Optimal combi boiler for rainfall shower head

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Hello, we are having a rainfall shower installed - it's 200mm so bigger than usual size shower head, but not HUGE (I know there are some 300mm+).

We will be installing a new boiler. Does anyone know what boiler specs would be ideal to deliver enough water for the rainfall showerhead? We were looking at Vaillant ecoTEC models? But not sure what kw would be needed. I know they also have a combi boiler which has a water store included in addition, which is an option if needed (but would prefer not to, as this one is bulkier).

The rest of the space is fairly small - 2 bedroom flat, just the one bathroom and kitchen, but don't expect to be using lots of water at the same time (except UFH). Our incoming water supply is on a 25mm pipe, I believe.
 
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Look at the spec of the shower and measure your incoming water pressure/ flow rate then look at boilers,you also need to look at boiler’s that can have the heating output adjusted to suit your heating output ,your heating will probably only be around 10 kw.
 
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My choice would NOT be a combi boiler.

water store included in addition

The volume of stored hot water would only run the shower for just a few seconds.

With a high flow rate rain shower you would need a combi of 36kW or more to have a good shower.

The rest of the space is fairly small - 2 bedroom flat, just the one bathroom and kitchen,

which could possibly be kept warm with less than 5kW of heat from the boiler. That limits your choice of boiler to one that can modulate ( reduce ) it's output down to 5 kW or less and still be efficient at that very much reduced output. Or your boiler turns itself ON and OFF so that the average output is low enough. That cycling is inefficient and increases the wear and tear on the boiler ( shortens the boiler's life ).
 
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@cross thread the shower spec is the below - but I'm not 100% sure how to match this up to the boiler spec - does this suggest, at a MINIMUM, we'd need 17.5l/min water flow - in which case the boiler needs to provide at least 17.5 DHW l/m? The boiler I've been looking at have a Max DHW of 10.5-15.9 (even with the 35kW models - with exception of the Vaillant ecoTEC plus combination store which allows up to 20l/m). But is that really true - that even the most powerful Vaillant couldn't deliver enough?
upload_2021-12-29_10-50-11.png


I'm not sure however how to measure incoming water pressure/flow rate? We are, however, on ground floor, and the distance the water will have to travel from the front of the property to the boiler is around 10metres, if that's any indication?

When you say a boiler that can have the output adjusted to suit the heating output - do you mean that the heating will require less power than the shower? So we want an adjustable rate for heating compared to shower, so as to not overspec? If so, do you know what I should be looking for on the boiler specification that would indicate this is possible?

Thanks!
 
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@bernardgreen thank you - being in a small flat, we have quite a limited space (a large cupboard for boiler + manifold), so I think a combi is all we have the space for. I am hoping there is a combi that meets our needs! When you say modulate/reduce, do you know what terminology I should be looking for on the boiler specification?
 
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@bernardgreen The Vaillant EcoTec plus says: "The ecoTEC plus is smart: when it knows you don’t need as much heating power, it can drop to using 16% of its maximum power saving on fuel and money." would this be the modulating aspect?
 
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All down to pressure - most combi's will deliver enough HW flow for a rain shower head but if the pressure isn't up to it then given the narrow waterways in a typical plate shower head, the flow will be poor. That being said given the flow figures @ 1 bar, then as long as that's sustained (dynamic) then that's your starting point, a quick test of the mains will tell you what yours mains will deliver.

Obtain a pressure gauge from TS or SF, attach it to an outside tap of a washing machine valve, run a cold tap or two and check the reading. Flow is measured by timing cold water at the highest flowing tap into a bucket over a minute and then measure the water volume.

Ideally a pressure of around 2 bar dynamic would be the min IMO for large shower heads, so the hot isn't impacted too much as the cold runs at the same time. Your min delivery would be calculated at an approx ration of 2:1 (H>C), so it you wanted 17L/min then you would be looking @ a HW output of ~11L/Min. The ratio's vary during the year of course as water temps fluctuate.
 
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@Madrab Thanks, that's useful! I hadn't realised the impact of H&C water, so that's useful to understand better.

As an indication, we have recently installed new 25mm diameter pipe to the Thames Water supply line - do you think there would be any reason why the water pressure would not be enough?
 
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"The ecoTEC plus is smart: when it knows you don’t need as much heating power, it can drop to using 16% of its maximum power saving on fuel and money." would this be the modulating aspect?

Yes that is the modulating aspect. 16% of 36kW is 5.7kW so it may be low enough for keeping your flat comfortable warm without a 36kW boiler having to cycle ON and OFF
 
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@bernardgreen thanks so much, I understand that part much better now. I will send the spec of this boiler to our plumber for their review, but it's great to help me narrow down options.
 
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we have recently installed new 25mm diameter pipe to the Thames Water supply line - do you think there would be any reason why the water pressure would not be enough?

It all depends on what the mains can deliver, rather than how large the supply pipe is. I would like to think that as you now have a nice 25mm pipe supplying the cold mains your flow and dynamic pressure would be suitable but until it's checked then it's still and unknown.

Did you get a report from Thames when they upgraded the mains, they sometime reference the delivery before and after.
 
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@Madrab We didn't, but will get in touch with them to see if they can provide this - thanks for your help!
 
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Flow is easily checked with a bucket of known capacity and a stopwatch measure in litres per minute.
Pressure you need a gauge perhaps your installer will have one
 
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I've bought, but not yet installed, an "air" rainfall head, think it's Crosswater. Supposed to use less water but deliver a satisfactory shower experience - might be worth investigating.
 
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