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

[myxiplx]

Hey all,

We're planning to revamp our main bathroom and I'm keen to ensure we use this to boost the flow rate out of the shower for my wife and daughter, without us running out of hot water when my teenage daugher spends 30 minutes attempting to turn the bathroom into a sauna.

Basically I'm looking for a way to boost the flow rate from a new shower, whilst minimising disruption to the rest of the house.

Today we have a mixer shower fed from our combi boiler, and a second mixer shower in a new extension. We checked the combi spec as the extension was built and two separate plumbers advised that the combi should be more than powerful enough for both, but our main shower has moderate performance at best and when the new shower turns on, the flow in the main bathroom drops to a trickle (possibly because the new extension has 22mm pipework for the main runs, whereas in the old house nearly everything runs from 15mm).

I've also used a jug to measure the flow rates in the main bathroom and we're getting 12l/min from the cold tap, 5.2 from the hot tap, and 4.9 from the shower. I really want the shower to be able to achieve much better flow rates, ideally around 12l/min.

My own best guess (and I don't know how practical this is), is:

Unvented, direct 250-350l tank in the loft
This will give us 30 minutes of hot water for a good mixer shower.
We already have a high current feed in the loft as we had an electric shower before, and this effectively gives us an electrically heated shower if the combi ever fails.
It also means we should be able to run both showers at once, with decent flow to them both.

Warm water feed to cylinder
This is the bit I'm not sure is practical or even possible. We already have hot & cold feeds running to the loft for the shower, can we feed a cylinder from a 50/50 mix of cold water and hot water from the combi? The only output from the cylinder will be this shower, so could we connect hot water from the combi to the secondary return, and cold water to the base of the tank as normal?

Even with our current pipework, this should give us around 10l/min of warm water feeding into the cylinder, which should support a good flow rate to the shower and using the existing hot water pipes from the combi should mean even my teenage daughters can't ever run us completely out of hot water.

Long term - 22mm piping
In the much longer term, the next time we redecorate the bedroom we'll upgrade the main feed from the stop tap from 15mm to 22mm. That should help the whole house but also give us a much better flow into the tank, potentially boosting the main shower to 15l/min. Later on, as we redecorate the hallway we'll run that 22mm feed through to most of the house, improving the feed to the combi and kitchen also.

I'm not sure how practical this idea is, or whether there's a simpler option I'm not aware of. Would love any and all input.

Thanks all.

 

[ericmark]

Any instant heat shower, be it gas or electric has a limited amount of water it can heat, only by storing energy with some hot water tank can you exceed the maximum output of a boiler.

Un-vented cylinders need annual testing, but you can get high pressures either with a heat exchanger or pump, the cylinder does not need to be at cold water pressure. Idea shown here the hot coil is at mains pressure not the whole cylinder.

However I would suspect it is the shower rose at fault, if the rose is too small the boiler may not kick in, but if too large it uses too much water for the boiler to supply, so if your boiler can provide 6 kW to 28 kW then two showers using 12 kW each would work OK, but if your boiler is 12 kW to 28 kW the use of one shower could result in the boiler cutting out.

In practice not quite so cut an dried, as it depends on cold water temperature, but smaller shower roses may cure the problem. I had a very early instant gas DHW boiler, a main 7 before the two functions were combined into one boiler, and shower rose size was critical to shower working well. Fit a head designed for electric and the boiler would cut out, today the combi boiler tends to have a better range of output between min and max that is assuming using gas, oil does not really modulate, it turns on/off and has a heat store so cylinder is inside the boiler, never tried fitting a shower to a oil combi boiler.

two separate plumbers advised that the combi should be more than powerful enough for both,

So why ask on here, I am an electrician, and the same complaint is often made about electric showers, they have no pressure and water is more like a dribble, often find the shower head has been swapped and does not match the heater. At some point the flex pipe to head has started to leak, so the whole head and pipe swapped, to a head which is too big.

 

[myxiplx]

So why ask on here,

Because the combi boiler clearly isn't the problem, but that's not my question. I mentioned the combi since with two plumbers saying it has a high enough rating we can rule that out as the root cause.

But I still need some input on ways to achieve my goal, which is doubling the flow rate through this shower with minimal disruption to the whole house. Swapping the shower rose isn't going to achieve that, as mentioned in my post I only get a moderate amount of hot water flow to the bathroom, there fundamentally isn't enough hot water available, hence my thoughts around a storage tank allowing me to boost the flow rates.

There's almost certainly a restriction in the pipework throughout the house, but that's not something that can be fixed as part of a bathroom refit, however there is a good flow rate of cold water to the bathroom (it's one of the first branches off the main water supply), so an unvented tank sounds like a good option to provide me with a store of hot water and a good flow rate.

But given I have a teenage daughter who takes 30 minute showers, I also want to try avoiding the hot water running out if possible, which is why I'm wondering whether we can use the combi to feed hot water into the tank via the secondary return.

That should give me around 5l/min of hot water flowing in near the top of the cylinder, with 5l/min of cold water feeding in from the bottom. There's enough flow rate there to supply a much better shower, and it should double the amount of time before the shower starts to cool down, and it will almost never go completely ice cold.

But I'm not a plumber, so this is (barely) educated guesswork. I'm looking for input on whether this would work, and whether there's a simpler option I might have missed.

 

[Madrab]

We checked the combi spec as the extension was built and two separate plumbers advised that the combi should be more than powerful enough for both

Even when the mains dynamic pressure and flow is sufficient then unless it's the largest combi then no combi will ever supply full output to 2 HW outlets at a time. Only way the largest would be able to supply that would be with showers that have water saving (flow restricted) outlets, which defeats what you are trying to achieve here, sorry but your plumbers didn't know what they were talking about.

Firstly you need to understand what the mains supply can deliver - dynamic flow and pressure needs to be measured - loads of posts on here that describes on how to do that properly. That is fundamental to knowing what your options are and without that any recommendation would be moot.

12l/min from the cold tap

If that is indicative of the dynamic mains flow then unvented would not be suitable though I would guess that it may not be.

Just to be clear 15mm pipe will happily supply high flow rates (rather noisily) but it need enough dynamic pressure to do so, if the pressure isn't available then it won't really matter whether it's 15mm or 22mm.

So back to the start again - find a cold outlet close to the mains and do the tests for dynamic pressure and flow (gauge on and 2 or more cold outlets running) and see what's available and go from there.

 

[ericmark]

There are four types of storage tank, those where DHW drawn from tank, and heated heated normally in a back boiler, with no built in heat exchangers (hot coils) those where the central heating water is sent through a hot so water does not mix, both result with lot pressure DHW unless a pump is used, then where like previous central heating trough hot coil, but whole tank at cold water pressure, but they need an annual test, and those where there are multi heat exchangers so the DHW and central heating both use a hot coil. Latter normally used where there are multi methods to heat water like solar power and back boilers.

It is in theroy easy, the last method is best, however that is without looking at price, once one looks at price the electric shower starts to look better, but the load is high, at typically 9 kW (40 amp) where often the whole house only has a 60 amp supply, clearly can only have one, however saying that I have two on a 60 amp supply, and DNO fuse has not ruptured, but then we continue I also have solar panels and a battery, so the 9 kW (assuming only one in use at any one time) is often only drawing 6 kW from the DNO.

You can't pump from the mains supply, you can from a storage tank, so once a cylinder and header tank is fitted, you can use a power shower, but there is so much involved, the forum can only tell you the systems exist.

My brother-in-law has this although one tank shown in real life twin 80 gallon tanks, which are heavy, so the new build had special floors upstairs to take the weight, the system worked very well, he went to visit daughters a lot in Germany, and from the airport could turn up his central heating, and solar kept the house above freezing while away and reheated on return, it was really good, so when he moved wanted it in his next house, the quote was over £25k to fit it, same age as me, so he would never get his money back, just not worth fitting, and this is the problem, loads of systems can be fitted, but the question is at what price, and that price may include reinforcing the floor.

So all we can do is tell you the system exists, we can't work out a price to fit in your house, I like the ability to store energy, and storing as hot water is a cheap method, and I was surprised at cost electric v oil, I had always assumed oil cheaper than electric, and on paper it is, but when it came down to actual measuring how much used to heat DHW I found losses using the boiler were way over what I had expected, I have no tank thermostat connected to central heating, so by experiment found I needed to run the boiler 4 times a week, and it ran for approx 20 minutes each time it ran. So boiler 20 kW so that was around 25 kWh to keep DHW hot. When solar panels were fitted I fitted an iboost+ to use the solar energy, it records how much energy used, and last 7 days, just looked, I used 2.74 kWh to keep DHW hot. Not quite one tenth, but neither oil or gas are cheap enough to match a simple immersion heater.

We had considered a 7 litre water tank and heater in the kitchen, to reduce the time to get hot water to tap, but not worth it, mainly as they take up too much room, like you are saying I have a problem running new pipes or wires, the floor will not come up in a hurry, likely any work would involve dropping ceiling. So my house is likely unique, and any work to improve my DHW supply would need costing for this house, and likely will not compare with others. So improvements require some out of the box thought, for example to get a better spread of light in the kitchen, normally we would use down lights, but in this house lighting track works better as no access into the ceiling.

 

[myxiplx]

Hey all, I picked up a pressure gauge off Amazon to see what I'm working with. The only tap with an accessible screw thread is the garden tap which possibly isn't ideal, but is located right behind our combi boiler. Static pressure is around 2.3 bar, with the kitchen tap running that drops to 2 bar.

And as mentioned before I'm getting 12l/min flow rate from the cold tap in the upstairs bathroom, and around 5l/min from the hot tap there. Boiler is an Ideal Logic 30. There used to be an electric shower fitted, so there's a high current line available and plenty of easily accessible loft space above.

Our current Mira mixer shower isn't terrible by any means, but if we're going to the expense of re-doing the bathroom and replacing the bath with a shower I really want this to be a noticeable upgrade if at all possible.

Thanks again for all the comments so far!

 

[Madrab]

The only tap with an accessible screw thread is the garden tap which possibly isn't ideal

Perfect, it's a full flow tap so will give the most honest reading and will probably be pretty close to the mains.

Run another tap too, as well as the kitchen one, say one upstairs and see how much further, if any it drops. Do the flow test at the same outside tap tap with the kitchen tap and upstairs tap on and see how you get on. We want to test how the cold mains can deliver and not after all the bends/elbows and twists to get it upstairs.

The hot flow may be because of the combi output and it may have a aerator/water saver nozzle etc that will further reduce the flow.

 

[Johntheo5]

Hey all, I picked up a pressure gauge off Amazon to see what I'm working with. The only tap with an accessible screw thread is the garden tap which possibly isn't ideal, but is located right behind our combi boiler. Static pressure is around 2.3 bar, with the kitchen tap running that drops to 2 bar.


And as mentioned before I'm getting 12l/min flow rate from the cold tap in the upstairs bathroom, and around 5l/min from the hot tap there. Boiler is an Ideal Logic 30. There used to be an electric shower fitted, so there's a high current line available and plenty of easily accessible loft space above.

What is the kitchen tap flowrate at 2bar dynamic and what is the dynamic pressure with a flowrate of 12LPM from the upstairs cold tap?

 

[myxiplx]

Ok, more measurements: This morning I'm getting 2.5bar static, and still 2.5bar with both the bath & kitchen tap running. I took flow rates from both taps and we're getting 14.4l/min combined. 3.6l/min from kitchen cold tap and 10.8l/min from bath cold.

It seems to me that there's plenty of water, the limitation is that for a really good shower I don't have enough hot water flow going to the bathroom from the combi.

 

[Johntheo5]

You got a dP of 0.3bar (2.3-2.0) with the kitchen tap only opened, (post #6), flowrate (now) 3.6LPM, but no dP at a flow rate of 14.4LPM, the only explanation for this is that there is a pressure reducing valve installed somewhere but there shoud still be some fall off in pressure from its setpoint, if set to 2.5bar. Can you measure the flowrate from the outside garden tap even though you won't be able to monitor its pressure.

 

[myxiplx]

Getting a little over 12l/min from the outside tap. It filled a 2L jug to the brim in 10s.

 

[myxiplx]

You got a dP of 0.3bar (2.3-2.0) with the kitchen tap only opened, (post #6), flowrate (now) 3.6LPM, but no dP at a flow rate of 14.4LPM, the only explanation for this is that there is a pressure reducing valve installed somewhere but there shoud still be some fall off in pressure from its setpoint, if set to 2.5bar. Can you measure the flowrate from the outside garden tap even though you won't be able to monitor its pressure.

Weirdly I had the same kitchen tap running when I saw the drop in pressure yesterday. It's a different time of day, I wonder if we're seeing higher water pressure this morning from the mains supply.

 

[Madrab]

3.6L/Min @ the kitchen could be due to a flow limiting nozzle/aerator on it but even then that's quite a drop, are there ISO valves to the kitchen tap and are they fully open? The differences in mains dynamic pressure could be where the district pressure is better maintained at peak times, basically they bring more pumps online.

You may be getting the max flow to the bath taps as they have the least restricted pipework running to them, is the bath cold 22mm pipe?

If 12L/Min is your max then you are limited as to your options - 12L/Min would be borderline for an unvented and only if that figure is truly dynamic. Again you need to check the flow at the outside tap with say 2 other outlets in the house open, just measure the flow at the outside tap.

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.

 

[Johntheo5]

I would suggest repeating those tests again at the combined flow of 14.4LPM, take the static pressure immediately before commencing the tests, then take the dynamic pressure, then immediately take the static pressure again when you close all taps, that will tell a lot IMO, also, as suggested above, the other tests.

 

[myxiplx]

I did a quick test running the bathroom tap and garden tap together. That gives me 15.9l/min combined, 8.7 from the garden plus 7.2 from the bath.

Madrab, to answer your question, the whole house is fed off 15mm. There's a stopcock in the lounge with 22mm coming up from the floor, but it comes out of that as 15mm, and goes straight up to the first floor, under the bedroom floor, and splits out from there.

The bathroom comes straight from that, so gets the best flow rates overall, the garden tap and kitchen sink are effectively off the same branch so any restrictions there are almost certainly due to the kitchen tap. The new extension does have 22mm plumbing, but again that connects to the 15mm supply under our bath.