DIY wiki: Increase Hot Water Capacity

There's an article of that title that's appeared on the wiki http://wiki.diyfaq.org.uk/index.php?title=Increase_Hot_Water_Capacity
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" Does the hot water run out too soon? 7 ways to increase hot water capacity ==================================== Bigger Tank ----------- The obvious option is a bigger tank. Thankfully there are easier ways too.
Higher Thermostat Temperature ----------------------------- The simplest way to increase capacity is to turn the HW thermostat up. This means the hot water is used more slowly, as less of it is needed to bring the shower or bath upto the required temp. Thus your HW lasts longer.
Taking this method to extremes, setting the stat to 95C gives a lot more hot capacity, but high temperatures are a serious burn risk, and best avoided.
Drain Heat Exchanger -------------------- A lot of heat goes down shower drains. Nearly the entire contents of the hot water tank in fact.
A Drain Heat Exchanger recovers a good percentage of this heat, returning it to the shower cold feed. So less hot water is needed to bring it upto temp, and the tankful lasts longer.
By reducing energy use these exchangers can pay back their cost in under a year in some cases. For information see the main Drain Heat Exchanger article.
Move Thermostat Lower --------------------- HW tank thermostats are typically 2/3 the way down the tank, but are sometimes higher up. HW tanks heat up from the top downwards, and water below the stat will be at lower temp than the stat setting. Often this cooler water is no more than lukewarm, and this could be heated to gain more HW capacity.
Moving the thermostat further down will increase the quantity of hot water in the tank. The implications depend on where the stat is in relation to the heating element or built in exchanger.
* If the stat was high up and is moved to 2/3 down, things will behave normally after moving * If the stat is moved much lower than the exchanger or element, heating that extra bottom zone of the HW tank will take longer, and the top water will get hotter than the stat setting.
Solar Preheater --------------- A solar preheater produces a batch of warm water which is fed into the tank when hot water is used, rather than the HW tank drawing cold water in as happens with most HW tanks. The result is less energy use and to some extent increased HW output. The amount of output increase will depend on the temperature of the preheat water.
There are several designs of solar preheater, with performance varying significantly from one design to another.
With any solar thermal equipment, it is strongly recommended to assess any proposed system properly before construction, since many designs are unable to pay back their cost. Professionally supplied systems are worse than DIY ones in this respect.
Use CH Circuit Heat ------------------- The Central Heating radiator circuit contains hot water in winter and tepid Water in summer. It is possible to harvest this heat and add it to the HW, thereby giving greater HW capacity.
Ways to arrange this:
* With a heat bank or Thermal Store, The cold water supply goes through a heat exchanger on the CH circuit before going to the HW heat exchanger.
* With a combi, the cold water supply to the combi goes through the CH exchanger, thus boosting the heat output for a while.
* With a conventional HW tank, the cold feed to the HW tank goes through the CH exchanger.
Performance
The Heat bank option uses all the CH heat. It is the most effective option.
How much boost the combi option gives depends on system design and boiler characteristics. It may prove to be advantageous to restrict the CH heat exchanger size to ensure the boiler does not modulate at first during HW heating.
The conventional tank variant can only benefit from part of the CH circuit's heat capacity. Once the CH circuit drops below usable HW temp, the remaining heat will not be used in a way that increases HW output at the tap. Hence there will be no HW boost in summer. Although capacity is not improved as much, HW recovery times will be improved all year round, and more so in winter.
Electric Boost -------------- This one is just an idea for discussion, and has not been tried by the author. You should not try it without finding out whether it would work ok first.
Add a 3kW heater to the cold water feed at the shower & bath taps, along with a low temperature thermostat to avoid uncomfortable temperature rise.
If heater flow is restricted, it may be possible to split the feed into 2 parallel pipes, one that goes through the heater and one that contains an isolating valve. The 2 feeds are then reunited. Adjustment of the iso valve determines the flow sharing.
By prewarming the cold feed, less HW is needed, so the tanked HW lasts longer.
Another variant is to add a 3kW heater to the combi heated water output (or possibly cold water input), and trigger it by a flow switch on the shower & bath hot water feed.
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I have some questions which I posted to the article's discussion page: http://wiki.diyfaq.org.uk/index.php?title=Talk:Increase_Hot_Water_Capacity
However this is probably the better place to discuss them in order to arrive at a peer-reviewed article so may I invite interested parties to comment?
--
John Stumbles

Fundamentalist agnostic
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John Stumbles wrote:

I suppose it depends on how long you spend in the shower but I'd have thought that by the time the water has flowed over your body, the shower cabinet, the wall tiles & the shower tray, the amount of usefully recoverable heat left in what's going down the drain isn't a great deal. Besides, if my daughters knew I had a heat saving device they'd just shower for longer. ;-)
Tim
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Tim Downie wrote:

Where else would most of that 7-24kW go? How hot do the tiles get :)
NT
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They do work and pay for themselves.
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On 2008-02-07 08:33:55 +0000, "Tim Downie"

Exactly. Once there are females, forget it.
Having said that, method 8 works as follows:
- Said females wash their hair at length in the shower.
- This causes a suitably chosen drain to reduce in flow as a result of the collection of hair around the drain.
- Females turn down the shower valve to avoid the water running over the edge of the shower tray.
- This increases water capacity.
The factor not taken into consideration thus far is the male grief factor.
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On Fri, 08 Feb 2008 01:07:43 +0000, Andy Hall wrote:

On what planet do females do that?!
--
John Stumbles

Women always generalise
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Good point.
I suppose that if they are hot, they would make the drain thing work better.
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If they were genuinely that hot, they wouldn't be clogging the drain with all the mess of hair and shaving soap they produce from shaving in the shower in an effort to make themselves appear hot! :-)
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On Thu, 07 Feb 2008 00:26:50 +0000, John Stumbles wrote:

""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
title=Talk:Increase_Hot_Water_Capacity
I just foolishly deleted a long reply. I CBA to do it again. so I'll summarise and be much less diplomatic (but still above the DD level I hope).
Bigger tank - of course but that other ideas are trying to solve not obliterate the problem.
Up the temp. 65C or use blending valve with 75C+boiler(85C) or electric 85C.
Drain H.E. Might put restriction on cold feed to HW. Only helps showers.
Move t/stat lower: I guess 1/4 is lowest practical?
Solar preheat: Even more marginal that solar assist. Also solar panels need to be drain back or anti freeze.
Extract from Primary circuit: In winter robbing the radiators will be counter productive (especially with a combi system). In summer there should not be very much hot primary water on hand.
Add 3kW of electric: this adds 12.5% (on the lowest 24kW combi) and lesser on anything bigger including a stored system.
Could try: 1) restrict HW flow where it's not so needed eg. wash basin 2) Change usage and heating patterns. 3) Upgrade to cylinder with faster coil. 4) Down grade boost pump.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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There is a bath version. See gfx web site.
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Big difference between a bath water usage pattern and a shower though. At least with a shower, you're drawing water (that can be usefully preheated) at the same time as water is going down the drain.
With a bath though, it's fiill first, then empty. So, how are you going to capture & store all this low grade heat you've recovered unless you have system of pumps & separate heat storage tank? Seem hugely complicated for any potential gains.
Tim

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Ed Sirett wrote:

or drain down.

If CH has priority, yes, if HW gets priority, no.

right
NT
NT
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John Stumbles
Fundamentalist agnostic
====
I've always thought of myself as a militant apathetic.
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It's our Nigel again. He has delusions of expertise and is liable to get the hump with anyone who has the audacity to suggest that maybe some of his proposals are not a very good idea.


Yeah.
Scald risk, much higher standing energy losses. A lot more limescale deposits itself at higher temperatures. The usual recommendation, borne out of hands-on experience, is 60 degC storage.

I am sceptical. You'd need a tube-in-shell heat exchanger and these are becoming unusual because of the cost of copper. A plate heat exchanger is impractical due to fouling and primary-side resistance. Plastic's thermal conductivity makes it implausible. The fouling factor would be huge and it would require frequent cleaning to remove the hair, soap scum, pubes, urine, etc.. You'd also have stagnant water on the secondary side when not in use, warmed to blood heat when it was and a risk of contamination from the drains to the mains. So, having constructed a legionella breeding incubator, you spray the water out of a shower. Sounds great.
They've never been sold commercially. I wonder why?

Similar to turning the thermostat up. Higher standing losses, scald risk, limescale accumulations.

Storing warm water is not a good idea. It is verboten. You could be excommunicated from CIBSE, your slide-rule broken before your eyes and your silk tie cut into small pieces.
Just get a solar water heating system which stores hot water. But it needs a storage cylinder, a big one.

No, it contains hot water in winter that's needed for heating. If it contains tepid water in summer, there's something wrong with it.

A thermal store? Why not store the therms in a bigger DHWS tank? I must be missing something.

All the boiler water gets diverted to the DHWS heat exchanger. You can't utilise the heat going to the CH circuit because when there's a DHWS demand, there isn't any. The combi CH usually goes off while you run the hot water.
Just get a modern cylinder. The coil is nearer the bottom, it heats up quicker and the insulation is better. Or turn your home into a water heating laboratory. SWMBO will like that.
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wrote:

It's our Nigel again. He has delusions of expertise and is liable to get the hump with anyone who has the audacity to suggest that maybe some of his proposals are not a very good idea.

Yeah.
Scald risk, much higher standing energy losses. A lot more limescale deposits itself at higher temperatures. The usual recommendation, borne out of hands-on experience, is 60 degC storage.

I am sceptical. <<<
They work see gfx. Do a Google on this forum.

They are.

No, it contains hot water in winter that's needed for heating. If it contains tepid water in summer, there's something wrong with it.

A thermal store? Why not store the therms in a bigger DHWS tank? I must be missing something. <<<
This is good to improve the flow to combi. The CH circuit is a thermal store. He has been reading my posts (good boy :)). I have actually implemented this. Works well.

All the boiler water gets diverted to the DHWS heat exchanger. You can't utilise the heat going to the CH circuit because when there's a DHWS demand, there isn't any. The combi CH usually goes off while you run the hot water. <<<<<
With a combi using a plate heat X, flowswitch, a pump and a check valve or two, when DHW is called the cold mains gores through a plate which pre-heats from heat that is stored in the rads. Works well in summer too, and cools a house slightly as the rads get cooler in summer.
Do a google on my name I explain how to do it. Cheap and easy to fill a bath quickly. The rads go cold, but the boiler quickly re-hearty them when the taps are off. Very effective. TRVs can restrict performance
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found it....
You could use the heat stored in radiators and use then radiators as a store of heat. they would cool rapidly and when switched to CH re-heat rapidly too. They will give 20-25C in summer and maybe even cool the house.
The heat in a full heating system can be used to pre-heat cold mains pressure hot water. This can be done. I have seen this done using a plate heat exchanger, pump, flowswitch and two check valves. The system should not have thermostatic rad valves, or few of them. On a combi system:
- a by-pass pipe between the flow and return at the boiler - on this pipe a re-heat plate heat exchanger is fitted - The pump is fitted between the plate and the return by-pass pipe tee to the return. - A check valve between the pump and the plate - A check valve on the boiler flow before the tee to the by-pass pipe. - A flow switch on the cold mains water before the pre-heated plate heat exchanger.
The check valve on the flow pipe ensure no flow back into the boiler, although the internal 3-way valve should do this. The check valve on the by-pass pipe ensures no short circuit in normal CH operation.
The cold mains water runs through the pre-heat plate heat exchanger. This pre-heated main water then runs into the combi as normal. When calling for DHW the combi diverts to DHW only to heat the incoming cold water (but this is pre-heated). The flow of mains water is detected by the flow switch and switches on the by-pass pump. This pumps water from the rads into the pre-heat plate heat exchanger. This will raise the mains water substantially and the combi tops up.
You can fill a bath up in a few minutes doing it this way. The rads cool down a lot. This doesn't matter as when the system switches over to CH, the boiler re-heats the rads ASAP with alarge combi boiler ouput, with loss in room temp so small it is not noticeable to the occupants.
The combi flow rate in summer, when the CH is off is better than an average flowrate combi as the water in the rads will be around 20 - 25C when the CH is off. This stored 20C plus heat is used to pre-heat the cold mains water, which is around 10-12C. Depending on the efficiency of the plate heat exchanger and power of the boiler, the flow rate may be very good, even in summer. Cooling the rads also helps to cool the house in summer too.
A simple and cheap way to vastly improve the output of a combi.
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Irrelevant because they are tiny in the first place. If the loss increases from 50W to 100W in the context of 5kW of energy use it is not even a discussion point. Scald risk is ameliorated with a thermostatic valve. Even the NHS can manage to provide a spec. for this and they are totally incompetent.

Irrelevant with a water softener.

Thermal stores operate at around 80 degrees, so this is a non issue.

Because they are pretty much pointless.

Non issues.
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From the cylinder. And from the boiler, pump, pipework etc., those increased standing losses are how much?

Who mentioned a softener? Now you need a softener to increase your dhw storage capacity?

Er, yeah. They do. Bit they're not DHW stores; they make hot water with an immersed internal pipe coil or an external plate heat exchanger. And, guess what? They scale up. Unless you have a softener, of course.

Yeah, it was a hypothetical question, see, coming after I'd explained in detail why they were bolleaux.

Same issues as turning up the storage temperature.
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Negligible because the boiler modulates down and switches of when there is no demand.

No. It's simply a means to make the scaling argument irrelevant.

So get a water softener. Good grief.

So either do it or don't
If you want more hot water, it means more storage volume, higher temperature or less temperature rise.
mass, specific heat, temperature rise.
First form physics when I went to school. Doctorate these days, after the Big Mac degree has been completed.
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Negligible because the boiler modulates down and switches of when there is no demand.

No. It's simply a means to make the scaling argument irrelevant.

So get a water softener. Good grief.

So either do it or don't
If you want more hot water, it means more storage volume, higher temperature or less temperature rise.
mass, specific heat, temperature rise.
First form physics when I went to school. Doctorate these days, after the Big Mac degree has been completed.
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