DIY wiki: Increase Hot Water Capacity

[meow2222]

or drain down.

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

right

NT

NT

[meow2222]

oes through the CH

I don't see the cost of the 2 copper pipes outweighing payback here.

they're vertical, not horiz, which helps.

the cold feed is only warmed while its flowing. At other times it cools down quickly, avoiding any bugfest.

the mains water is under positive pressure.

They've long been used in industry, where there is a large amount of heat to recover. They're now getting used in hotels where payback potential is good. In some cases they can pay back in DIY use too. The article links to a commercial seller for the hotel market.

NT

[Onetap]

I studied thermodynamics, I do.

Heat transfer depends on surface area. A pipe-in-pipe heat exchanger won't give you much heat treansfer area; you won't recover much heat.

Commercial heat exchangers were usually a tube-in-shell type ( a 'bundle' of copper tubes inside a pipe shell); these cost lots (cost of copper, manual fabrication).

Recently plate heat exchangers have become much more common (consisting of a 'stack' of plates).

In both types, note the large heat transfer area.

Heat transfer depends also on the temperature differential or LMTD. The above commercially types work with large temperature differences between primary and secondary, for DHWS something of the order of

80/60 and 10/60. What you propose has something like 30/10 and 10/20.

So, you've got sod-all surface area, bugger-all temperature difference, giving you FA heat recovery and a huge maintenance liability to boot.

I'm a little sceptical.

Oh, good, that makes all the difference.

It would be insulated and you'd have stationary warm water, lying there for a (potentially) long time between use. Lest you think this is no problem, there is concern that the expansion vessel on (UK) unvented water heaters may be a legionella incubator and it was proposed to introduce flow-through types.

You could make it work commercially, where you would have a near continuous waste water stream, but the controls & monitoring would be costly. The controls for a domestic sized one would be the same, unless you sacrificed the safety.

[Onetap]

Recently plate heat exchangers have become much more common

> In both types, note the large heat transfer area.

[meow2222]

understood. Are gfx's claims phoney then?

I don't see much likelihood of that, not for a domestic one

Isnt that a dead leg though?

Like any product there are good and bad designs about. Double walled exchangers are whats wanted, where there is no joint relied on for water isolation. Ad example would be a central round pipe wrapped with multiple parallel microbore pipes connected at each end to a manifold.

Why would you need control or monitoring for domestic use?

NT

[Doctor Drivel]

The Ethos combis has a tube-in-shell heat exchanger. 4 pipes inside one big pipe. About the only combo that doesn't have a plate heat exchanger.

These are compact and highly efficient.

You obviously didn't do what I said in previous posts. Look at:

IS commercially viable. The gfx has been around for about 8 to 9 years. The US government did tests and proved it economically viable.

[Doctor Drivel]

Look at these:

units can be easily made by DIYers. The longer the copper drain tube with a 15mm soft copper coil around it the better. The storage in the 15mm soft copper is enough alone. Payback is between 2 - 5 years. That is very good.

From US Dept of Energy: Economics

A recent field evaluation of the GFX conducted by Pennsylvania Power and Light found the simple payback of a residential GFX system to range from 2 to 5 years. This was based on an installed GFX cost of $500 and electricity savings ranging from 800 kWh/y to 2300 kWh/y depending on the average number of daily showers in each home. The economics of the GFX improved with the number of daily showers in the residence as expected. In general, buildings that require large amounts of hot water for showers (e.g., homes of families with several children, multifamily apartments, or barracks with showers on a common drain line) would be ideal candidates for the GFX and would lead to shorter paybacks.

In addition to operating cost reductions based on energy savings alone, the GFX provides additional benefits. By recovering heat from drainwater and simultaneously using this heat for preheating water to the water heater, the GFX effectively shortens the time needed for the water heater to recover. This is important if the existing water heater is undersized or if there are more showers than usual taken back-to-back. Because heat is extracted from drainwater by the GFX, the capacity of the water heating system is increased. This means that it is possible to lower the thermostat setting on the water heater without directly affecting the capacity of the water heating system. These benefits, however, depend on hot water consumption patterns and the fraction of overall hot water consumption that is amenable to heat recovery by the GFX.

[meow2222]

OTOH you get film flow with the vertical pipes, which is several times as effective at heat transfer as bulk flow.

Probably a good place to get unbiased figures is alt.solar.thermal.

not sure exactly how effective the ocasional dose of hot caustic would be. It sure clears blockages well.

But these are really issues that, while of significance, dont greatly change the increase capacity article. any other remaining issues with the article?

NT

[Onetap]

LTHW to DHWS heat exchanger, hence large LMTD. We are speaking of Waste Water/DHWS heat exchangers, which have small LMTD. Hence a larger surface area is required per unit of heat transferrred. You still need a secondary heat source to bring your warmed DHW up to a useable temperature, and it's a breeding territory for legionella and every other bug in it until you have done so.

IMHO, it's probably not worth the effort on a domestic scale. You go ahead and build one if you feel inclined.

echnology.com

Drivel, I rarely read your posts and would never do what you recommend. This is because you are a clueless, moronic troll.

[Doctor Drivel]