#1105 Temp sensor tags with respect to coils

Mike Robbins Mon 11 Mar

Is there a recommended way to tag air temperature sensors that describe their function with respect to coils? How should you tag these sensors to show their multiple functions?

My examples involve upstream and downstream AHU coil temperature sensors. A mixed air temp can also be the inlet temp for the preheat or cooling coil. The discharge of the preheat coil is often the inlet of the cooling coil. The supply air temp can be the discharge of the cooling or reheat coil.

I've skimmed the comments but didn't see this question, sorry if it is a duplicate.

Stephen Frank Tue 12 Mar

If you have a non-standard AHU configuration, one possible way to approach it is to model the individual coils as coil equip records, with equipRef to the parent AHU, then model the entering and leaving air temp sensor under each coil.

Paul Quinn Wed 13 Mar

I use the tags: Coil Type: preheat, hot, precool, chilled, reheat, evapIndirect, heatRecovery coil Medium: air or water Measuring Point: entering or leaving

These have covered all of my AHU air and water temp needs related to coils.

Mike Robbins Thu 14 Mar

Thanks for the recommendations. I want to keep all tags as close to the standard as possible. My work around was to add definitions for "coilInlet" tags on air temperature sensors (coilInletPreheat, coilInletCooling, etc.). @Paul Quinn, if you have a preheat coil leaving air temp, which is also the cooling coil entering air temp, do you give it a coilType of preheat and cooling?

annie dehghani Thu 14 Mar

In OAP, we have defined the following points which may be useful to you:

Mixed Air Temperature

Preheat Coil Discharge Air Temperature

Cooling Coil Discharge Air Temperature

Heating Coil Discharge Air Temperature

Discharge Air Temperature (this would be the actual temp leaving the AHU, not any specific coil. This could be the same as a coil leaving temp, in which case we would use this rather than the coil point)

Essentially all temperature points related to coils are defined based on where they are leaving.

In general, we can get away with this because usually the order is Mixed Air Temp, Preheat Coil, Cooling Coil, Heating Coil and our analytics will still work if there is a non-standard order. However you are 100% correct that the preheat coil could actually be before the mixed air plenum which happens from time to time. The heating coil could also be upstream of the cooling coil. The fan could be at the inlet or outlet of the set of coils, etc.

The order of components in an AHU is not something we usually precisely model, but there are definitely use cases for this level of detail.

Frank Smit Fri 15 Mar


Thanks for sharing the link to OAP. This is very helpfull For mine point of view I always look how a mechanical or electrical installtion is build but there you see that it depends on the engineer how they call it. For example we sell AHU and according the Eurovent certification we need to use ODA,SUP,ETA and EHA but Haystack we talk about discharge air instead of supply air and return air instead of extract air. Sometimes it is confusing because if I say to mech eng you need to replace discharge air sensor then I hear what? oh I mean supply air sensor. I am here in our company busy to make a much as possible standards and it is sometimes diffcult to use the right name due to difference between EU and US in will use OAP as reference.

Leroy Simms Fri 15 Mar


I'm curious, does your team alway model coils as a sub-equip or whould the preheat coil discharge air temp point fall under an ahu? I would think the later would make it very difficult to get to the air handlers discharge air temp without a lot of "and not".

For that reason I typically use entering and leaving for coils as Stephen mentioned.

annie dehghani Fri 15 Mar

@Frank, no problem and I'm so glad to hear that is helpful!

I understand what you are saying about "supply" vs. "discharge". I agree that it is common convention to call the temperature leaving the AHU "supply air temperature" and name the point "Supply Air Temperature" or "SAT". But the name of the point and the semantic model need not be one in the same. That's part of the reason for the semantic model in the first place (to standardize on a data model irrespective of name). For airside equipment, we have settled on the convention of inlet/discharge and avoid use of the word/tag "supply".

Just for further interest, some ontologies such as Google's Digital Building Ontology actually DO use the word "supply" distinctly from "discharge".

Lastly on your point about differences between EU and US, we are working to add localization to OAP. Email us directly if you would like to chat more about that effort -- [email protected]. Part of the challenge is that even in North America, people have different names for the same thing, e.g supply vs discharge, RTU vs AC vs PAC, exhaust fan vs extract fan, etc!

annie dehghani Fri 15 Mar

@Leroy, no, we typically do not model AHU sub equipment such as fans or coils. Rather, we just use the point tags to indicate what a point is relative to e.g. "Cooling Coil Discharge Air Temp" has tags to indicate that it's at the discharge of the cooling coil. As you pointed out, this approach obviously has limitations. And yes, we do sometimes have to do a good amount of exclusion for a point such as Discharge Air Temperature (and not coolingCoil and not heatingCoil...), although we generally handle this by using modular bits of code to look up points.

The only time we model sub equipment for AHUs is when there are multiple points for the same type of equipment and there would be ambiguity by having multiple of the same point with the same tag set. For example if there are 2 compressors each with a "Compressor Status", we would model them as sub equipment with a status point.

I completely agree with your suggested approach and concede that modelling the coils as sub equips and adding the entering/leaving temps creates a more precise model. If you use airRefs to link the coils together you can also precisely model the order of the components as well which seems useful! However, we try to balance this need for precision with the need for expediency in data modelling. It's a trade off.

Paul Quinn Wed 20 Mar

@Mike If there is only one sensor between coils and I have rules or reports focused on a specific coil, I will manually add the tags needed for the point to be found as the entering or leaving air temperature of each coil.

Anytime there is a specific piece or sub-piece of equipment, like a coil, chiller, boiler, heatExchanger, etc., I always use entering and leaving for air or water. The only exception is discharge and return for an air handling piece of equipment to be compliant with the Haystack tagging model.

Mike Robbins Thu 21 Mar

@Paul, if your discharge air temp is after the cooling coil in one AHU and after a reheat coil in another. Do you add tags to differentiate the coil the air is leaving?

I like entering and leaving too. From the tag definitions, it seems like there was supposed to be a distinction between piping and ductwork, entering/leaving pipes and inlet/discharge for ducts, but it seems easier to stick with one pair of terms.

Paul Quinn Fri 22 Mar

Yes, I would add tags to distinguish entering and leaving relative to each coil. I would add "discharge" on the last sensor indicating the temperature leaving the AHU.

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