selection of water heater

[lzh007]

Normally for apartment building, should we select water tank for a domestic water heater? (for example there are 70 suits inside)
As I read in Ashrae handbook, 2 data are used to select heater and tank, recovery rate GPH and size of storage capacity Galon.
While my boss always selects a heater with a huge gas input but a very small storage capacity without providing additional storage tank. Is this the new design concept or is he wrong?

THX

 

[quark]

The idea of going for storage(be it any storage) is to have the process running at full load (so that better efficiency and better unit power consumption)

In the absense of storage, you have to run the process/machine even if you have only one user point requiring it.

 

[IRstuff]

The downside is that the tank must be extremely well insulated and if the actual usage rate is low, you'll spend lots of your heat in maintaining the temperature.

Also, since there's a sawtooth temperature profile, there will also be variations in the delivered water temperature.

TTFN

 

[Joesteam]

For domestic hot water many new installations are moving away from large storage. The reason is two fold, the energy savings considering keeping large volumes of water hot during off hours, and the safety aspect of growths in the water such as Legoinella.
Why not look into the new condensing boilers that give excellent effeciencies going to smaller storage that is over 160deg F and then mixing back down to operating temperature??

 

[PEDARRIN2]

I would hesitate to use such a high operating temperature on the water heater. There is just too much room for problems.

The scenario I see is somebody in an apartment a long distance from the heater complains that he doesn't have enough hot water. The maintenance guy, without really diagnosing the problem, goes in and turns the thermostat up either on the water heater or the mixing valve. Now the system is outside its design parameters which usually means that an apartment close to the heater gets water that is way too hot, possibly 160.

Also, at least in the apartments I have seen, the showers/tubs do not have thermostatic mixing valves to prevent or limit the above problem from escalating, so scalding water is now in the tub or the shower. If the occupants are elderly or have younger children, the results can be deadly.

For a residential application, I would never have the system produce water above 130. While it can still scald, the exposure time is much less before damage occurs.

 

[TBP]

I don't think you can get 160* water out of a condensing boiler.

 

[KiwiMace]

Often the size of the water heater in residential applications is determined by code requirements. Check your local code. In highrise apartments in Asia, the water heater is typically suspended from the soffit, meaning an unsightly or obstructive bulge that the architect would love to disappear - this may be the driver for your small cap tank.

 

[walkes]

TBP you can get 160°F hot water from a condensing boiler. The flue gas temperature will be a function of the entering water temperature. The water heater is essentially a counterflow heat exchanger, entering water in closest proximity to leaving flue gas.

http://www.aerco.com/cgi-bin/docsserv.cgi?idDocument=144

 

[TBP]

walkes - are we talking boilers, or water heaters? Typically a boiler with 160*F out will have 140*F in for water temps.

 

[Joesteam]

Because of the counter flow of condensing boiler it is the INLET water temperature that determines the efficiency. It can do practically any temperature gain as long as you are within the BTU and flow ratings of the boiler.
So, for domestic HW applications, the condensing boilers get pretty much the full rated efficiency, and for heating applications the efficiency goes down because of the higher return temperature.
Look at:

http://www.ajaxboiler.com/documentation/technical/product_specs/atlas_series/atlas_efficiency.pdf

 

[TBP]

If the boiler is JUST doing the domestic hot water, then it can be arranged such that it will condense. In this case, I'd call it a water heater, rather than a boiler. If it's doing space heating as well as DHW though a heat exchanger, then it won't regularly condense (if at all) with a 160*F outlet temp, unless there are huge heat emitters, such as radiant infloor heating. That's unlikely in an apartment building. The initial cost of heating systems like that is very high (excellent comfort & economy of operation, however) but I just don't see developers and landlords ever spending that kind of money on the heating systems for rental properties.

 

[Joesteam]

This type of heater is more expensive than a standard one, however I have even sold one to a laudramat! We have them installed in public housing as well.