Combi boiler Hot water pressure

[Paul_Mc26]

I have a Combi boiler but when i run the hot water for a bath the hot water doesn't seem to have much pressure, why is this?

Many thanks

[ianmunt]

The water can only pass through at a controlled rate in order for it to heat up.The colder the supply water is the slower the flow will be to achieve the same outlet temperature.Also the input pressure will control the outlet pressure,with some pressure loss in passing through the boiler.A good installation will have a larger bore feed than outlet so 22mm in and 15mm out.

[gucciman]

ive never seen a combi with a 22mm cold feed, always 15mm, 22 mm for the gas and the heating flow and returns but not the cold feed. and the flow through the boiler as nothing to do with the temperature of the cold water going in. the only way you can control the flow of water through the boiler to make it hotter is to turn down the flow of your hot tap.

are you new to a combi ? most people are very dissapointed when they switch from a conventional system with tanks and cylinders to a combi. a combi will never deliver water as hot and fast as a cylinder. the only thing you can do is to buy a powerful a combi as possible to power the hot water i.e 37 kw.

the inlet pressure of the cold water does to some extent control the outlet pressure of the hot water but many boilers have a flow restrictor built in to slow the flow of water to achieve higher temperatures.

you can call in a heating engineer to check the boiler over, he can use weir guages and a temp probe to ensure the boiler is doing what its designed to i.e. it is giving the specified temp rise at the specified flow rate, if it isn't then he may be able to adjust it but my feelings are your going to be stuck with what you've got as it probably is doing what its meant to.

what kilowatt is your boiler?

[Paul_Mc26]

yeah i'm new to combi it say max heating input 26.3 kw. I was looking at getting a new shower am i right in saying the mira 415 combiforce is best for a combi system

[ianmunt]

By turning down the flow from your hot tap you ARE reducing the flow rate through the boiler !

[gucciman]

thats not what you said, you said "the colder the supply water is the slower the flow will be to achieve the same outlet temperature" thus implying that the boiler could somehow magically slow down the flow rate to match the incoming water temperature.
what you should have said is the colder the incoming water supply the colder the outlet temperature will be so you will have to slow the flow down by closing the tap slightly to get hotter water.

[gucciman]

im not an expert on combi powered showers yet, gasman is the best man to answer that. i will say however that i reccomend you buy an electric shower as combis are pretty unreliable and if or when it breaks the electric shower is invaluable as a back-up.
do you know what the actual kw of the boiler is, it normally says on the front of the case or control panel i.e. it may say vaillant ecotec plus 31kw or something like that.
if its say a 24, 26, or even 28 kw then your going to struggle for hot water, 31, 35 or 37 kw would be much better.

[VICTIMS]

We had a combi fitted couple of yrs ago, and have the same problem. Had the engineers back several times, and they say its working ok and nothing can be done.
While using the sink / wash basin its not good but adequate, but having a bath is hopeless. You need to leave the tap running slowly to get hot water, so the bath takes forever to fill, and so has started to go cold before it fills up.
We use an electric shower, but the kids enjoy a bath, they like to splash around.
The only advantage we have found with the combi is, we have lost the tank in the bathroom, and so have more room, and do not need any water supply in the loft, so do worry about burst pipes in the winter, as much.

[gucciman]

classic underpowered combi, always go for the most powerful one you can afford.
you could look at having an indirect hot water cylinder plumbed into your heating system which would give you a good store of hot water but at great expense and once again its a tank in the loft job.

[gasman]

You cannot go wrong with a Mira shower. I would recomend The Mira Combiflow or the CombiForce 415 both are pressure balanced shower controls which maintain a constant outlet temperature irrespective of changes in inlet pressures as long as the inlet water temperatures remain the same. It is not a thermostatic shower control and does not sense supply temperature variations. Therefore, inlet water temperatures
especially the hot, should remain relatively constant. Making them perfect for use in a combination boiler set up.