Optimal combi boiler for rainfall shower head

[Madrab]

Even then, most combi's, especially @ 36Kw I find will easily reach >45Deg at the HW outlet, many I have commissioned do exactly that. All comes down to flow through the boiler, especially if the input is lower than the specc'd L/min figures, > 45Deg can be happily attained.

... and just to confirm, I happily shower at the temp stop @ 38Deg, sometime a little lower ..... 45deg would be way too hot for me. I know a lot of my customers do the same, especially when I calibrate their showers and they are receiving 'true' temps. A lot of showers out of the box need calibrating properly. I have even come across TMV2 valves out of the box that have been out by a few deg.

Wonder how many installers don't actually check the shower output temps and adjust as necessary

 

[bernardgreen]

All comes down to flow through the boiler, especially if the input is lower than the specc'd L/min figures, > 45Deg can be happily attained.

Do any domestic gas boilers measure the flow rate of the water and adjust the gas flow rate so as to achieve the required temperature of hot water leaving the boiler ?.

The algorithm controlling the aperture in the gas valve would also need to know the temperature of the cold water entering the boiler.

 

[Madrab]

Not specifically, not to that level I wouldn't expect but a lot of cold mains supplies (L/Min) don't reach the max that the boiler can achieve therefore by default they can heat that up to a higher temp (or to what it's set to).
The quoted @35(25)deg rise in most MI's is a target figure and allows them to quote a standardised delivery figure at that rise but boilers will happily heat the HW up to a max of 55>60deg, if flow is lower.
Some have flow limiters fitted through the boiler so they can reach those specified deltas at the specified L/Min .... aaaaand inlet flows can be physically turned down to increase the outlet temps. A lot obviously control the output temps via sensors/stats and modulate to suit.

 

[fixitflav]

Not specifically, not to that level I wouldn't expect but a lot of cold mains supplies (L/Min) don't reach the max that the boiler can achieve therefore by default they can heat that up to a higher temp (or to what it's set to).
The quoted @35(25)deg rise in most MI's is a target figure and allows them to quote a standardised delivery figure at that rise but boilers will happily heat the HW up to a max of 55>60deg, if flow is lower.
Some have flow limiters fitted through the boiler so they can reach those specified deltas at the specified L/Min .... aaaaand inlet flows can be physically turned down to increase the outlet temps. A lot obviously control the output temps via sensors/stats and modulate to suit.

Correct me if I’m wrong, but I assume the HW temperature from the boiler is controlled at the boiler, the gas flow being modulated to maintain the temperature at varying HW flow (up to a limit depending on the boiler rating).

The temperature would usually be well above an acceptable shower temperature. The temperature at the shower outlet set by the shower controls, preferably by a thermostatic mixing valve, otherwise a manual mixer. Obviously, cold water is also piped to the mixer. In that respect it works the same as a stored water system, though in my limited experience not as easy to maintain a stable shower temperature. That’s one of the reasons I wouldn’t have a combi.

 

[Madrab]

Of course, the outlet (thermo shower) controls itself by constantly balancing hot and cold, the boiler controls the temp of the HW input.

A thermostatic valve, preferably TMV2/3 rated would be the ideal. Manual valves are OK as long as the input temps are stable but there is no fail safe or automatic control from scalding if the cold supply is interrupted.

 

[bernardgreen]

Of course, the outlet (thermo shower) controls itself by constantly balancing hot and cold, the boiler controls the temp of the HW input.

though in my limited experience not as easy to maintain a stable shower temperature. That’s one of the reasons I wouldn’t have a combi.

When the water from the shower head is too hot the thermo shower will reduce the amount of hot water taken from the boiler and increase the amount of cold water. That would appear to be the solution to keeping the water from the shower head at the right temperature.

So how could the temperature of the water from the shower head become unstable ?

If the flow of hot water to the shower is reduced then the flow through the boiler is reduced, this could result in the temperature of the hot water increasing, the thermo shower will then further reduce the amount of hot water it takes from the boiler.

The flow of water through the boiler may be reduced even more by the reduction of cold water pressure when the thermo shower takes more cold water to compensate for the increasing temperature of the hot water from the boiler.

 

[Madrab]

So how could the temperature of the water from the shower head become unstable

If it's a manual shower, it won't adjust itself therefore the output temperature can fluctuate if other hot or cold outlets are open.

If the flow of hot water to the shower is reduced then the flow through the boiler is reduced, this could result in the temperature of the hot water increasing, the thermo shower will then further reduce the amount of hot water it takes from the boiler.

A thermostatic shower will eventually settle down in concert with the boiler to deliver the desired outlet temp, based primarily on the HW output temp setting on the boiler. A thermo shower will always take the max amount of HW it can and then as it warms adds cold to maintain the set temp. Usually it will only reduce HW draw from the boiler if it can't reach it's set temp by just adding cold. As a normal shower temp is reached with a 2:1 (H>C) ratio then the Hot will invariably be on full (as much as the shower can be full on given it's narrow waterways etc) and if the hot is reduced then the boiler control will modulate down to maintain its set temp.

The flow of water through the boiler may be reduced even more by the reduction of cold water pressure when the thermo shower takes more cold water to compensate for the increasing temperature of the hot water from the boiler.

Indeed, if the mains dynamic pressure isn't capable of maintaining a suitable head of pressure then that can always be a drawback. Of course that should always be considered by the professional installer when a combi is considered as an option, as should be the case when considering any mains water driven HW delivery system.

 

[fixitflav]

So how could the temperature of the water from the shower head become unstable ?

No problem if the HW temperature from the boiler is well controlled, but I've seen cases where it reaches temperature, then the gas cuts off, and restarts at a lower temperature (with a non-adjustable differential!). That gives a fluctuating, and very irritating, temperature variation at the shower. Maybe the latest combis are better.

 

[bernardgreen]

A thermo shower will always take the max amount of HW it can and then as it warms adds cold to maintain the set temp.

Which means the flow rate from the shower head has to increase when cold water is added to the maximum flow of hot water, will the manual flow control allow that ?

 

[fixitflav]

Which means the flow rate from the shower head has to increase when cold water is added to the maximum flow of hot water, will the manual flow control allow that ?

I don't see why. When the (total) flow is set it should stay constant. When starting from cold the thermovalve takes all hot water (as Madrab said), and as it warms varies the proportion of hot and cold to give correct temperature, with the same total flow.

 

[Agile]

The OP seems to have ducked out of the discussion.

I should add that if the proposed "plumber" who is expected to install the boiler cannot advise and answer all these questions then I would wonder if he has sufficient knowledge and experience to do the job.

A shower head will produce a flow that depends on the applied water pressure. That will vary with the settings of the controls.

Pressure is related to the flow rate when a flow is taken and so also varies. The dynamic flow rate from the mains needs to be measured. That needs a little understanding.

I would never usually recommend a combi boiler to supply a high flow rate shower head because the temperature varies as the flow rate varies and so when someone else uses hot water then the shower will get much colder.

That is why a hot water cylinder is always preferable to supply a high flow rate shower.

I suspect a potential installation disaster based on the information given so far.

 

[Madrab]

Which means the flow rate from the shower head has to increase when cold water is added to the maximum flow of hot water, will the manual flow control allow that ?

Depends on the output of the shower and like everything water related, at full flow, it will flow as much as it can at a specific pressure. Given both are at mains pressure then they will balance each other out and find an equilibrium.

Manual showers tend to have a higher flow output as they don't have a thermostatic valve and as narrow waterways for the water to work its way through, even then most showers will probably be able to flow more than the output of most combi boilers

 

[Madrab]

I suspect a potential installation disaster based on the information given so far

??

 

[Agile]

??

Well we have an OP who wants to specify a shower head without understanding or even knowing its flow and pressure requirements.

It then seems that, rather than the plumber being involved in choosing a suitable boiler, he is not having any input at all and again the OP is trying to choose the boiler.

The fact that no one has even made the first step, before choosing anything, of measuring the dynamic flow rate from the mains, further compounds the likelihood of a disaster.

 

[Madrab]

Well we have an OP who wants to specify a shower head without understanding or even knowing its flow and pressure requirements.

It then seems that, rather than the plumber being involved in choosing a suitable boiler, he is not having any input at all and again the OP is trying to choose the boiler.

The fact that no one has even made the first step, before choosing anything, of measuring the dynamic flow rate from the mains, further compounds the likelihood of a disaster.

Ah OK, I thought you were referring to the advice being given .... and comments which as you suggest have kind of ran off at a tangent