Hello all,

I am modeling a building with eQUEST that has a tankless, a.k.a.

instantaneous, gas-fired hot water heater. I am uncertain of how to

baseline the system according to ASHRAE 90.1-2004 Appendix G. The standard

states that for a 180,000 BTU/h system the EF (Energy Factor) = 0.62-0.0019V.

So, I set V equal to zero for the tankless/instantaneous gas water heater

and get an EF =0.62.

The question I have for everyone is: How do I translate this baseline EF to

HIR (Heat-input-ratio) for eQUEST? Is there an accepted method for doing

so? I saw in a previous post from David Eldridge on 3/21/07 that

calculating HIR = 1/EF leads to unreasonably high values for HIR (In my

example, 1/0.62 = 1.61), but I'm having trouble seeing how else to come up

with a baseline.

Thanks in advance for your help.

Best,

Andrew McNamara, LEED AP

Andrew,

It is probably incorrect to baseline the instantaneous water heater

using the current minimum efficiency standard for the residential size

gas instantaneous water heaters (EF=0.62). According to GAMA's

Consumer's Directory of Certified Efficiency Ratings for Heating and

Water Heating Equipment's directory, very few gas instantaneous water

heater models have EF below 0.80.

Alex Lekov, Ph.D., P.E.

Thank you for your reply. Your response leads me to two more questions:

1) In the case of an 180 kBTU/h instantaneous heater is it standard practice

to use EF equal to 0.80 (or some other number based on a "bottom of the

heap" instantaneous heater) as you suggested rather than the 0.62 listed in

the standard?

2) How does one translate EF (this or any other) to HIR (Heat-input-ratio)

for use in eQUEST? Or might it be acceptable to simply look up the "bottom

of the heap" HIR values for instantaneous heaters and use one of them as a

baseline?

I would appreciate responses from any and all - thanks in advance for your

help!

Best,

Andrew McNamara, LEED AP

Andy,

Did you catch the footnote on the bottom of Table 7.8 that says,

"Instantaneous water heaters with input rates below 200,000 Btu/hr must

comply with [80% thermal efficiency] if the water heater is designed to

heat water to temperatures 180F or higher."?

So, if you're heater does not heat water to 180 F, you can use the EF =

0.62 as your Baseline efficiency in your 90.1 model. Otherwise, you

must use 80% thermal efficiency as your Baseline.

As far as translating the EF into HIR...

EF include standby losses, while thermal efficiency does not. In

eQUEST, the HIR does not include standby losses since this is modeled by

specifying the tank volume and UA. Since the EF of a tankless unit is

essentially zero, it seems perfectly acceptable to model the HIR as 1/EF

and then set the tank volume to zero in eQUEST.

Hope this helps!

Kendra Tupper

Greetings All,

I've run up against an issue for which I'd appreciate some advice. I'm trying to model the staging of dilution fans which run at 100% even if the building exhaust flow is varying. At times when the building exhaust flow is reduced sufficiently, we will turn one or more of the fans off. This is essentially the same as stepped dimming for daylight applications.

I've attempted to create a part-load curve using data points, but the cubic formula that is generated does not even come close to reflecting the "curve" or steps that I actually need. I've also played around a bit with writing a user expression, but to no avail. I was attempting to "get" the hourly return fan CFM and use IF/THEN statements to establish a new kW/CFM. But, I don't that the kW/CFM is used after design and for determining the starting point on the part-load curve. It does seem like I can get the hourly value for return CFM.

Has anyone had any success with staging constant speed fans? If you have suggestions for creating a curve, or on how to use multiple linear curves (a expression for curve selection), I would be grateful. Or, if you have ideas on syntax for writing the appropriate user expression, I'd be thankful as well.

Obliged,

Paul Erickson

Andrew,

It is probably incorrect to baseline the instantaneous water heater

using the current minimum efficiency standard for the residential size

gas instantaneous water heaters (EF=0.62). According to GAMA's

Consumer's Directory of Certified Efficiency Ratings for Heating and

Water Heating Equipment's directory, very few gas instantaneous water

heater models have EF below 0.80.

Alex Lekov, Ph.D., P.E.

Salutations Paul,

The solution I'm about to propose may be somewhat wanting, but nonetheless it is a solution.

Perhaps the quick/easy answer to your boggle is to hand calculate the energy use of the exhaust fans. Simply use the hourly reports of your system exhaust to tally how many hours you have 6 exhaust fans running versus 5 fans running versus 4 fans running....etc. Then once you have the hourly summation bins you could easily hand calculate the electrical consumption because your exhaust fans are running at constant speed.

Of course by hand calculating the exhaust fan energy use this will then somewhat skew the results of the peak electrical demand charge.

Best wishes on your endeavor.

Regards,

mark