Plumbing - Shower question
Discussion
I have a combi boiler. The flow rate is OK. I can get very hot water out at a fairly fast rate but not mega fast.
As my electric shower has just broken I am considering getting a mixer shower instead. However, I fear that my boiler cannot pump out hot water quick enough to provide a decent shower.
I believe the pipe from the boiler to the bath is 22mm. This allows a larger volume of water to go from boiler to bath than 15mm. But having read around it seems a 15mm pipe is better for a combi boiler as there will be less water hanging around in the pipes getting cold and having smaller pipes will reduce the amount of water coming out of the tap and MAY mean the water that comes out will come out hotter.
Would it herefore make sense to put in a flow restrictor or a piece of 15mm pipe into the 22m pipe before it reaches the tap to slow things down? But would this not effectivley be the same as just putting the on at 'less than full speed'
I think I am confused now! Any plumbers on here advise?
If I put a mxier shwoe in I guess it will control the incoming hotwater flow anyway so there would be no need to mess with the pipes?
As my electric shower has just broken I am considering getting a mixer shower instead. However, I fear that my boiler cannot pump out hot water quick enough to provide a decent shower.
I believe the pipe from the boiler to the bath is 22mm. This allows a larger volume of water to go from boiler to bath than 15mm. But having read around it seems a 15mm pipe is better for a combi boiler as there will be less water hanging around in the pipes getting cold and having smaller pipes will reduce the amount of water coming out of the tap and MAY mean the water that comes out will come out hotter.
Would it herefore make sense to put in a flow restrictor or a piece of 15mm pipe into the 22m pipe before it reaches the tap to slow things down? But would this not effectivley be the same as just putting the on at 'less than full speed'
I think I am confused now! Any plumbers on here advise?
If I put a mxier shwoe in I guess it will control the incoming hotwater flow anyway so there would be no need to mess with the pipes?
Your combi shouldn't have any problems providing a decent shower, and will be substantially better than electric shower.
Even a powerful 10kW electric will struggle to output more than 5 to 6 ltr/min.
A small combi should be able to output 8 to 9 ltr/min of hot water. Since you'll be blending that with cold, you'll be able to get ~15 ltr/min from the shower, which is plenty for a really nice shower from a single shower head.
Water pressure and flow rate from the mains is also important. One would hope that since you've had a combi fitted these were checked prior to the installation and are fine.
You can easily check the hot flow rate from yours by measuring the output from existing taps running off the combi.
Because the combi is working off of mains pressure, 15mm pipe to outlets would normally be used over 22mm because the increased pressure and maximum flow rate achievable from the combi means that 15mm is perfectly adequate, and is cheaper to install than 22mm.
The only real disadvantage of using existing 22mm pipe is that it's going to take that much longer for the hot water to reach the outlet, as the volume held in the pipe is greater. This also means that you'll be heating a little extra water each time just to fill the pipe. Once the hot water has reached the outlet, there'll not be any noticeable difference in performance.
Realistically, given the cost of copper pipe, and the labour costs associated with changing it over, it'll take an awful lot of showers heating a little extra water each time to make it cheaper to swap over to 15mm.
Even a powerful 10kW electric will struggle to output more than 5 to 6 ltr/min.
A small combi should be able to output 8 to 9 ltr/min of hot water. Since you'll be blending that with cold, you'll be able to get ~15 ltr/min from the shower, which is plenty for a really nice shower from a single shower head.
Water pressure and flow rate from the mains is also important. One would hope that since you've had a combi fitted these were checked prior to the installation and are fine.
You can easily check the hot flow rate from yours by measuring the output from existing taps running off the combi.
Because the combi is working off of mains pressure, 15mm pipe to outlets would normally be used over 22mm because the increased pressure and maximum flow rate achievable from the combi means that 15mm is perfectly adequate, and is cheaper to install than 22mm.
The only real disadvantage of using existing 22mm pipe is that it's going to take that much longer for the hot water to reach the outlet, as the volume held in the pipe is greater. This also means that you'll be heating a little extra water each time just to fill the pipe. Once the hot water has reached the outlet, there'll not be any noticeable difference in performance.
Realistically, given the cost of copper pipe, and the labour costs associated with changing it over, it'll take an awful lot of showers heating a little extra water each time to make it cheaper to swap over to 15mm.
I have very high mains water pressure.
My fear is that the max hot water flow rate isn't hot enough.
How do you measure hot water flowrate though? Do you set the tap at the maximum it can output (only warm) or maybe mid way (really hot). There will be a significant difference in flow rate.
Presumably if you have a thermatatic mixer shower it will take the hot water at its hottest and mix it with cold to add pressure but cool it down ?
My fear is that the max hot water flow rate isn't hot enough.
How do you measure hot water flowrate though? Do you set the tap at the maximum it can output (only warm) or maybe mid way (really hot). There will be a significant difference in flow rate.
Presumably if you have a thermatatic mixer shower it will take the hot water at its hottest and mix it with cold to add pressure but cool it down ?
Yes, the thermostatic mixer will blend cold water with the hot to reach the desired temperature. The pressure will remain the same, since both are coming off the mains, but the flow will increase since you're now adding an additional source to the hot water supply.
To measure the flow, open the tap up completely. The temperature of the water will be cooler than a partially opened tap, but remember that by the time you force the water through a thermostatic cartridge and out of a shower head, the flow rate will be lower than that of a fully open tap anyway, slowing the hot flow, raising the temperature and blending in cold. If you fit one of these new eco shower heads that blend air with the water your flow rate demand will drop even more, yet give just as effective a shower.
Tbh, if your mains pressure and flow are fine, the combi isn't on its last legs, and you're not trying to feed a shower with multiple outlet jets, I can't see the combi not being able to supply a decent shower. If it can't then it's really not fit for purpose.
A shower can only supply so much water anyway, and even a small combi will have no problem meeting that demand. Too much flow and it can kill the shower cartridge - there was one installation I attended where the shower was being fed by mains cold and pumped hot, both at 3 bar and with very high flow rates. I had to fit flow restrictors in the shower elbows as the thermostatic cartridge kept getting jammed and failing.
To measure the flow, open the tap up completely. The temperature of the water will be cooler than a partially opened tap, but remember that by the time you force the water through a thermostatic cartridge and out of a shower head, the flow rate will be lower than that of a fully open tap anyway, slowing the hot flow, raising the temperature and blending in cold. If you fit one of these new eco shower heads that blend air with the water your flow rate demand will drop even more, yet give just as effective a shower.
Tbh, if your mains pressure and flow are fine, the combi isn't on its last legs, and you're not trying to feed a shower with multiple outlet jets, I can't see the combi not being able to supply a decent shower. If it can't then it's really not fit for purpose.
A shower can only supply so much water anyway, and even a small combi will have no problem meeting that demand. Too much flow and it can kill the shower cartridge - there was one installation I attended where the shower was being fed by mains cold and pumped hot, both at 3 bar and with very high flow rates. I had to fit flow restrictors in the shower elbows as the thermostatic cartridge kept getting jammed and failing.
Thanks for your help
My electric shower broke so I ordered a replacement. I am wondering whether I should try out a thermostatic mxier shower instead (although I will lose my safety net of electric shower should the boiler break!)
Maybe I will try a dual system for a while!
Due to pipe placements I will probably now go with having a mixer shower that uses the bath pipes and becomes a part of the tap rather than have a new hot water feed. Are these as good as wall ones? If so, is it just a case of taking off the old bath taps and attaching the shower?
My electric shower broke so I ordered a replacement. I am wondering whether I should try out a thermostatic mxier shower instead (although I will lose my safety net of electric shower should the boiler break!)
Maybe I will try a dual system for a while!
Due to pipe placements I will probably now go with having a mixer shower that uses the bath pipes and becomes a part of the tap rather than have a new hot water feed. Are these as good as wall ones? If so, is it just a case of taking off the old bath taps and attaching the shower?
No harm having a backup plan 
There's not many bath filler/shower mixers with thermostatic control - most are just manual mixers. Those that are tend to be a little less aesthetically pleasing.
Having said that, a manual mixer isn't bad - it's just prone to temperature fluctuations if someone opens an outlet elsewhere in the house at the same time.
It'll give you a good idea of what the shower will be like from the combi, and is significantly quicker and easier to fit than a new dedicated thermostatic shower. It is just a question of removing the existing taps and swapping for the new one - assuming you don't have oddly spaced holes in the bath.
There's not many bath filler/shower mixers with thermostatic control - most are just manual mixers. Those that are tend to be a little less aesthetically pleasing.
Having said that, a manual mixer isn't bad - it's just prone to temperature fluctuations if someone opens an outlet elsewhere in the house at the same time.
It'll give you a good idea of what the shower will be like from the combi, and is significantly quicker and easier to fit than a new dedicated thermostatic shower. It is just a question of removing the existing taps and swapping for the new one - assuming you don't have oddly spaced holes in the bath.
I have measure my maximum flowrate on my bath - measured in litres per minute
Cold - 22.5
Hot - 15.9 (although at this rate its barely war - very hot is prob closer to 8-9)
Hot - 11.4 (with the cold sink tap running at full speed - likely to happen if someone uses kitchen tap downstairs whilst showering).
Is a thermo shower more compelx than just changing over the taps then? It looked to me liek they were effective self contained.
Cold - 22.5
Hot - 15.9 (although at this rate its barely war - very hot is prob closer to 8-9)
Hot - 11.4 (with the cold sink tap running at full speed - likely to happen if someone uses kitchen tap downstairs whilst showering).
Is a thermo shower more compelx than just changing over the taps then? It looked to me liek they were effective self contained.
Mojooo said:
I have measure my maximum flowrate on my bath - measured in litres per minute
Cold - 22.5
Hot - 15.9 (although at this rate its barely war - very hot is prob closer to 8-9)
Hot - 11.4 (with the cold sink tap running at full speed - likely to happen if someone uses kitchen tap downstairs whilst showering).
Is a thermo shower more compelx than just changing over the taps then? It looked to me liek they were effective self contained.
Those numbers are fine. My elderly combi produces very hot water at about 7ltr/min and has no trouble at all producing a very respectable shower, and one far better than any electric shower that I've seen.Cold - 22.5
Hot - 15.9 (although at this rate its barely war - very hot is prob closer to 8-9)
Hot - 11.4 (with the cold sink tap running at full speed - likely to happen if someone uses kitchen tap downstairs whilst showering).
Is a thermo shower more compelx than just changing over the taps then? It looked to me liek they were effective self contained.
The issue with manual mixers and temperature fluctuations occurs more with people using a hot outlet elsewhere rather than the cold. Unless your mains flow rate is borderline, supplying cold to multiple outlets isn't normally an issue.
Again, referring to my flat, we've no problems if one of us in the (manual mixer) shower and someone flushes the WC in the other room, but woe betide me if I try running a kitchen sink full of hot water for the washing up if the OH is in the shower
A thermostatic shower/bath filler isn't any more complex to fit, it's still just a question of changing the taps over. A simple mixer can be designed with a slimmer body as it doesn't have to fit the thermostat gubbins inside, so tend to come in a wider variety of styles and designs. It's just aesthetics though, so if you find a thermostatic one that you like the look of, then you're on to a win win situation.
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