Modeling Reverse Acting TStat EF

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Hi eQUESTers,

First off, you probably will not likely appreciate this and I'll preface
this with saying that I'm a total bandwagon jumper but I am okay with that.

GO JAYS!!!

Now that that's out of the way I am trying to model a very simple system
that I'm sure many of you have modeled before.

I have a zone that has an exhaust fan in it. When the temp. reaches above
80 degF, an exhaust fan turns on and a damper opens to draw OA into the
zone.

I've been fooling around with a few systems trying to figure out a good way
to model this but haven't come up with one yet.

Let me know if have any recommendations.

Thanks!
Darryl

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Zonal exhaust fan has an option to control that on a temp schedule if I recall correctly.

Open the zone, tab "outside air" then at the right you can specify a zonal exhaust fan. (All this from memory so might differ slightly)

Cheers,
Julien

Envoy? de mon iPhone

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Does this mean the exhaust fan can cycle based on zone temperature? ? I had thought that the exhaust fan schedule was just on/off, on/off/flag and fraction. That means you have to pre-define the expected schedule rather than having the EF respond to the hourly space timestep temperature.?

What we have done in the past is to model the EF as a PSZ with the fan power as the supply fan power and the "indoor fan mode" as intermittent. Cooling capacity set to a small non-zero value (as zero invokes auto-sizing).?

? Original Message ?

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I have modeled this situation using a PSZ system with the supply flow the same as the exhaust flow. The fan schedule is always off with night cycle on any. Cooling is always available but no cooling capacity. The fan power is equal to the exhaust fan power. The minimum OA fraction is 1.

The only issue is that the system won?t cycle on for cooling unless there is a realistic cooling load in the space. That is the trickiest part to figure out.

Christopher Jones, P. Eng.
Rowan Williams Davies & Irwin Inc.
Consulting Engineers & Scientists
901 King Street West, Suite 400, Toronto, Ontario, M5V 3H5
T: (519) 823-1311 ext 2052
M: (416) 697-0056

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Hey Brian,

You're absolutely right - I should have double-checked before posting. In my mind's eye, I recalled assigning a temperature schedule to zonal exhaust inputs, but it does appear I misremembered and that isn't a direct option.

Tricking an airside system as you're describing into the intended behavior must be how I've managed this in the past. A thermostat schedule is minimally needed at the system to define the intended "trigger temperatures," and one could optionally also specify a fan schedule to go beyond intermittent and enforce off/on hours as might happen in the real world via tstat + relays.

~Nick

[cid:image002.png at 01D2286D.03968C70]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

[cid:image001.png at 01D189AB.58634A10]

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Hi everyone - thank you so much for the replies. Just wanted to provide as
a summary as to what I've modeled thus far - pls comment if you have any:

*SYSTEM LEVEL:*

- Modeled the reverse acting t-stat system as a PSZ with the following
inputs at the system level:
- Control zone is the zone the supply fan serves
- Supply fan kW/CFM equals that of the EF
- Delta T across supply fan = 0
- Supply fan schedule is on/off type with default off for all hours
- Night Cycle Control: On Any
- min outside air ratio = 1.0
- CLG Cap left blank
- EIR = 0

*ZONE LEVEL:*

- HTG/CLG design temps are 60/80 degF;
- E/A control is "No Airflow Tracking"
- E/A flow matching the flow of the EF (11,000 CFM)
- E/A source: infiltration
- Max clg rate left blank

*QUESTIONS REMAINING:*

- Can I simply leave the CLG capacity to be auto-sized? Brian you had
mentioned to put in a minimal value for this input but I do not understand
why.
- This zone also has HW UHs serving it - how to I implement them? I see at
the system level there's an option for the Zone Heat Source to be furnace
which provides the opportunity to account for a UH's fan energy but looks
like a furnace cannot be connected to a HW loop.
- Any advice of thermostat type?

Thanks again
Darryl Kasun

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Hey Darryl,

This turned into a big response, but it seems the problem we?re trying to help solve is a moving target!

1. The advice to enter something nominal but tiny for cooling capacity, to avoid autosizing & prevent airstream cooling, is based on the earlier statement that you do not intend to cool the airstream making up the exhaust. Going the other route of just making it ?free cooling? (EIR=0) goes against your goal of modeling reality as now the exhaust ?makeup? airstream is being conditioned on its way in, resulting in milder interior temperatures for determining hourly zonal heat transfers. An equally viable alternative I generally prefer to ?lock out? cooling operations by the way is an airside system COOLING-SCHEDULE with hourly values of zero. An expected side effect of of crippling/breaking your PSZ?s cooling capacity with a tiny number is that you will end up with a justifiable quantity of unmet cooling hours ? so expect & make note of that swing in unmet hours so you can quantify & parse it out later for your own QC/documentation.

2. Based on the reference help entries, I?m concerned the night-cycle-ctrl inputs are intended specifically for the inverse case only (reacting only to provide heating when the system is off. I think the effect you?re looking for (turning on only when thermostat calls request it) can be achieved for PSZ by simply not defining a fan schedule (right-click, default that dropdown field). Either way, you can & should check/verify intended system operation via custom hourly reports for the associated system & control zone ? include hourly zone temperatures, associated tstat setpoints, and something like fan operation/cfm to verify if the system is only operating when intended.

3. I?m not certain zonal exhaust inputs are necessary at all if you are modeling a 100% OA PSZ, but be cautious you aren?t ending up unintentionally ?double-counting? fan energies between your PSZ fan power inputs and your zonal exhaust power inputs (or lack thereof). I think you can ?zero out? the zonal kW/cfm or else make it a very tiny number if you do decide zonal exhaust inputs are needed.

Ah? The fact you intend to model hot water unit heaters probably throws a wrench into the advice/discussion so far?

?Furnace? generally implies gas combustion heating in doe2/eQUEST. That?s not what you want for hot water heating.

If it?s fair to assume the space is well-mixed during heating scenarios (i.e. not much stratification), then you can probably get by with accounting for the associated unit heater by specifying hot water baseboards with appropriate heating capacity in the zone. Have them controlled by a heating temperature schedule (sounds like you want this set to 60F all the time). You might want to specify the associated loop and boilers/meters first so that you can assign those in the same step. Downside to layering in baseboards as an approach is the unit heater fan energies won?t get picked up explicitly ? the heat those contribute will instead be supplied by the HHW loop, and the energies associated with their fan operations for air circulation will not be picked up automatically. You could layer that in however (if it?s pertinent) by doing an hourly report to identify when baseboards are in operation (delivering heat), then assigning that to the space as an equipment load, capturing the heat contributions and electricity consumption.

If accounting for exhaust and unit heater fan energies operations separately/distinctly is a major priority? you?ll probably need to re-think the broader approach & system type selection. UHT is an attractive option for picking up unit heater operations explicitly and clearly on its own, but that system type affords no capacity to handle OA/exhaust. You?d either need to accomplish the exhaust/damper operations at the zonal level (spoon-feeding expected operations as a fractional or ON/OFF schedule since we?ve established thermostat schedules are not presently an option). You could alternatively pair UHT?s with an airside-system-based approach to the exhaust/damper operations (like the PSZ we?ve been discussing) if you split your zone/spaces geometrically to afford separations to assign different systems to, thermally connected with airwalls. Advantage to this approach is the flexibility to separate the conditioned volume if air mixing is not going to be stellar (stratification expected) during heating.

Hope this helps steer the ship!

~Nick

[cid:image001.png at 01D22928.AFA3BD60]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

[cid:image001.png at 01D189AB.58634A10]

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Hi everyone,

Thanks again for all your input.

I've run into a snag. I cannot seem to get the fan to turn on when the
temp > 80 degF. When I look at some of the reports I see that there is no
reported unmet clg hours for this zone which is a red flag - not sure why
this is. There is a sizable clg load in this zone: 65 MBH. I do see on
the SS-F that the max zone temp hits 86.1 degF. Any thoughts?

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Immediate thought: Have you set a cooling thermostat schedule for the zone with 80F at all hours? That would explain both the lack of unmet hours and no calls for conditioning (fan operation).

If there is a thermostat schedule, did you confirm the fan is never operating via custom hourly output reporting? Do you have a system fan schedule forcing OFF behavior with zeroes and/or -1?s? If so, try forcing ON with 1?s and/or just not defining a fan schedule at all to permit system fan operations when called for.

[cid:image001.png at 01D22A17.3063D8B0]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

[cid:image001.png at 01D189AB.58634A10]

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I do have a clg t-stat schedule set at 80F for all hours. I have tried
changing the fan schedule to undefined and to on for all hours and am still
met with the fan not turning on at all. The original fan schedule to keep
the fan off had zeros for all hours.

How to I check the custom hourly output reporting?

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Hi everyone,

Have these systems modeled now that they are showing unmet clg hours and
the exhaust fans are turning on when temp is above 80F.

Here's a summary of the inputs required to achieve:

- fan schedule at BOTH system and zone level to be set to always off (0)
- night cycle fan control set to "cycle on any"
- clg rate very low (1 btu/hr)
- clg t-stat required

Thanks again everyone for your help.

Darryl

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I have been playing around with a test case using a PSZ system with cooling capacity = 1. The fans are set off with night cycle on any. The zone outdoor air is set to 1 cfm. If I use no outdoor air in the zone, the economizer doesn?t kick on. The system fan cycles on for free cooling The space temperatures are maintained. The only issue is that the economizer is variable. In reality, the outdoor air should go to 100% when the fan cycles on for free cooling. I also can?t figure out how to have the exhaust fan cycle on when the fans cycle on.

Christopher Jones, P. Eng.
Rowan Williams Davies & Irwin Inc.
Consulting Engineers & Scientists
901 King Street West, Suite 400, Toronto, Ontario, M5V 3H5
T: (519) 823-1311 ext 2052
M: (416) 697-0056

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Darryl,
The place to learn about the available hourly reports is the DOE22Vol4-Libraries_48y.pdf manual. The Hourly-Report section of this manual lists all available hourly reports. For outdoor air rates, you report the system supply flow (17) and the ratio of outside air flow to total supply flow (39).

Christopher Jones, P. Eng.
Rowan Williams Davies & Irwin Inc.
Consulting Engineers & Scientists
901 King Street West, Suite 400, Toronto, Ontario, M5V 3H5
T: (519) 823-1311 ext 2052
M: (416) 697-0056

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Joined: 2016-07-15
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