combi boiler flow rate

[Stephen]

Hello,

I have moved into a house with an old non-condensing combi boiler.

I am thinking of replacing it before the winter but I am unsure what to replace it with. There is a loft conversion with an en suite, so my first thought was that there was no room for a header tank for a non-combi boiler. However there is some space above the plasterboard ceiling in the apex of the loft conversion and the joists look a decent thickness, so it is possible that I could fit a header tank up there.

The advantage of a combi boiler is that I would only heat what I need but the disadvantage would be no immersion heater in a cylinder as a back-up if the boiler failed.

My last house had gravity fed HW in 22mm pipe and I feel that the bath filled much more quickly there. I like baths and I find that in the new house it takes at least 10 minutes to run the bath. The bath is plumbed in 15mm pipe. Is this normal with combi boilers? Is it that because the water is at mains pressure rather than gravity fed, a smaller pipe diameter can be used? Would I see no benefit from using

22mm pipe?

I find the combi boiler pretty useless for washing my hands. If I turn on the tap in the kitchen, I can hear the fan start but the gas has not ignited before I switch the tap off. Are all combi boilers this slow to light?

If I go down the route of replacing the combi with another combi, what flow rate figures should I look for to ensure a quick filling bath or a decent shower?

What are these ERP ratings that manufacturers are quoting?

The old boiler is under the stairs. Is this still allowed? I thought you had to have a 30 minute barrier if you installed there?

What about micro CHP boiler. I presume it is much more expensive? Has anyone got one?

Thanks, Stephen.

[RJH]

Electric shower?

Mine lights within a second or two, but takes a good 10s to feed hot water. Complete waste - so much for 'efficiency'.

[sm_jamieson]

If it works fine, does it need replacing? Is the extra efficiency of a cond ensing boiler worth throwing away the existing one? And do you have radiato r sizings such that the new boiler will actually run in condensing mode?

Be very careful about putting a tank up there. Joists may look strong but d on't underestimate the weight of a water tank.

Wide pipe is used to get sufficient flow from a low pressure system (gravit y). With combis the pressure is not the problem, so 15mm is fine. Regarding filling a bath, just get it going and do something else for 10 mi nutes.

Mine is a bit faster than that. You can also get combis that include a smal l water tank to tide you over until the direct heated water is up to temper ature. Another common setting is a "stay warm" function where the heat exch anger is kept warm by periodic boiler firings, so the hot water comes soone r.

Basically, the higher the heating power, the faster it can heat water. Actu al flow rater will depend on the incoming water temperature. But it will ne ver fill a bath as fast as the entire contents of a hot water tank being du mped through 22mm pipe.

Simon.

[David]

This is probably the length of the pipe run from the boiler to the tap - I had a stored water system with a long pipe run and it took ages for the hot water to get to the tap.

Flow rates from combis are bound to be slower than gravity fed through large bore pipe - there is a limit to how much heat you can get into water that is charging past. Limited by not just the boiler but the gas supply as well.

I now have a combi, but the best system I have had is a large hot water tank and header tank with a power shower pump.

Glorious showers, and it also filled the bath very quickly!

Cheers

Dave R

[John Rumm]

You are confusing combi boiler (i.e. one that heats hot water on demand as well as running the CH) with a sealed system boiler. The two are unrelated. Most combis use a sealed system (no header tank) but not all. Other boiler types are also available in bot sealed and vented forms.

Why would you want one?

and your flow rate of hot water will be limited by the power of the boiler...

Yes, most are not powerful enough to need a larger pipe size. Plus they run at higher pressure and so its less of an issue.

Yup, that and also the fact that even a 35kW combi can only manage 15 lpm of water at final temperature in the winter - and that will not be a problem with 15mm pipe.

No, you would just experience more delay for the hot water to come through.

Some keep a small store of preheated water so as to minimise the delay in production.

35kW as a minimum. In reality you will never match a stored water system for flow rate - some of which will deliver at a rate that a combi could only manage with an output of >100kW. Since you gas meter is not rated for even half that, its a non starter.

pass

Its still allowed. Although you will need a route for the condensate drain with a new boiler.

I am not sure I really see the logic of these (other than the normal reason of attempting to extract incentives and paybacks from various daft government schemes). All they do is divert some electricity generation from a highly efficient combined cycle gas turbine power station to a less efficient single cycle boiler.

Other options to consider if you have space, would be either a heatbank or an unvented cylinder. Both can give high flow rates, immersion backup and mains pressure delivery with no need for header tanks.

See:

and

[David]

Just to add to John's excellent list, AFAIK without a header tank or other cold water storage method you are ultimately limited to the maximum flow rate of your mains water supply to supply both hot and cold to your bath.

I remember that I could empty the large loft tank after a while if running both showers off the shower pump. The flow rate was far higher than the maximum flow in from the mains.

So you need to have some idea of the maximum delivery of cold water to the house before you can be sure that you can fill your bath in the time desired.

Cheers

Dave R

[Stephen]

So if I redo some plumbing if I want to have the new boiler in a different location, I can just run one 15mm pipe and tee off to the bathroom, kitchen, etc. It won't be a problem even if all taps are open at once?

[Chris French]

In message , Stephen writes

John meant, that 15mm pipe won't have a problem delivering 15 lpm of HW.

It will still be a problem in that if you have a big draw - filling the bath or having a shower, then turning on say the kitchen tap will reduce the flow to the bath or whatever.

But that will happen however you arrange the plumbing. All the HW water has to come from the boiler and that is the limiting factor.

You should get a decent shower from a combi, but filling a bath is always going to be slow

[John Rumm]

Once you are drawing whatever flow rate the combi can heat adequately, trying to take more will (depending on the combi, and whether it has flow regulation or not) either get you more cooler water or the same amount of water shared between the taps.

[John Rumm]

Keep in mind there is nothing to stop you using a combi to heat a traditional vented cylinder as well as having its own supply of hot water. Some may do that to have good bath filling performance, but also mains pressure hot water direct from the combi either for a shower, or a short dead leg to a particular tap etc.