Unmet Load Hours - VAV with no re-heat

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Hi folks,

We are modeling a large facility operating with multiple VAV AHUs with central heating and cooling coils, but no re-heat at the zones. There are high heat loads in the spaces and therefore higher return air temperatures than the norm.

When I remove the re-heat from the systems I have excessive unmet load hours (heating). I have specified "HEAT-SET-T" to activate the central heating coil. I also have cooling control set to "warmest" and heat control set to "coldest". If I do not set the coil capacity it is still not meeting the loads. The only way I have been able to eliminate these unmet hours is by adding the re-heat back in or changing it to a multi-zone (MZS) system with all the same parameters.

If I model this scenario using the multi-zone system in eQuest to get around this would that be vastly different than VAV with no re-heat? Has anyone had this issue before? Any insight is appreciated. Thanks!

-Dana

Mr. Dana Etherington, E.I.T., LEED AP BD+C | CRB 

Dana Etherington, CEM, LEED AP
email: dana.etherington@crbusa.com
cell: 617-583-3009

Dana Etherington's picture
Joined: 2011-09-30
Reputation: 0

I am assuming this is an existing building. I have always had this issue. I
do all the stuff that you have and use the reheat also, if the unmet hours
are too many. I also look at the SS-F report to see how low the temps are
and when (hourly report) - if they are acceptable, then I leave the reheat
out. You could also widen the throttling range - there might be unmet hours
in the actual building - people just don't complain if it is a few degrees
less/more than the thermostat. You should look at the hourly report to see
if the heating coil is coming on (these reports are not very clean - since
it is not clear for what point in the air stream eQuest is reporting the
temps). Do you have VAV boxes - sometimes I have seen that a constant
volume system has a VFD added to the fan, without VAV boxes in the space.
You could reduce the min flows to zero and see if that helps. I haven't dug
deeper into it, but I think the VAV system is going to set the supply temps
low when even one zone needs cooling/less heating and so others will need
reheat. If things are working smoothly in the existing building, then the
zones must be similar and may not need too much variation in supply temps
for different zones. I haven't used MZ - but that would probably work -
what is the difference in energy consumption between the two options? I
have not been comfortable with the economizer and MZ together.

I will be curious to know what you find.
-Rohini

R B's picture
R B
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Joined: 2011-09-30
Reputation: 4

If you have heavy heating loads in the space, then should try to replicate
that by putting in a heat load in the internal energy tab of the spaces.
This will force the higher return temperatures and make it unnecessary to
have re-heat coils.

This may work for you, because it seems like in reality, the spaces don't
require the reheat coils and the spaces are kept to temperature.

Hope this is helpful,

Rob

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Joined: 2011-02-14
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Thanks for the input guys.

So this is actually a new construction project and we're performing the modeling for LEED certification. Rob, I originally entered process equipment electrical usage as eq. w/sf to the space and varied sensible heat gain % to the space based on what was reasonable. The corresponding process hot water and chilled water loads are accounted for via a detailed schedule and load the respective circulation loops. I'm hesitant to add internal energy sources, I feel like I'd be double-dipping..

The units themselves are VAV and I do not believe there are individual VAV boxes at the zone level. So I just ran the model with VAV and included the re-heat coils, keeping everything else exactly the same as with the multi-zone model.

Results for VAV with re-heat, compared to MZS with no re-heat: overall energy cost for the year increased by only .2% and unmet load hours were slightly higher (applied a uniform re-heat delta T). Not sure which would be more acceptable by LEED reviewers...

-Dana

Dana Etherington, CEM, LEED AP
email: dana.etherington@crbusa.com
cell: 617-583-3009

Dana Etherington's picture
Joined: 2011-09-30
Reputation: 0

Dana,
Did you make sure that the central gratin coils are actually operating when you remove reheat? Sometimes you have to input the hot deck max leaving temperature for the central and reheat coils to turn on.

Sent from my iPhone

Shruti Kasarekar's picture
Joined: 2011-10-02
Reputation: 0

If you do not have VAV boxes in the space, then all the zones in reality
will get the flows that are proportionally reduced based on the main supply
flow, whereas in eQuest VAV system, the flows will depend on the zones and
vary according to what is needed. This may not be a big deal since the fan
energy is the important part in terms of energy consumption. One work
around, which I don't think is the best and have never used is (but you can
try and see what it does) is to run the system as constant volume and set
the flow at 60-70% (or whatever you think is reasonable - most VAV fans are
in this or lower plr region - look up some of the EDR graphs on VAV).

-Rohini

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R B
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Joined: 2011-09-30
Reputation: 4

Dana -

A few questions:

1) What are your expected return temperatures during cooling? If they are significantly higher than normal, would it be appropriate to increase your space cooling setpoint to match your expected return temperatures?

2) Is the air distribution designed to produce a thermally stratified zone? (eg displacement). If so, there is a nifty guide from Energy Design Resources that will help you raise your return air temp while maintaining more traditional space air temps.

3) Does the facility as designed actually have multiple temperature control "zones" per air handler - or one temperature control zone per air handler? If there is just one control zone per air handler - then perhaps per Table G3.1 entry 7 / proposed, you could justify less zones in your model. Changing the number of zones in a model is a lot of work - I've been through it before, and sometimes it means a from-scratch rebuild - but it is sometimes the only and/or best option to reduce unmet load hours.

Aaron

Aaron Dahlstrom , PE, LEED? AP

Dahlstrom, Aaron2's picture
Joined: 2011-09-30
Reputation: 4

Hey Aaron,

I think the solution here is most related to your third question. The design was originally VAV systems with re-heat everywhere. The re-heats have since been VE'd out with local exceptions. The units are still variable volume but are only controlled by a single thermostat in most cases. If I were to model this from scratch, I would combine the spaces into single zones. I can go back and make the changes now but I'd rather not.. :)

I just tested out one theory and it seems to do what I'm looking for, with zero unmet load hours. For any spaces that are not the "control" zone, I simply set the cooling/heating tstat schedules to "undefined". This tells the program that there is no zonal heating or cooling control, which is how the systems are actually designed. I checked the max and min temperatures for each of these spaces for the year and they seem reasonable, and unmet load hours are zero. Some exterior spaces are seeing minimum temperatures as low as 51 deg F in the winter, but this is accepted and understood by the client.

For the baseline model I will define all tstat schedules to simulate the zonal re-heat requirement per ASHRAE. I think this solution seems okay?

Dana

Dana Etherington, CEM, LEED AP
email: dana.etherington@crbusa.com
cell: 617-583-3009

Dana Etherington's picture
Joined: 2011-09-30
Reputation: 0

Dana:

An interesting approach - I haven't tried this before in a LEED model.

A few "vetting" questions:

1) When the tstat schedule is undefined, does DOE-2 run the uncontrolled zones at a flow rate proportion to the main control zone, or do they sit at their minimum flows all hours?

2) Why not have identical thermostat schedules in the baseline and the proposed? I might protest that having no tstat control in the proposed versus tstat control in the baseline shows unfair savings. (Table G3.1 / entry 4 "schedules" / baseline.)

Regardless of #1, I think keeping the zone tstat schedules undefined is a novel way to approach this problem, and I'll be interested to test it out on my own projects (for both proposed/baseline) if needed.

Aaron Dahlstrom , PE, LEED? AP

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Joined: 2011-09-30
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