Bathroom remodel.

ohreallƳ

I intend removing my existing shower cubicle (1200x900 quad) and installing a newer 1400x800 walk in job.

I currently have a 10.8kW elec shower supplied by a 10mm t&E cable.

Now a thought entering my empty head is can I connect the DHW output from my Worcester combi to the inlet solenoid on the shower?  

I realise that contravenes the manufacturers installation instructions however what are the implications of doing so?  

The reason for considering this is the boss would like a power shower however that may be a pain in the !!!!!! to accommodate so thinking if I can increase the incoming temp she will get far greater throughout of hot water than is presently possible.

I won't have any desire to take a cold shower.

Downsides?

I've had half a bottle of red wine so.......

bjorn_toby_wilde

If you have a Combi, why not install a mains mixer shower?

I don’t get why you think feeding the shower with hot water at the same pressure as the cold water through an identical bore pipe would somehow increase the flow.

Definitely too much red wine 😉

WIAWSNB

This Q has been asked more than once, OhreallY, but at least you have an excuse.
It can't be done because, um, er, well I'm pretty sure it can't be done...
Are you talking about blending the hot and cold supply to the electric shower, or just full DHW from the combi?
The latter certainly wouldn't be sensible, as you wouldn't be able to control the shower temp and you'd likely scald, and the shower would almost certainly overheat and trip the safety 'stat. 
But, in theory, if you could blend in some 'hot' to effectively raise the incoming water temp by, say, 10o or so, then I'd imagine you'd have a much healthier shower flow, and the shower should cope ok.
A blending thermostat - commonly available - should be able to do this. 
However, it would require a relative trickle of hot from your combi, and I'm not sure I see one running that slowly. And if the combi shuts off, your shower temp would cycle.
In short, in theory it will work, but you'd be bonkers to try.
Since you have a combi boiler, the obvious answer is to run a 15mm DHW pipe to accompany the existing cold, and fit a thermostatic mixer shower. That will give you an effective 'power shower', far superior to even a mod'ed leccy jobbie.
Combis usually give great showers, provided your incoming mains is decent. A 30kW model should provide approaching 20lpm at the shower head, which compares rather well to the ~7lpm max from an electric job.

sheenas

As others have said engineering you one shower system is going to be a fail. I would go the route of installing a mixer tap and using the combi boiler. Safe and well proven.

ohreallƳ

bjorn_toby_wilde said:

If you have a Combi, why not install a mains mixer shower?

I don’t get why you think feeding the shower with hot water at the same pressure as the cold water through an identical bore pipe would somehow increase the flow.

Definitely too much red wine 😉

The rationale was, the flow rate, anywhere near maximum, can't be heated quickly enough as it passes over the element far too quickly and is only a few degrees warmer that ambient.

My mains pressure is excellent and far greater than what the combi can supply.

So thinking, and I accept I may not be equipped for that, increasing the incoming temp by connecting to the combi DHW output will relieve the workload on the element cannister thus heating the already hot water, set to say ~44 degrees to a higher temp at far greater flow rate.

If temp requires adjustment, say between summer and winter incoming temps surely that can be accommodated by adjusting the DHW setting at the boiler and or increasing/ decreasing the flow passing over the element, even reducing the electric power on the shower by selecting 'low', I suspect may be around 5kW, (can check with a clamp meter), in warmer seasons.

Ty all.

WIAWSNB

ohreallƳ said:

bjorn_toby_wilde said:

If you have a Combi, why not install a mains mixer shower?

I don’t get why you think feeding the shower with hot water at the same pressure as the cold water through an identical bore pipe would somehow increase the flow.

Definitely too much red wine 😉

The rationale was, the flow rate, anywhere near maximum, can't be heated quickly enough as it passes over the element far too quickly and is only a few degrees warmer that ambient.

My mains pressure is excellent and far greater than what the combi can supply.

So thinking, and I accept I may not be equipped for that, increasing the incoming temp by connecting to the combi DHW output will relieve the workload on the element cannister thus heating the already hot water, set to say ~44 degrees to a higher temp at far greater flow rate.

If temp requires adjustment, say between summer and winter incoming temps surely that can be accommodated by adjusting the DHW setting at the boiler and or increasing/ decreasing the flow passing over the element, even reducing the electric power on the shower by selecting 'low', I suspect may be around 5kW, (can check with a clamp meter), in warmer seasons.

Ty all.

Have you read my reply? 
In any case, if you set the supply temp at 44oC - already painfully hot - what do you expect the leccy shower to add to this? Other than blisters.
And if you set the combi output at a low 44oC, you'll find it disappointing for things like washing dishes and filling baths. 
If you are daft enough to pursue this, you should use a thermostatic blender to provide, I imagine, a ~20oC-ish supply, possibly a bit higher. Yes, in theory it'll work wonders, but you'll be having both a boiler and electric shower running simultaneously. Absolutely bonkers.
Anyhoo, you've been given the best advice - a combi will give a far superior shower on its own, and will be cheaper to run. 

FreeBear

ohreallƳ said: Now a thought entering my empty head is can I connect the DHW output from my Worcester combi to the inlet solenoid on the shower?  

I realise that contravenes the manufacturers installation instructions however what are the implications of doing so?

Probably breaches HSE guidance on such things, and I dare say Building Control would have a few things to say about it. Any plumber wanting to protect himself from legal claims will refuse to carry out the work.

You are spending ££££ on a new bathroom, just budget another £200 or so for a decent thermostatic shower connected to both the cold supply and the hot feed from the combi.

ohreallƳ

Have you read my reply

I Have

In any case, if you set the supply temp at 44oC - already painfully hot - what do you expect the leccy shower to add to this? Other than blisters.

Ability to provide greater flow rate on reduced power (shower on low setting) (I stated this)  44C is certainly not painfully hot and is in the region NHS set mixing taps and baths to. 


And if you set the combi output at a low 44oC, you'll find it disappointing for things like washing dishes and filling baths. 

I have a dishwasher. Bath filling not required


Anyhoo, you've been given the best advice - a combi will give a far superior shower on its own, and will be cheaper to run. 


The combi DHW output is limited by the boiler not my incoming supply which is considerably greater. If the shower flow rate control valve is fully opened, the shower hose and head just about take off - the combi out pit is nowhere approaching that.

I could also ask if you read my reply but I won't; I can supply copious sarcasm myself.

I was simply seeking reasons why this was going to be a problem.


Addendum: HSE states hot water should reach a temp of 55C within 1 minute at outlets  https://www.hse.gov.uk/legionnaires/hot-and-cold.htm

I can also provide link to the NHS HTM if you require but that is getting away from my question.

Ty.

ohreallƳ

FreeBear said:

Probably breaches HSE guidance on such things, and I dare say Building Control would have a few things to say about it. Any plumber wanting to protect himself from legal claims will refuse to carry out the work.

You are spending ££££ on a new bathroom, just budget another £200 or so for a decent thermostatic shower connected to both the cold supply and the hot feed from the combi.

Probably breaches HSE guidance on such things

Tell me more on this.

Any plumber wanting to protect himself from legal claims will refuse to carry out the work.

I would agree, however I'm doing the work myself whether this or a thermostatic shower.

I've explained, I have cold water pressure most would kill for, the combi DHW output is limited by what the combi can supply not the incoming pressure, which if connected to the elec shower input solenoid opens the possibility of a higher pressure/ greater flow output shower output to that of a pumped system.

For everyones clarity, I am not hell bent on doing this, I simply was wondering, if it wont work - why?

WIAWSNB

ohreallƳ said:

Have you read my reply. I Have.
In any case, if you set the supply temp at 44oC - already painfully hot - what do you expect the leccy shower to add to this? Other than blisters.
Ability to provide greater flow rate on reduced power (shower on low setting) (I stated this)  44C is certainly not painfully hot and is in the region NHS set mixing taps and baths to. 
And if you set the combi output at a low 44oC, you'll find it disappointing for things like washing dishes and filling baths. 
I have a dishwasher. Bath filling not required.

Anyhoo, you've been given the best advice - a combi will give a far superior shower on its own, and will be cheaper to run. 
The combi DHW output is limited by the boiler not my incoming supply which is considerably greater. If the shower flow rate control valve is fully opened, the shower hose and head just about take off - the combi out pit is nowhere approaching that.
I could also ask if you read my reply but I won't; I can supply copious sarcasm myself.
I was simply seeking reasons why this was going to be a problem.
Addendum: HSE states hot water should reach a temp of 55C within 1 minute at outlets  https://www.hse.gov.uk/legionnaires/hot-and-cold.htm
I can also provide link to the NHS HTM if you require but that is getting away from my question. Ty.

:-)

I'll ignore 80% of your reply, and cut to the important bits.

"The combi DHW output is limited by the boiler not my incoming supply which is considerably greater. If the shower flow rate control valve is fully opened, the shower hose and head just about take off - the combi output is nowhere approaching that." 

I get what you are saying. You have great incoming mains, and it's the boiler that limits how much DHW can be produced. This is correct. What size combi do you have? If it's a typical 30kW jobbie, then it'll provide around 12lpm at near 60oC - which is what mine does. When I jump under my shower, I get approaching 20lpm of thermostatically-controlled water at around 38oC, which is enough to bounce off my head and make a mess, obliging me to tweak the flow control down. In this balmy weather, I usually turn the temp down too so it's more like mid-30s, which would mean the water flow would nearly knock me off my feet - probably around 25lpm - if I didn't turn it down. This is from a handset which has sizeable holes in it, and not the teeny wee things in a leccy shower's shower head like yours. 

Your shower seems powerful, but it ain't. It's because you have great pressure, which is pushing the water through TINY wee holes = jets. The actual quantity of water coming out is waaaay less than you think. If you don't believe me, then set your shower to provide a nice 40oC output, and measure the litres per minute being produced at this temp; it'll be around 6.  It is literally like a pressure-washer; these push out water at around a staggering 100psi, but the quantity is less than that from the open tap that's supplying it. 

So, my combi gives me around a 20lpm shower, more if I want, and at a thermostatically-stabilised temp, which is constantly adjustable. Your combi will almost certainly give you a similar performance to this = a cracking shower.

This, on the other hand, is what a typical 10.8kW shower gives you; around 7lpm at a flow temp of around 40oC. And not thermostatically controlled - unless you choose a rare model that is.

That's the comparison. This should not come as any surprise, since a combi is roughly three times as powerful, but at no extra running cost, since gas is roughly thee times cheaper than leccy. 

Then you have your idea - and as I said, you ain't the first to wonder (as have I, many moons ago). Yes, if you can raise the incoming temp, it'll have a direct and positive impact on the flow rate at the showerhead. Have a look:

 

At a typical incoming temp of around 10oC, you'll get just over 5lpm with a 40oC shower. Raise the incoming temp to 20oC, and the flow should increase to nearly 8lpm. 30oC, and it'll be significantly higher still.

The Q is, how do you provide a stable supply at 20 or 30oC from a combi? The answer is as I said before - you use a thermostatic blender. But it begs the Q - why? Why use a combi to tweak up the supply temp to a leccy shower, when it'll do a much better job all by itself?

And then you take it to its logical (mis)conclusion; why not supply the leccy shower with 40oC water from the combi? Well, the answer is multi, I think;

1) If you supply 40oC to the shower, what 'work' will the shower do? It cannot raise the temp any higher or you'll scald. 

2) How do you turn your leccy shower temp down? You cannot.

3) In theory you can increase the flow through the shower by it adding more leccy heat to the incoming 40oC provided the water flow is commensurately higher. But it'll be working in a hard-to-control state as it isn't thermostatic; if you turn the flow rate down, the temp will increase to above 40oC. Any alteration in the supply temp - someone flushes the loo - expect an unpleasant outcome; the incoming temp will still be 40oC, the shower's electric elements will still be adding their bit, but the overall water flow rate will have fallen = potentially dangerous increase in temp. The temp certainly ain't going to fall below the just-about-bearably hot 40oC - it can only go higher. And that is dangerous.

4) Have you checked with the manufacturer what max supply temp your shower can take? I'm going to guess that - once they stop laughing - they'll say 20 or so degrees. 

5) Your leccy shower still has piddly wee showerhead nozzle holes, designed for a piddly leccy shower rate of around 8lpm max. You'll need a new showerhead with much larger holes. And if you use that showehead without the combi adding its bit, then the flow appearance will be awful - a gentle dribbly shower.

I think the issue here is that you don't actually appreciate just how effective a combi boiler is at giving a really good shower. It will be at least three times better than your existing leccy shower even when running at its best. 

ohreallƳ

Ok, point taken, I'll drop the idea.

I'll work on the basis of a thermostatic shower is way to go.

Any to avoid?