How big a new HW cyclinder?

More ponderings on the bathroom install/refit front.
The new ensuite we are planning will have longer pipes runs from the HW cylinder than the current main bathroom, which takes ages it seems.
(House is an L shape, kitchen/utility on one leg, with the HW tank above and just a bit along from the kitchen in a room which is technically a bathroom, but not used as such - though is to be refitted and used again. The ensuite and current house bathroom, are on the other bit of the L . If I was starting from scratch no I wouldn't start from here, but i'm not, so I am)
So, considering putting an additional HW cylinder for the ensuite.
Going for a bit of luxury, (the room is much bigger than ur old bathrrom in the old house) We will have a decent pumped shower (no, I'm not interested in a electric shower), and a nice big bath (I like having the space for a proper ). So maximum draw in normal use is likely to be a couple of consecutive showers, or a bath fill.
How big a tank do you reckon for this? My gut feeling is that the marginal cost of a bigger tank, or heating a bit more water is such that erring on the bigger side is a good idea.
Space for the tank is not an issue, we ahve a cupboard in the room with planty of space
--
Chris French


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On 08/11/2010 13:16, chris French wrote:

A big part of the question is how often will it be reheated (and by what)?
What sort of boiler have you got? If its a modern condenser, can it support dual (or more) temperature operation? Ideally you want a boiler that can run a lower flow temp for the rads to improve efficiency, but then ramp up the temperature for the final stages of hot water heating.
With simple fixed output temperature boilers, there is no harm in having the water reheated by its stat on demand, meaning you can cope with a smaller tank. If however you need to arrange set aside times for reheating when there is likely to be less demand for room heating, then its handy to go for more storage (i.e. 250L or more).
--
Cheers,

John.

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Thanks John,
The boiler is a gas fired, floor standing, conventional flue, Ideal Mexico. I don't have the documentation handy (it was in the house when we bought it), but ISTR it is about 10 - 15 years old. There aren't plans to replace it at the moment.
A heat loss calculation for the heating system is on list of round tuits, but AFAICT there is enough capacity to supply the HW at the same time as the heating. Currently we have a 165 -ish L HW cylinder on a gravity fed circuit from the boiler. That will all get upgraded to pumped of course. The route for the primary circuit to the new cylinder will be a bit convoluted, but doable.
so I guess sizing to fill the bath as a minimum is a place to start, what sort of percentage of water will come from the tank (say at 60 C) to run a nice hot bath?
--
Chris French


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writes

Depends on the house layout, but if there is room for a combi near the bathroom put one in. Do not connect up the CH and do that later when a the old gas guzzling Mexico is ditched. Later you could again fit another combi at the other end of the "L".
Then re-arrange to have one combi do upstairs CH and one bathroom, one do downstairs and the other bathroom. Combis are cheap and easy. They do not run out of hot water.
You could fit an insulated secondary circulation loop. It could be activated by a PIR and turn on the pump only when someone is in the en-suit. This prevents heating the house with the loop unnecessarily if running all day.
You could fit an instant electric heater in the new bathroom at the basin fed from the cylinder, so it cuts out when hot water comes through. The shower and bath do not matter as you get undressed after turning on. The en-suite will not be used as much as the main bathroom, so high electric bills are not a worry.
Two cylinders will take up space and add to control complexity - valves, etc.
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I did think about something like that - though using a gas instant water heater rather than a combi. But the bath filling performance of combi's is poor.
One of the frustrations of the current bathroom is the poor filling performance of the bath. This is from the tank, but was badly done. It's a fairly convoluted pipe run, and done in 15mm pipe and it just can't supply water faster enough (and if you decide to wash your hands in the handbasin or something whilst running it, you get an airlock in the bath supply). The bath in the old bathroom that we don't really use much fills much faster as it's near the cylinder and plumbed in 22mm.
Anyway BTDTGTTS I fitted one in the old house, it was an acceptable compromise there for space reasons, but I much prefer a storage system. I'm not convinced our flow rate from that mains is that great either. They aren't cheap AFAIC, esp as this time it would probably be a plumber install, and getting gas up there would be non-trivial. I don't forsee running out of water to be an issue in normal use.

That is the other idea that bobs around. I'd have to upgrade the existing cylinder to a bigger one though as it's quite easy for us to use up all the HW at the moment.

This just feels like a bit of a kludge really.

As I said, space isn't really an issue (and a combi would take up space as well, and the cupboard would not be such a good space for it.), we have a big cupboard that can be used, and it will give us a nice airing cupboard by our bedroom. I don't really see the controls ( a timer and a valve) as anything complicated, certainly not compared to having two boilers for the CH :-)
--
Chris French


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writes

It depends on the model and flow rate. High flowrate combis are very good. I am on about minimum 35kW.

You have to measure the flowrate and pressure of the mains pipe.

But an effective work-around.

If the water pressure is
1. A high static pressure 2. Low dynamic pressure 3. With a low flowrate
Then consider a cold water accumulator. This is the size of a DHW cylinder and stores cold water at peak dynamic pressure. It has a pressurised ball inside. It improves the cold water flowrate greatly without using pumps or open tanks.
Use one with a highflow combi, they are cost effective and deliver high flowrates to hot and cold. So, space saving. One cylinder and one boiler and no silly tank. And a great flowrate wit the combi and great flow ad pressure all around.
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writes

Two boilers were mentioned. Read the thread. It is cost effective to do so. That may be over what your small brain can absorb. Maybe you want to spend lots of money. Be my guest. Spend all your money. Spend it all.
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On 08/11/2010 16:11, chris French wrote:

Yup, I know the beast (one came with our place as well). Its fixed high temperature cast iron lump of a boiler so no condensing efficiency to worry about!

Yup, with a conventional indirect coil in tank type arrangement, the cylinder will probably only be able to swallow heat at about 5kW tops. So you are better running the hot water reheat with the heating, since otherwise the boiler will just cycle[1]
If you specified a fast recovery coil, then you could probably get the cylinder to take the full output of the boiler.

You can work out the mix volumes based on the ratio of temperature differences between the final temperature required and the hot and cold water temperatures (a weighted average if you prefer).
So say you need 150L of bath water at 45 degrees, and the cold water is at 10 degrees, and the hot water is at 60 you get a ratio of cold to hot water of :
60 - 45 : 45 - 10 = 15:35
So if you split the 150L required into those proportions you get 45L of cold and 105L of hot. So that will give you one bath from your tank, but that is about it without reheating.
[1] Short cycling on hot water reheat is not as bad as on CH, since pump will stay on based on the stat demand, and the boiler will cycle on its internal stat. That at least gives it a chance to dump most residual heat in the boiler into the cylinder rather than venting it out the flue.
--
Cheers,

John.

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On 08/11/2010 13:16, chris French wrote:

Depends whether you have teenagers in the house, or are likely to do so soon.
Problem is that once the discover the concept of 'washing', they then go in the shower and stand there until the hot tank runs cold. It's a physical impossibilty to find a tank big enough to outlast them, which means that the bigger the tank you install, the more hot water you will waste. And there'll still be none for your own shower.
hth David
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No teenagers yet, but they will morph sooner or later.
But this tank is for our ensuite, no kids allowed :-)
--
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On 08/11/2010 18:56, chris French wrote:

or have a nice tall cylinder for both, but with their hot water tapping coming off some 150L lower than the top! ;-)
--
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John.

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Then you use an instant water heater or combi and they NEVER run out of hot water. Body Jet showesrs neded instant water, as nothing else can deliver the DHW needed.
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