Why is this a bad idea?

Hi all,
I'm guessing that this is a pretty daft idea as I have seen no-one else suggest it. I'm looking at shower options at the moment, and I live in an area with fairly low water pressure. One of the easiest options would be to have an electric shower, but I have hear that the water pressure would be even worse in winter as the shower would need to heat the water from a lover ambient temperature.
So, why not mix the water with hot water from the combi? I could have a feed fron the hot and cold water supply going into one pipe and then feeding the electric shower. The water would be pre-warmed and therefore could be made the same all year round. If the electric shower wasn't raising the temperature too much it woudn't have such a problem delivering more water would it?
Like I say, this is based purely on my own "sort of" logic rather than any fact, so shoot me down gently!
Cheers Mike
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Mike Hibbert wrote:

There is no reason you couldn't feed the hot and cold into a thermostatic mixer and then into an electric shower. I wanted some more pressure for my shower so ran a large feed from the cold tank (yes I know you haven't got one, but you could add one) into a pump and then into a high power shower unit, this works fine and you can't get scalded by someone flushing the loo.
MrCheerful
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Hi,
when you say a "high power shower unit" do you mean an electric shower? If so, what sort of wattage is it and what sort of flow rate can you get?
cheers mike
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output of the combi to a cold supply at mains pressure, you won't have any flow through the combi - because the input and output will be at the same pressure.
If the combi delivers an adequate supply of hot water, all you need is a shower mixing valve to blend hot and cold - rather than an electric shower.
Roger
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Not sure I understand your first point, what I thought about doing would be a "Y" shaped connection with the hot (from combi) and the mains cold coming in at the top bits of the Y and then the shower being supplied from the the down bit. The water should draw from both supplies?
Unfortunately the supply from the combi isnt that good, the mains cold is only 11 litres per minute at best.
Cheers Mike
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Er, no! As I understand it, the only thing that makes water flow through the combi is the pressure difference between input and output. In other words, the input is at mains pressure and the output is at a much lower pressure - equal to the back pressure generated by the flow going through the pipework and open tap(s).
However, if you connect the combi output to a source of mains pressure through a Y-piece, you immediately raise the output to mains pressure - thus removing the differential. Don't forget that pressure is homogeneous - and acts equally in all directions - so there's no inherent reason why flow can't come down one branch of the Y and go back up the other branch rather than along the down pipe.
You might still get *some* flow through the combi, because there will be a small amount of back pressure in the cold feed to the Y-piece, but the vast majority of the water is going to take the line of least resistance - which is the direct feed to the Y-piece, by-passing the combi. So you will be little better off than if you simply took the cold mains straight to the electric shower.
Roger
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Yeah, this is a good point, but (and this is a big but....) we are currently not on a meter and really dont want to be on one. We have a fairly large pond and what with evaporation, this would be expensive to keep topped up. If we upgraded anything, the water board would (I expect) want us to go onto a meter.
Mike
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Do some water consumption calculations. Most people are pleasantly surprised at how cheap being on a meter can be. Inquire, you may not be forced onto a meter.
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A further question on the same topic. We have fantastic pressure as far as I can tell. Only the kitchen tap is directly connected to the mains at the minute and you can turn it full on without soaking yourself. However I have asked Waterline what the pressure is, to check everything out for a combi, and am waiting 3 weeks later for some-one to get back to me... This may be a stupid question , but is it possible we would need to get our mains supply uprated for a combi supplying 2 showers, 1 bath and all the other usual stuff? I wonder if we have good pressure but a small pipe and performance can't be measured by the kitchen tap. How do I check what size ours is? Sorry I know this is really really basic for you guys, but I know so little, and am a bit of a give-it-a-go Jane.
Ta Suzanne
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There is an assortment of information at
http://www.ofwat.gov.uk/aptrix/ofwat/publish.nsf/Content/waterpressure
The key point regarding what the water supplier has to do is
"The DG2 measure is ten metres head of pressure, at the external stop tap, at a flow of nine litres per minute. This should be sufficient to fill a one-gallon container in 30 seconds. This level of pressure does not override the duty to supply water constantly at a pressure to reach the upper floors of properties. "
The important point is to differentiate between static and dynamic pressure and flow rate.
The static pressure at a given point is the pressure with no water flow. If you were to connect a pressure gauge at your kitchen tap and there is no flow into the house the (static) pressure will be the same as at the external stop tap. If you then start drawing water into your house, the pressure at the kitchen tap will drop, possibly markedly. At the external stop tap and especially the water main, it may drop a little, but if the distributionsystem is well engineered, it should not be by very much. The longer and the thinner the communication pipe from the water main to your kitchen tap, the greater the pressure drop under *dynamic* conditions will be. The greater the flow that you draw into your house, the greater the pressure drop as well.
So this is why it can be interesting to upgrade the communication pipe, but only if the pressure outside at the main is worthwhile. You will notice that Ofwat only makes the water suppliers deliver 9 litres/minute at 10 metres head of pressure at the external stop cock. This is a very poor standard indeed and is probably something that they can easily meet. It would be just about enough to supply one shower directly, but not brilliantly, and certainly not two.

What rate are you getting at the kitchen tap?
It is possible that the communication pipe is small and would benefit from being upgraded. However, this can be expensive to do since you have to pay. You would really need to have measurements taken to be sure, but I wouldn't commit to anything in terms of upgrades or boiler changes until you know where you stand.

You can check the pipe size by using a caliper gauge to measure the diameter directly or you can wrap a thread around the pipe and carefully measure the length. Divide by pi (3.14) to get the external diameter of the pipe.

.andy
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Once again Andy, thanks a million.
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Check the flow rate by using a bucket. How many litres per minute? Pressure in some combi's is not a major issue as they can operate on very low pressure. As long as the flow rate is there they will operate very well.
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On Wed, 27 Aug 2003 20:11:32 GMT, "Mike Hibbert"

I'm not sure that it's going to help you.
Electric showers are normally rated at up to about 10kW and the pipework and shower head designed to give a fairly low flow rate. This is because 10kW is a relatively small about of heat to add to water to raise it from cold water mains temperature to the normal working temperature of around 40 degrees.
You can actually do the sums. Let's say the water is at 10 degrees coming in. You need to raise the temperature by 30 degrees, so using Heat energy = mass x specific heat x temperature rise, we have
10000 = M x 4200 x 30
since the specific heat of water is 4200 Joules/kg
A litre of water weighs approx. 1kg so you can work out that this amount of energy will be enough to heat approx. 0.08 litres per second or about 4.75 litres/minute.
This equates to a disappointing shower.
A typical combi may have a power rating of 20-30 (possibly even as much as 45kW), so under the same circumstances of needing to raise the water temperature by 30 degrees will give you 2 to 4.5 times the flow rate, which becomes a lot more sensible.
However, if the issue is that the water pressure, or more importantly the flow rate is poor, then adding in more heating of the water is not going to help.
If the issue were that the combi is under-rated and water supply is adequate, then theoretically, adding in another source of heat would be interesting. However, I am not sure that water regulations or the manufacturers of electric showers would make this a feasible option.
With a flow rate issue because of the water supply, the options would be to upgrade the water supply pipe from the street main (you would have to pay for this unless the supply rate is *extremely* low, or to have some form of cold water storage tank and a shower pump.
.andy
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wrote:

Thanks Andy, I have already looked at getting a cold water supply and then pumping it into the combi, this could well be the best bet, but there are lots of pitfalls for me with that (primaily being a total lack of plumbing skills - but a willingness to learn!). I would still want to have mains tap water at some of the sinks so would have concerns about the hot being under pressure and the cold being much less (at the moment it is about 1 bar or 11 litres per min). Your other option though sounds good, would I have to get intouch with the water board and they would do the work? Presumably, this woudl mean I have to go onto a water meter afterwards?
Cheers for the help
Mike
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On Wed, 27 Aug 2003 20:54:08 GMT, "Mike Hibbert"

Another thing that you could do, if you have the space is to add in what amounts to a conventional hot water system on a small scale.
This would consist of a roof tank for the cold water and a smallish hot water cylinder. You would need to run the coil of the hot water tank by connecting it across the central heating circuit and using a motorised diverter valve. Your existing CH controller may be able to handle the control aspect of this but otherwise a fairly inexpensive one will do. The principle would be that the CH would work as it does today, but that a thermostat on the cylinder which would operate when the water needs heating, would move the motorised valve over so that the CH water goes through the cylinder coil, and would also fire up the boiler. In this way of working (which is standard for non-combi systems) the boiler really doesn't know that it's being asked to provide heat that's going to a HW cylinder rather than the radiators.
You could use the hot water from this cylinder just to run the shower or possibly the bath as well if you wanted. Both the hot and the cold water for the shower would be derived from the roof tank and a pump could be included to boost the pressure and flow at the shower - all completely safely. The combi output could continue to feed all the other taps that in any case don't usually need as much flow rate, and also you would have a mains derived supply at both taps in such cases - i.e. equal pressures. I agree with you that it doesn'[t seem to safe an idea to have high pressure hot and low pressure cold.
The results that you could get at the shower would be limited only by the size of tank and cylinder - so you need to think about what flow rate you want (10 -15 litres/min is good) and how long you want the shower to run before the cylinder ran out of hot water. Another thing that this arrangement does is to allow you to have the hot water stored at 60 degrees. Thus even if the boiler is a bit undersized it doesn't really matter too much and if you mix hot water at 60 with cold water at 10 or less, you are not limited by what the combi can do directly.
All told in materials, not including shower and pump, this should be under 200 - 250. A good shower pump by Stuart Turner can be had for about 150.
In effect, this approach removes the limitations of the water supply and of the combi, if any.
It's worth seeing whether the water supplier will do anything, although the requirement from Ofwat is minimal on what they are required to provide. If you want more then you would have to pay for new pipework from the road, which depending on what's involved may not be cheap. As you say, if they can find a way to get you on a meter, they will.
.andy
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If the output from the boiler and main water supply are openly connected together in a tee fitting, you'll also get cold water flowing back toward any hot taps you turn on.
Can't you just install a wider diameter pipe, say 22mm, directly from the main supply to the shower and then reduce it to 15mm as close to the shower as possible. This should give you enough pressure to run an instant shower from all year round.
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Mike,
You have the making of a good idea with some trimming. There have been some odd suggestions from people on this thread. A combi will do the shower no problem even in winter, as they put in a lot kWs. So first option is a thermostatic mixer valve fed from the combi's hot supply. Also gas is 1/4 of the price of electricity.
For backup you could have between the combi's hot outlet and the taps and shower mixer an instant in-line electric heater. These have thermosstatically controlled water temp. When on electric this will give a trickle of a shower, and service one tap at a time, but fine for backup. Normally you keep this switched off and the hot water produced by the combi will run straight through it (any water throttle on the electric heater just keep fully open). If incoming cold mains temperature is an issue then the electric in-line heat can be switched on to boost the combi. I doubt if this will ever occur.
In-line electric heaters are about 80-100 from major suppliers and I believe Screwfix now do one. They can be fitted in the loft, or under sinks, etc. they are not that big.
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If you go to the bother of installing a tank in the loft, then have the pump supply the combi. Simple. A double feed pump that will have two tappings off the tank and one leg supplies only the combi and the other the cold water oulets, except the cold water tap at the kitchen.
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Mike Hibbert wrote:

Get you combi boiler to do the work. Even a poor quality one half filled with chalk will still out perform any domestic electric shower.
Put you money into upgrading the shower mixer to a thermostatic unit.
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