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What's the most Cost Effecitive way to heat water for great Shower.

Postby lifeUsedToBeSimpler » Sun Jul 10, 2022 4:54 pm

Hi everyone,
I would really appreciate some practical advice please.

My current heating system:

CWST on the rafters in the loft
HWST on the first floor, but because it's above the stairs, it's about 3ft off the floor.
Gas Central Heating (Regular) Boiler: Greenstar Ri (Ri18)
Current Electric Shower: Instantaneous, off the CW mains supply.

Head from Shower-to bottom of CWST: about 10"

So the boiler heats the rads, and indirectly heats the hot water in the HWST.

So what is the best way to heat our domestic hot water, so that it will provide a more affordable good flow rate to a new shower.

The Problem:
Currently, we use an Instantaneous Electric Shower,
which has never been ideal because it's too hot in the summer, and too cold in the winter with a very low flow rate.

And now since the recent huge electricity price rise,
has made this too expensive to use.

This is aggravated even more because I like to take long showers.

So I need to change to a shower that has a good flow rate all year round, where the water is heated by gas, because gas is currently much, much cheaper than electricity.

A shower with the water heated by gas.
Good flow rate all year round.
Plenty of hot water for long showers.

Option 1: Shower + Water Pump
My first thought was to install a new shower + water pump + larger CWST in loft.
But feel that our current HWST capacity may not be big enough, and we don’t really have the space to install a larger one.

Option 2: Combi Boiler
Replace existing (Regular) Boiler, with Combi Boiler.
But these don’t heat the radiators when the shower is in use,
so not ideal in the winter when taking a long shower.

Option 3: Instantaneous Gas Water Heater
So keep my current (Regular) central heating boiler to heat the radiators, but add IGWH to heat the water.
So this would be like having a 'Combi Boiler' but when taking a shower, both boilers could function at the same time.

So far, Option 3, seems to make sense.
And I can’t think of any other options, so I would appreciate anyone’s help to arrive at the best solution.

So does Option 3, make sense?
Would it fulfil my requirements, or are there pitfalls that I've not thought of?

Are there any other suitable options?

Many thanks.
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Simply Build It

Postby ericmark » Tue Jul 12, 2022 9:25 pm

The electric shower depends on having a shower head which suits the heater, there is a set amount of heat given out, so temperature depends on flow rate, and size of holes in the rose controls flow rate.

The combi boiler has a variable output, however it needs to warm up, so big delay from turning on to getting hot water, often there is a small reservoir but it is not big enough for a shower, so shower starts cold goes warm then cold again and then warm again.

From the hot water cylinder the pressure is often lower, but ample hot water, you can get power showers with a pump to increase pressure, but need to be careful plumbing so it can't cause the immersion heater not to be covered.

The power shower must be the best, with instant electric the worst, but cost depends on the plumbing required.
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Postby lifeUsedToBeSimpler » Wed Jul 13, 2022 2:26 pm

Hi ericmark,
Many thanks for your reply:

So you seem to:
* Be in favour of my Option 1...
But as I said, I don't think that would work well for me, due to the limited amount of stored hot water.
And adding a pump (which I would need to do) would use up the water even faster.

* And against Option 2...
But I'm not sure why the water temp would be fluctuating?

And I can only assume you think Option 3, would have the same problems as Option 2 ?

Perhaps you could clarify?

Many thanks.
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Postby ericmark » Thu Jul 14, 2022 2:19 pm

I have never measured how much water is used having a shower, clearly less than a bath, as we would run out of hot water running a bath, never done it with a shower.

However when caravanning we did get one with a built in shower, not often used, but the shower did work well, and we never ran out of water, the cold water was an aquaroll so 40 litres. And the carver cascade 2 water heater holds just 9 litres, so I would guess we were using no more than 10 litres on a shower, OK aware we had to carry water, so did not want to use too much, but likely we don't use more than 20 litres of water having a shower.

The problem is most water heater heat at the bottom, so the whole tank needs warming before we can start to have a shower, only the Irish seemed to have worked out how to only heat just enough, and we say they are thick?

The Willis water heater heats from the top down, so a simple timer can set how much hot water you have, or some thermostats on the side of the tank of course.

If you live in Northern Ireland great, the plumbers know how to fit and set up the Willis system, but mainland UK, not sure you will find a plumber who has ever fitted one and set it up.

However we have used twin element immersion heaters for years, the short element for the hand basin and long element for when you want a bath, and when Economy 7 came out we had tanks with two elements top one set for low temperature and bottom one higher, so we have the same idea, done a different way.

There was a big move to remove the water cistern, and replace with combi boilers, but the advent of solar panels has reversed that, as want some way to use the spare energy, and easy way to store is hot water, much cheaper than large Tesla batteries.
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Postby lifeUsedToBeSimpler » Thu Jul 14, 2022 3:38 pm

ericmark: Manty thanks for the reply.
I don't dispute your experiences for one minute, if it works for you.

I'm sure most people don't ever think about how much water they use, certainly me included, until they have to.

As in my case, I need to make sure that I fit a system that can cope with our current needs.

And my research so far, suggests that a typical shower can consume anywhere from about 6 litres, to 12 or even 20 litres per minute, for a power shower etc.

Our useless current instantaneous electric shower, which runs off the CW mains supply, which I've yet to measure, probably uses at least 6 litres per minute.

So a HOT tank of 40 litres would only run for about 6 minutes, and that's assuming that the cold water coming into the tank didn’t cool the stored hot water to quickly, which it probably would.

I would have to fit a shower pump to our current gravity system, in order to have any usable flow rate, which could be in the region of 9 litres per minute.

So that's why my option 1, eg: using our existing HWST will not work for me.

So I keep coming back to my Option 3, eg: Instantaneous Gas Water Heater

So is there anyone else who can offer their advice ??
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Postby ericmark » Fri Jul 15, 2022 7:40 am

My last house had an instantaneous gas water heater, seem to remember it was fitted around 1985, and was a Main 7, idea was to turn the airing cupboard into a bedroom.

This was the first variable output gas water heater I had seen, it had no electric supply, just pilot flame. And it did have a hick-up when it caused the pipe to burst as did not turn off fast enough, but it did a good job until removed around 2 years ago, as the central heating boiler started to leak, and it seemed sensible to combine both boilers into one.

But any gas boiler which can modulate is only going to work between two points, this range has increased over the years, now around 6 kW to 28 kW, my shower was on the edge for the old boiler, was not really using enough, and in summer would go cold every so often, and flow rate not enough.

It seems odd but central heating boilers way over what is needed for central heating are fitted in order to fill the bath, think in last 20 years I can count the numbers of baths I have had on my fingers, but a house able to be heated with a 20 kW boiler ends up with a 28 kW for the old time you want to fill a bath.

But if using a combi boiler it must suit both CH and DHW, so I look at the central heating first.

What we want is for the boiler to run in the most cost effective way, to do that the return water needs to be cool enough to gain the latent heat, most boilers modulate by monitoring return water temperature, there are some which have a wall thermostat connected to the e-bus, but in the main return water temperature is used.

So the TRV is king, these are the devices which decide when the radiator needs to switch on, and as they close the by-pass valve opens letting hot water return which in turn causes the boiler to modulate (turn down). There may be a wall thermostat (well I am told some law requires them) but it's job in the main is to turn off the boiler rather than leaving it cycling on/off.

The system can be further refined by using programmable TRV's, and where one goes to work for example you can arrange the TRV's so they heat the rooms in the order used, this gives a faster recovery time, but the whole system needs to be designed to do this.

So if the boilers minimum output is 6 kW and you are due to come home at 6 pm, using say kitchen and dinning room, then the radiators in kitchen and dinning room need to be able to sink 6 kW, the radiator temperature is lower today to be able to gain the latent heat, so they need to be larger, and looking at radiator sizes they need to be really large today, some thing to do with Delta T rating, which with the old non condensing boiler was around T65 and new boilers T30, I am an electrician not a heating engineer, so not my field, but the fact remains with a combi boiler you need the output to go low enough to suit the radiators installed and likely to be used at a time.

Yes it can be done, but at what expense? And do you size boiler for central heating or domestic hot water?

As the home gets better and better insulated we need less and less heat to maintain the home, my old house before having double glazing and cavity wall insulation or loft insulation was designed to be heated with a single 4.5 kW gas fire in the living room, OK it did not really get the kitchen warm enough, and a pain as not thermostatic controlled, I fitted a 18 kW boiler back in late 1970's to heat the home, and it was ample. Today with sealed doors, double glazing, cavity wall and loft insulation that three bedroom house needs a lot less, so the boiler required for central heating is far smaller than needed for DHW, but we can use the extra output allowing the home to cool and reheat only when required, but only if the radiators are big enough.
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