of varying the flow rate

[fred]

In article , DerbyBorn writes

I'm confused, WB say that current CDi Classics (big case) come in 29,

34, 38 and 42, CDi Compacts come in 28, 32 and 36 and Si Compacts come in 25 and 30.

So is it an 30 Si Compact or perhaps one of the older CDi models?

Most combis I have come across (not many as I'm not a fan) have some means of DHW flow control so that you don't end up with a huge flow at only 5degC rise. Most I have seen are manual but I see a note in the installation manual of the Si Compact saying:

"Greenstar Si Compact boilers are fitted with a flow regulator set to achieve a 40°C temperature rise. This ensures comfortable bathing during the colder winter months."

Even if it is an older CDi then perhaps it has a similar flow regulator that has reduced flow in the face of a really cold incoming mains water.

The installation manual should let you know if you have some kind of flow control (even if hidden) and will be avail from the WB website.

[David]

Can someone confirm/deny this? It seems remarkably crude for a sophisticated modulating boiler.

Assuming that flow rate is not throttled but heat is modulated, then setting a high temperature seems the best way to go.

If the flow is throttled, then is there a better strategy? Fast flow of 40C water against slower flow of 60C water, for example?

Cheers

Dave R

[Roger Mills]

Dunno. There's likely to be more restriction in the hot flow coming through the boiler than in the cold flow coming straight from the mains. If you want to maximise the outlet flow from the mixer shower, it might be better to have the hot as hot as the boiler can deliver, and mix it with more cold.

[John Rumm]

Not only that, a 30kW boiler ought to be able to do a good deal more than 7 lpm.

[PeterC]

The shower unit on minimum flow at 3.75kW in Summer and 4.75kW in Winter is

2.4 l/min. Allowing for differences in efficiency of heat transfer to the water that should give some guidance for the flow from a boiler for the same rise in temperature. I'm not going to work it out as I don't know the figures.

[John Rumm]

Its fairly straight forward to do...

Lets say the 30kW is the actual heat input rate to the water, that gives us 30K J/sec or 1.8MJ/min to play with.

Each litre of water will need about 4200 J of energy per deg C temperature rise. So if we take the starting temp at say 5 deg, and the finish at 42 deg, then that's a rise of 37 degs.

So total energy = mass x shc x Tdelta

1.8m = mass x 4200 x 37

mass = 1.8m / 4200 / 37

= 11.6 lpm

[PeterC]

Thanks John. That's 5x the rate from the electric one for a bit over 5x the power, but of course the loss in the heatX explains that.

[John Rumm]

Its probably different assumptions about start and final temperatures that account for the difference since I was calculating based on the assumption that the 30kW was the actual heat input to the water (i.e. ignoring the losses).

[John Rumm]

From those I have met, I would probably estimate that more combis don't have automatic flow regulation than do.... So basically you set a HW temperature on the boiler and it will modulate / cycle as required to ensure that this upper limit is not exceeded for low demand rates, but for higher rates the temperature will begin to fall. There may come a point where at full flow, the temperature rise is actually inadequate and the water not hot enough. The quick fix solution being simply to turn the tap down, and the slightly more technical one being to throttle the cold water input to the boiler with the inlet service valve to a point where the water is "hot enough" at max flow.

Indeed, these WBs are a little more sophisticated in that they close the loop and modulate the maximum flow to keep the temperature within an acceptable lower minimum.

I must admit its not always obvious from the manuals what regulation (if any) the boilers have.

[PeterC]

With the electric shower there weren't those factors of course: it's on min. flow for most of the year (in May I remove the connection to the 4.75kW element and run on just the 3.75kW one), with a bit more in hot weather and just before/after going onto the lower element. The temperature varies a bit but is within an acceptable range. The shower head is modded to reduce flow a bit, so increasing throughput does result in rather 'sharp' jets!

[DerbyBorn]

Update. I realised the boiler is under warranty and called in the engineer after convincing myself that it must be a boiler problem. He stripped out t he water turbine and the flow restrictor and found that an "O" Ring had dis torted and was partly blocking the flow restrictor. I am delighted that a p roblem was found and easily resolved.