Venturi Showers

[civory]

I've seen one or two posts on the subject of Venturi Showers, but could do with some unbiased opinions.

I want to install a shower and like the idea of a Venturi - we have low-ish hot water pressure and normal-ish cold water pressure (I'll be measuring more accurately hopefully tomorrow with a mains pressure water gauge).

Anyone have any ideas/thoughts/warnings etc.?

Many thanks

 

[Softus]

I've just installed a Trevi Boost venturi shower for a customer.

The user said that it is "brilliant".

I chose the Trevi over the two alternative brands because:

1. My local supplier sells them;
2. The guarantee was longer;
3. There was more technical information available on the Trevi web site.

I followed the manufacturer's instructions to the letter, including the 22mm full bore single check valves.

I didn't use any isolating valves because:

a) I didn't want to add any other constriction to the pipework;

b) the bathroom was next to the airing cupboard that contained both the mains stop cock and the DHW gate valve, therefore isolating in an emergency would be easy;

c) the cold storage cistern was of the 4 gallon F&E type, so even if the DHW GV failed to hold then, after shutting off the mains, no more than a couple of gallons would be deposited on the flat below.

The head in my application was about -5cm; given the mains pressure of about 3 bars, this head was well within the Trevi specified limit of -50cm.

Hope this helps.

 

[civory]

Hi,

Thanks for the info - I notice that the Trevi Boost specifies a maximum cold water pressure of 3.0 bar.

We've got hot water pressure of 0.3 bar and cold water of 5.8, according to my mains water pressure gauge.

Can we use a Venturi shower in this situation? The bottom of the hot water cylinder is about 1m above shower head.

thanks

 

[Softus]

Hi civery

Hi,
Thanks for the info - I notice that the Trevi Boost specifies a maximum cold water pressure of 3.0 bar.

We've got hot water pressure of 0.3 bar and cold water of 5.8, according to my mains water pressure gauge.

Can we use a Venturi shower in this situation?

Yes, but you'll need to fit a pressure limiting device on the cold supply to the shower. Only you will know the most convenient location for this.

The bottom of the hot water cylinder is about 1m above shower head.

thanks

The minimum head for the hot supply is negative 0.05 bar, but I don't believe that there is any [relevant] upper limit, so you're OK with 1.0m (== 0.1 bar).

Basically, the greater the head for the hot water the better the aggregate flow you'll get. The Trevi instructions contain a graph that illustrates how you need to compendate for a 'poor' head for hot by increasing the temperate of the stored hot water (i.e. immersion stat or cylinder stat). In your case, with good hot and cold pressures, you won't have any delicate adjustments to make.

I think you can find the graph and other technical details on the Ideal/Trevi web site.

Good luck!
S.

 

[civory]

Thanks S - hopefully my final query! What happens if the hot water gets too hot? There seem to be some maxima with regard to temperature.

Regards,

Civory

 

[Softus]

Good question.

The Trevi (and, I believe, the others) don't have a thermostat in the mixer valve, so your only control over the temperature is the setting on the cylinder stat (or immersion stat - whichever you use).

Does this answer the question?

 

[civory]

....kind of answers the question, except that I don't have a cylinder or immersion stat! Should have, I know, but not sure how easy it is to put one in.

Some of these types of showers seem to incorporate a safety cut-out if the temperature gets over a certain level?

CI

 

[Softus]

....kind of answers the question, except that I don't have a cylinder or immersion stat! Should have, I know, but not sure how easy it is to put one in.

Nothing hard about it, but could be made marginally if you don't have enough wiring to start with. Do you have a programmer? If so, what is it?

Some of these types of showers seem to incorporate a safety cut-out if the temperature gets over a certain level?

Er, didn't know that. Or perhaps had forgotten it. Have you looked at the Trevi web site?

 

[civory]

Yes I have a programmer - it's a Potterton with fairly simple settings of once, twice, 24 hours and off and can only be set for the same times every day.

While we're on - a plumber acquaintance of mine who admits no experience of the Venturi system, wondered whether the cylinder could be accidentally drained using one of these showers and what would happen?

 

[Softus]

Yes I have a programmer - it's a Potterton with fairly simple settings of once, twice, 24 hours and off and can only be set for the same times every day.

Firstly, any work you do on the system should be with it isolated from the electrical supply. If the timer is plugged in, then uplug it. If it's on a fused spur then remove the fuse.

I believe that your timer is plain and simple - single-channel, and just a timer rather than a programmer. If so, then you don't have any system controls that act to maintain the temperature of either stored hot water or the living space.

You might be able to introduce both a cylinder stat and a room stat, break into the supply from timer to boiler, and wire the stats in parallel such that either acts to provide the demand to the boiler. I don't know which boiler you have, but be mindful of the possibility that the pump is controller by the boiler (rather than by the timer) for the purpose of an overrun feature, in which case you would need to be careful not to change that arrangement.

Whilst this would limit the temperature of the water, it would stilll be relatively crude because the water would continue to be heated whenever the central heating was on - definately not up to the Building Regulations' recommendations.

To introduce a 'proper' degree of control, you'd need to add a three port motorized zone valve (or two two port MZVs), so that CH and DHW are independant.

This is getting into the realms of upgrading the system, which is generally beyond the remit of this kind of posting - sorry to have to say it but this is a job for your plumber friend (or, perhaps, for him to show you how).

While we're on - a plumber acquaintance of mine who admits no experience of the Venturi system, wondered whether the cylinder could be accidentally drained using one of these showers and what would happen?

The concept of a copper cylinder actually being drained by the shower isn't valid, since the hot water is taken from the top, not the bottom - either your friend didn't mean quite that, or he/she isn't actually a plumber (or is one but shouldn't be!).

However....

if the venturi shower starved the cold storage cistern of its cold feed,
AND
if the quantity of stored cold water is less than the quantity of hot water needed for a shower

(or if the cold cistern just couldn't fill quickly enough)

...then the hot water would cease mid-shower, giving you a bit of a surprise!

Does the hot water peter out when you draw a bath?

 

[civory]

Your replies are most informative thankyou!

To summarise, I'm intending to do the following, hopefully it'll be OK:-

- install a Venturi shower
- leave the current hot water storage arrangements as they are, i.e. without cylinder thermostat (there is a room stat)
- put in a pressure reducing valve for the cold feed
- hope for the best!

The reason I don't wish to go for any major upgrade of the hot water system is that it's in a second home, limited cash to do renovations at present, and in future we may decide to put in a Rayburn or similar to replace the current 'back-boiler' set-up. Meantime, however, we need the shower.

Do you think there are any risks to the above approach?

 

[AdamW]

I have no experience with these showers, but I do know something about the venturi effect, and I saw this type of shower demonstrated by "Howard" on Tomorrow's World many years ago!

I reckon that the lack of a thermostatic valve wouldn't be an issue: If the cold water pressure drops (e.g. through flushing a toilet) then the hot water pressure will also drop accordingly. Seeing as thermostatic valves seem to be there mainly to avoid scalding rather than a cold blast, this should also be safe.

The only real downer to them that I can see is that, like electric showers and combi showers, the flow rate you experience will be proportional to the temperature of the incoming cold water. After all, in winter time you will then turn down the flow of cold water in order to get the same temperature, the venturi-assisted hot water flow will also be reduced. I'm not sure how much by though!

It's a very elegant engineering solution, brilliant idea!

Just wondering though, if it won't cause too much mess to install, would an electric shower give a better flow rate overall? with 5.8bar pressure you certainly have a good bit of oomph in your water pressure: much of London is at 1 bar, not enough to get to the loft in a 3-storey house!!!

 

[Softus]

Do you think there are any risks to the above approach?

The only one I can foresee is that the hot water in the cylinder gets too hot, but you're living with that risk already.

If the cold water pressure drops (e.g. through flushing a toilet) then the hot water pressure will also drop accordingly.

I'm not sure I agree with you there AdamW. I believe that the cold supply to the shower is from the mains, whereas the hot water stored. Therefore a transient drop in mains pressure will not affect the hot pressure, althought it will affect the flow of mixed water from the shower head - this is because the flow of mixed water is an aggregate flow dictated by the 3.0 bar of mains cold and 0.3 bar of stored hot.

... the flow rate you experience will be proportional to the temperature of the incoming cold water.

Erm, not by the temperature, but by the pressure of the two supplies.

After all, in winter time you will then turn down the flow of cold water in order to get the same temperature, the venturi-assisted hot water flow will also be reduced.

Erm, with at least two of the three products that I've read about (one of which I've installed), you can't turn down the cold as you would do on a conventional shower valve.

Just wondering though, if it won't cause too much mess to install, would an electric shower give a better flow rate overall?

If you're talking about an instantaneous water heater type of electric shower, then I would expect one of this type would be worse than a venturi valve, because all such (electric) showers are quite poor.

...with 5.8bar pressure you certainly have a good bit of oomph in your water pressure...

Although there is a minimum pressure at which they will work, yer actual flow from the shower head is a function of the power rating of the element and the specific rise in temperature needed, not a function of the mains pressure. For a given position on the knob, the colder the mains is the poorer the flow will be.

On a final point, another issue with electric showers is the high maintenance cost, because the element cartridge has to be renewed periodically. In areas of relatively hard water this cost rises steeply. People choose them because they're incredibly cheap to buy and quick/cheap to install - only mains water and an appropriately rated electrical supply are needed.

 

[civory]

Now that I've decided to go ahead with the Trevi Boost shower, I wonder can anyone tell me how it's plumbed in? I may need to do the plumbing prior to actually getting delivery of the shower itself, so I would need to know:-

- what kind of connections
- size of pipe feeds (15 or 22mm?)
- simply 1 cold and 1 hot pipe feed into shower valve?
- positioning of plumbing (distance between hot and cold pipe centres)

I know this all seems strange but circumstances may dictate doing the plumbing 'blind' if you see what I mean...

Not least because initial order promised within 7 days is now apparently 4 weeks (trying to find another supplier who can get me one by Monday 31st October).

 

[Softus]

Now that I've decided to go ahead with the Trevi Boost shower, I wonder can anyone tell me how it's plumbed in? I may need to do the plumbing prior to actually getting delivery of the shower itself, so I would need to know:-

- what kind of connections

22mm compression elbows are provided with the valve

- size of pipe feeds (15 or 22mm?)

The instructions recommend 22mm.

- simply 1 cold and 1 hot pipe feed into shower valve?

Quite so. No electrics, no funny business, just cold from the mains and hot from a tank. Don't forget the single check valves - specify that you want full-flow (aka full-bore) check valves when you buy them, and don't be fobbed off with anything smaller.

- positioning of plumbing (distance between hot and cold pipe centres)

Er, you can get this from Ideal/Trevi, but it's in the order of 75mm.

I know this all seems strange but circumstances may dictate doing the plumbing 'blind' if you see what I mean...

Er, no; not sure what you mean. Sounds like a fault-prone way of doing it

Not least because initial order promised within 7 days is now apparently 4 weeks (trying to find another supplier who can get me one by Monday 31st October).

Unless there's some weird and sudden shortage, I would normally expect a two day delivery at my local supplier, so 4 weeks is plain crazy. Who are you trying to get one from? Whereabouts do you live?