Hello All
We're being audited on the energy model for the dhw saving. I'm just
wondering if this make sense and from your experience if it would be
acceptable to the LEED reviewer.
What I've done is taken numbers from ASHRAE Handbook HVAC Applications DHW
consumption per fixture for office use.
7.6 L / hr for lavatory fixtures
Showers, kitchen sinks, and Lavatory sinks.
Service sinks I'm not quite sure whether i should include all 5 service
sinks (one on each level) or just one sinks since there won't be any mopping
done since most of the building Under floor air distribution and i'll do a
write up of course to explain my reasoning.
I took the consumptions per fixture numbers multiplied by the amount of
fixture then by the demand factor given.
7.6 L/h x 55 # of fixtures x 0.3 demand factor
for each type i took the savings percentage based on each fixture. So
baseline is 2.5 and proposed case is .5 for lavatory fixtures
.5 proposed case / 2.5 base case
So the proposed domestic hot water demand is 25.08 L / hr or 0.11 gpm.
The reviewer had asked for percentage of hot water vs cold water, and
temperature at the fixtures, i found that harder to substantiate. So I
thought this process would be a lot better.
Tai Lieu
Hi Tai,
I'm going to attempt to answer this question for you even though I don't
really know what it is exactly that the reviewer wants to see. I think, in
general, that reviewers are pretty flexible as to the calculation
methodology you use as long as you are using a methodology that is
technically sound.
The excerpt from the ASHRAE Handbook HVAC Applications that you are using is
correct for designing the the design demand flow of your system. The demand
factor you are given is, I think, an estimate of when all of these fixtures
might be in use at the same time. The result of this calculation would be
the demand on your DHW system. Think of this "demand" as being somewhat the
same as "kW" on the rest of your energy systems. With that in mind, you will
see that you need to then apply the Usage Schedule that you have created in
your model to get the associated reduction in DHW usage, similar to kWh
saved.
Since we are ultimately interested in the reduction of energy use here, the
last bit of information I would provide the reviewer is your estimate of how
much of this water will be heated by the DHW heater and to what temperature.
Your mop sinks will contribute a bit to this if the janitorial staff like to
mop with hot water, but the majority of the heated water will be at the
lavatory sinks. You may have to make some educated guesses here so back them
up.
Lastly, always, always provide your units. For us and for the reviewers.
Best,
Carol
Tai:
In general, I tend to follow the procedure laid out in WEc3 for the hot water savings from low-flow fixtures. I've found their calculation procedure for total flow (gallons / year) to be accepted by LEED reviewers.
This does not address water flow from fixtures outside the WEc3 scope, like service sinks, so the ASHRAE handbook or a reasonably documented project-specific assumption sounds like the way to go to get total water use per year for these fixtures.
Like you mentioned, this also does not specify the water temperatures, which are needed for the calc.
Different hot water uses in a building often have different temperature needs (ie handwashing, dishwashing, showers, etc), so even if we know the total water use for each of these fixtures, we'd need to get the expected discharge temperature of each use in order to figure out how much hot water is required.
Most of the time I get the temperatures I need from the project plumbing engineer, who has a better familiarity with these targets than I do.
If you don't have access to a plumbing engineer -
- For the inlet cold water temp, per the eQUEST dictionary, if you don't specify the temp eQUEST uses the monthly average ground temp. I hope this would be available via an hourly report, although I haven't checked.
- For the discharge hot water temp, various plumbing codes (IPC, NPC) specify limits on the hot water temp to prevent scalding, and I've seen engineers take a factor off of that to estimate the average hot water use temp. I've heard 110 deg F for showers and 105 deg F for lavs in our office. Service sinks might have something higher (say 120?).
- For the water storage temp, this is also something that should be obtainable from the plumbing engineer. As a starting point I've heard 120 - 140 deg F in our office as well. I believe the IPC limits the maximum storage water temp to 140.
This should enable you to calculate the quantity (gal / year) of hot water leaving the water heater to serve the annual total flow needed.
Finally, you need to turn the gallons HW / year into a GPM PROCESS-FLOW, if you're inputting into eQUEST. One way to do this is to take whatever use-schedule you had been using (ie eQUEST's default, ASHRAE 90.1-2007 User Manual's, or project-specific) and integrate it, to determine the annual total equivalent full load hours. Dividing total annual gallons by annual full-load hours (and converting units), you should be able to arrive at a GPM to enter as a process flow.
Yours,
Aaron Dahlstrom , PE, LEED(r) AP
Thanks, Aaron, for providing much more detail than me. You have covered it
very well. For inlet cold water temperatures I have always seen 50-55 F
provided as the average temp: it may be hotter in the summer and colder in
the winter but overall I think those work.
Carol
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Yikes! Is Legionella actually growing in chlorinated public domestic water supplies? I recall hearing of it in non-potable water (cooling towers and drain pans, anyone?)... strong bugs we make these days.
The International Plumbing Code (and maybe others - check your state model codes and state amendments) requires the use of temperature mixing valves in domestic hot water supplies, so you could, theoretically, run 160F water all the way to the fixture (or bank of fixtures) and temper it there, but that's pretty expensive both in equipment and energy.
I'm going to bounce this off a public health researcher I've met - maybe he can share some insight.
MARK DARRALL, AIA, LEED AP BD+C, NCARB
Mark:
I would be interested to hear what the public health researcher has to say.
It is my understanding that Legionella forms in untreated cooling tower water.
I have not caught it yet even though I have serviced and treated many scummy
open cooling towers.
I have never heard of it being in the public domestic water supply.
Paul Diglio
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