Acceptable mains flow rate for combi boiler

[richard44]

Hi,

I'm looking to have a new boiler fitted, and I'm struggling to decide whether to go for a combi boiler or stick with my conventional heating system (hot water cylinder etc.) and just replace the boiler.

I'm tempted by a combi in the sense that it will mean I don't have to store a large amount of hot water (which I rarely use) yet still get a decent shower. However, I'm concerned that my mains water flow rate is not going to be good enough for a combi system, or at least to get good performance out of it. The best I can get out of any outlet (I've tried them all!) is 10l/min. I've not measured the static pressure yet, I guess could be the next step. I've checked the stop-cocks etc, the rising main looks to be 1/2" copper pipe, and I'm not in an area known for low water pressure.

I've had various plumbers round to quote, and so far it's about a 50:50 split between those who say it's not a problem and a combi will be fine and those who say it's not worth the risk.

Can anyone give me any guidance on whether 10l/min is ok/bad/terrible as a mains flow rate for starters, and whether you think it'd be ok for a combi boiler?

Really appreciate any help anyone can give me...

Thanks,
Richard

 

[Agile]

You have not really given the most important answers needed. How many people live there and do you use baths?

You really need a minimum dynamic flow rate of about 12 li/min to use a combi and even then ALL other water uses must be shut down when a shower is being used.

There may be possibilities of improving the mains flow rate.

Tony

 

[richard44]

Tony,

Thanks for the advice. It's a 3-bed, 1-bath house, with max two people in it and we very rarely have baths. At the moment we have an electric shower and if any other taps are turned on whilst that's in use the flow drops markedly - I'm expecting exactly the same result with a combi boiler. Unless of course something can be done to improve the flow rate... is it just a case of improving the incoming main etc? or are there other options?

Thanks,
Richard

 

[darren17]

what sort of shower is it ??
a power shower or a pumped shower??
that flow rate is'nt good enough for a combi.

 

[dtweek]

I'm interested in this too. I'm about to have a new combi put in.

Single bathroom, 3 bed house, 1 bath one shower.

Measured my flow rate last night, getting about 16 downstairs and 14 upstairs.

Just bought the house and am rennovating so if replacing the mains is needed I would be willing to do, however if not needed I'd love to save the cash.

There is a possibility that in 5/10 years that I may add an ensuite with shower to the loft.

would my flow rates give me a decent pressure shower or would it be like being sneezed on by a kitten?

 

[Agile]

Its the dynamic flow rate that matters.

However, an open pipe flow rate of 16 li/min would usually be an indication that a combi will work to an extent but a proper measurement would be made by a competent installer.

No installer should ever quote for a combi or unvented without measuring or assessing the dynamic flow rate.

Yesterday I went to an unvented cylinder what was giving problems and found that it has been installed on a supply with a static pressure of 2.2 Bar and an open pipe flor rate of just 12 li/min. Totally unsuitable!

Tony

 

[dtweek]

When you say to an extent would that mean it is enough to give me a decent shower. Again sorry to hijack the thread but it's still on topic.

I'm not talking rip the skin off your back strong, just to be on the strong side of average.

I guess what I'm after is some scale of shower with dribbling kitten at one end and ripping off your skin at the other

 

[Agile]

Its the dynamic flow rate that matters.

Tony

 

[dtweek]

Many people seem to get very confused by pressure, flow and dynamic flow! There should be an FAQ about it.

You need a flow rate of 13 li/min absolute minimum whilst leaving a residual pressure in the pipes of say 0.5 Bar as there is a pressure loss within the boiler.

An open tap flow of 13 li/min will possibly reduce to 5-10 li/min dynamic flow rate at 0.5 Bar.

Furthermore most showers need at least 0.5 Bar to produce any useful jet flow through the orifices.

Tony

Wasn't sure what you meant by dynamic flow so I googled it and you were the top result. Pasted your quote from the other thread to help the other noobs. Will bear this in mind.

 

[Agile]

A residual pressure on just 0.5 Bar should be seen as the most basic figure.

One shower spec I was reading recently was asking for a minimum dynamic pressure of 1.0 bar.

That posting was intended to be the absolute minimum but the actual requirements are best assessed by a competent professional.

A DIY minimum dynamic pressure might be better stated to be 13 li/min @ 1.0 bar. That would usually be adequate for many combi boilers up to 30 kW but would still mean that only the one outlet could be used for showering etc.

For an unvented to give good performance a DIY minimum should be about 22 li/min @ 2.0 Bar but even that would not provide the optimum performance.

Tony

 

[richard44]

Thanks for the advice.

I guess to measure the dynamic pressure I need to get a pressure gauge, attach it to one tap, then set another tap flowing at, say, 10l/min and look at what the pressure gauge is reading. And that will give me the dynamic pressure at 10l/min?

I suspect it's going to be very low!

Richard

 

[Agile]

You should do it the other way!

Set one tap to give say 1.0 Bar and then measure the flow rate from the other tap.

While doing it also measure the flow rate leaving 0.5 bar in the system as that gives further information.

The flow rate@ 1.0 Bar is the maximum what you can expect to get from a shower depending on the shower head etc. but would usually be good if over 10 li/min as long as you have an average shower and not a fancy big head.

Tony