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Mrs peaslaker and I are looking at a bathroom remodel. We've got a an "in warranty" Vaillant combi boiler (25kW) in a top floor flat of a mansion block (4th floor). Our existing bath mixer tap shower gets you wet but isn't great.
Water pressure isn't the best but cold water runs from the tap at 18 l/min and from the existing shower head at ~12 l/min. Hot water is 7.5 l/min at the tap and 6 l/min at the shower head. A shower at a bearable temperature needs a bit of cold mixed in, so probably 7 l/min.
The boiler's spec says 8.8 l/min for delta-T 42K. Boiler lives in a kitchen cabinet and we can't really go up a size.
There's no point in having a fancy new bathroom shower setup without fixing the flow rate. These numbers are also when the delivered pressure is good. There are definitely times when the pressure drops as we're the last flat at the top of a 15mm riser. Hot water delivery is the problem. Cold water appears to be sufficient.
So my simple thinking was that we could keep the existing boiler but run a Y-plan or S-plan, call for heat on the boiler and have a closed system hot water tank in the roof space. The combi hot water could still supply the kitchen (or not).
Is this a "permissible" or sensible thing to do with a combi boiler? My understanding is that the boiler wouldn't know the difference and would just be servicing a call for heat.
Or... what other options?
Seems a perfectly reasonable thing to do in my opinion (although I am not a plumber).
a 25 kw water heater should provide a decent shower. sound to me like something is wrong with the plumbing as you are not getting enough hot water flow.
You can get a "bigger" boiler without increasing the physical dimensions. You'll probably find that your 25kw model has 30 & 35 equivalents, they just have bigger heat exchangers inside.
Speak to Vaillant. A new shower set up might be enough but cold weather = colder water. A boiler swap is probably the simplest option with less head-scratching and teeth-sucking involved
IANAP-HE I'm tending to agree with TJ. Your H/W flow rate sounds like its way too low. 25KW is a lot of heat but it doesn't sound like that's what your getting at the shower head. Are there any filters in the shower valve you could examine/clean as a starting point?
Your H/W flow rate sounds like its way too low.
Not sure which boiler the OP has, but many combis have a built in flow restrictor on the hot water. (The one on our old Worcester was marked 'Summer / Winter' - it allowed higher flow in the 'Summer' position).
It was also 24kW max and struggled to provide a decent hot shower - 42°C rise only gives you ~50°C water for much of the year, and this is *at the boiler* - by the time it has travelled around the house it's a fair bit cooler. The trouble is that if the hot supply isn't hot enough, a thermostatic shower valve asks for more hot water which increases the flow through the boiler and reduces the temperature further. The only way to get a decent shower from our 24kW boiler was to use a shower head that restricted the flow.
The current 35kW boiler is far, far better.
You could try and increase the pipe size to reduce hydraulic losses on the hot.
Use an aerating shower head designed for low flow as well
As the others say it should be fine, sounds like something is restricting the flow. Electric showers only go up to around 10.5kw. Shower head is a good call, it's not all about quantity but also spray pattern.
Does the shower run with the same flow when it is only cold water running? Just checking that there are no flow inhibitors in the shower set up - lots of different types but often found as inserts at one end of the hose. Good for eco water saving but not so great for thr flow!