Suggestions for boosting shower flow rate (combi / cylinder / both)?

[myxiplx]

My initial goal is to undertand what I would need for the best possible shower, and just how good that could be. I know the 15mm piping and routing isn't ideal, but there's more flow and pressure available than we're using today, with the hot water being the main limitation in the bathroom.

The reason I was thinking hot water storage is that gives me the extra hot water flow to power a really good shower, and there's plenty of cold water flow rate to go with it.

A 10l/min shower would be double the performance we have today:
- I'm assuming 6l/min of hot and 4l/min of cold
- If I can have the combi running to the secondary return of a direct hot water storage tank I can top up the tank with hot water at the rate of 5l/min
- That's effectively only drawing 1l/min of hot water from storage, so even a 150l tank would give 2 hours of shower time.

A 15l/min double shower would be the ultimate upgrade:
- 9l/min of hot, 6l/min of cold
- Same as before, 5l/min from combi feeding into the tank means we're effectively using 4l/min of the stored hot water
- A 250l tank would give a 62.5 min shower time
- And even after that, with the combi feeding the top of the tank, dropping the rate to 10l/min would mean the shower still wouldn't be entirely freezing cold

Yes it's a luxury, and a chunk of money to spend on just a shower. But if we're paying for the bathroom refit anyway and expecting to stay in this house for the next 10-20 years, it's an option I'm definitely wanting to investigate fully.

In the longer term I will get the main water pipe upgraded to 22mm too:
- I'll get 22mm pipes fitted under the floor as part of this bathroom project.
- The master bedroom does want redecorating in a few more years, which will give me the opportunity to upgrade all the incoming pipework to 22mm and run that right through the upstairs hallway. That gives me a 22mm feed for both the main house and extension, and the shower will be coming directly off that.

Bearing in mind this storage tank isn't there to run the whole house, is that an option, or if not can anybody suggest an alternative? 20 years ago this house did have a cold water tank in the loft above the bath, and a hot water closet in the master bedroom, but we removed those when the combi was fitted and as we boarded out the loft for storage space.

 

[Johntheo5]

You couldn't have been very happy with the combi if it only gave/gives a flowrate of 5.0LPM, someone mentioned that the power output to HW is 28kw which means a flowrate (required) of 11.47LPM at the standard 35C temperature rise which will still give a showering temp of 40C in the debths of winter even if the mains temp is only 5C. You should still though have been able to get a flowrate of 8.57LPM (from mains at 5C) at a showering temp of 40C if the mixer could have added only 3.57LPM of cold at 5C to the 5.0LPM from the combi at its max temp of 65C. If you use a more realistic average mains temp of say 10C then you should be able to get 9.17LPM at 40C, (5lpm@65c+4.17LPM@10C).

 

[myxiplx]

As far as HW performance is concerned - that boiler should be able to deliver ~12L/Min @ 35deg rise so it has the capacity but if you are only getting ~5L/Min then something is throttling the flow through the boiler - again either water saving outlets or valves not opened fully.

Based on this I went and double checked and found one of the valves under the boiler is very stiff and wasn't fully open. After opening that I'm getting 8.7l/min of hot water from the bath tap. I then swapped the high pressure shower head my wife bought for the original Mira one and we're now getting 8.1-8.4l/min from the shower. A big improvement on the 4.9l/min from before!

That's a huge improvement, which now means my upgrade thoughts are firmly into the category of a luxury upgrade rather than necessity. If I re-run my maths:

- We get at least 15.9l/min of flow at 2bar dynamic pressure, with a peak seen of 2.5bar static
- We can get 8.5l/min of hot water from the combi today
- A top spec digital shower can consume 16l/min from dual heads
- Assuming worst case 2:1 hot to cold in the winter, that's around 10.5l/min of hot
- With a combi feeding hot water in via the secondary, we only need to pull an additional 2l/min of hot water from storage for that to work.
- A 150l tank will give well over an hour of piping hot shower
- At 15.9l/min measured today we just about have enough flow rate for that today, and upgrading the main water pipes in the future from 15ml to 22ml would likely improve that further

With the flow rates and pressures measured, is that at all feasible?

 

[Johntheo5]

If you are getting 8.57LPM at 40C then at the present mains temp of 18.5C, the boiler output required is only 12.86kw, if you can avail of the full 30kw output then even in the winter with a very minimum mains temp of say 5C you will still get a flowrate of 12.28LPM which might be quite acceptable, if you set the DHW temp at the boiler to its max, 65C??, then the flowrate through the boiler only has to be 7.16LPM which mixed with 5.12LPM @ 5C will give 12.28LPM @ 40C. Ideal state that there is a restrictor installed so maybe remove this to get the full benefit of a 30kw combi.

Not sure what this means, "With a combi feeding hot water in via the secondary, we only need to pull an additional 2l/min of hot water from storage for that to work."

 

[myxiplx]

Not sure what this means, "With a combi feeding hot water in via the secondary, we only need to pull an additional 2l/min of hot water from storage for that to work."

What I'm wondering is whether I can make use of the stored hot water and the combi boiler for the shower, to avoid the problem of my teenage daughter running the shower out of hot water.

The stored hot water means I can run a higher flow rate hot shower than the combi alone can supply, and should minimize the impact of our daughters turning on a tap or running the shower downstairs.

I don't know if that idea is actually possible though.

 

[Madrab]

No, you wouldn't use the combi in series with say an unvented cylinder, where the combi fed hot water into the cylinder, that would be far too complicated and they aren't really designed to work like that, not very efficient use of energy at all.

What you could do is create a HW priority system with a ultra high recovery coil to allow the cylinder to actually warm back up as it's being used and couple that with a timer that shuts the shower off after 10 minutes.

Sorry but there's absolutely no need for anyone to spend 30mins in a shower, it's actually bad for skin to be washing for that long. I used to limit our teenage daughter and now our typically smelly teenage son to a max of 15mins.

 

[Johntheo5]

What I'm wondering is whether I can make use of the stored hot water and the combi boiler for the shower, to avoid the problem of my teenage daughter running the shower out of hot water.

The stored hot water means I can run a higher flow rate hot shower than the combi alone can supply, and should minimize the impact of our daughters turning on a tap or running the shower downstairs.

I don't know if that idea is actually possible though.

Get your installer to remove the Flow regulator, item 124 and take it from there.

 

[myxiplx]

Oh, interesting. That took me down a rabbit hole of checking combi flow rates, leading me to high flow combis.

The Logic 30 should deliver up to 12l/min I believe, although the 8.7l/min we get today isn't bad.

Another option would be to refurb the bathroom, using a digital shower where the max flow rate is set via the app. I could leave that to a sensible 10l/min today, and if the future work to upgrade the house to 22l/min, or a future boiler upgrade improves our hot water flow I can adjust the shower down the line.

Definitely a cheaper option today, without any crazy ideas needed.

 

[Johntheo5]

What is the DHW temperature set to on the combi just now?,

 

[myxiplx]

It was set to 60, it's max is 65.

 

[Johntheo5]

It was set to 60, it's max is 65.

That means mixing 4.4LPM @ 60C from the combi with 4.1LPM @18.5C to give 8.5LPM @ (if) 40C, the dynamic pressure available should have been 2.35bar based on a dynamic pressure of 2.0bar at 15.9LPM (from static pressure of 2.5bar). You were getting 5.0LPM from the bathroom hot tap, post #6, but you have since opened a cock fully under the boiler, so, sometime you might measure the flow from this batroom hot tap again.

 

[myxiplx]

I did, that's where the new 8.7l/min figure for the hot water came from.

 

[Johntheo5]

8.7LPM at 60C isn't bad from a getting the full thermal boiler output, it has to output 25.2kw or 84%, shower is quite poor though , 12.75kw, due to the low HW flow, what temperature is the shower set to?.
How old is the boiler?, the plate heat exchanger might be getting fouled up, I doubt if the flow restrictor would reduce the flowrate through a fully open tap to 8.7LPM with a dynamic head of 2.35Bar, you could do one more test, switch off the boiler, turn the shower TMV to full hot and measure the flowrate.
I would still get that boiler flow regulator removed.

 

[Madrab]

Get your installer to remove the Flow regulator

TBH, if they are only getting 5.2L/Min @ the hot outlet then there's something else going on. I can't see it be anything to do with the flow regulator in the combi, unless it's all scaled up of course. That combi's regulator should allow at least 12L/Min @ 35Deg

 

[myxiplx]

Err, guys, I'm getting 8.7l/min now from the hot tap, and 8.4l/min from the shower. Your suggestion of checking for valve that was restricting the boiler was spot on.