Hi all,
New eQuest user, attempting to model some alternate HVAC system configurations for a new building. I set up the building, zones, internal loads etc., then created the first set of HVAC systems as packaged single zones with DX cooling and electric heat. Did a save-as, changed all the units to hot water heat and created a hot water loop. So far no issues. Now did another save-as and changed the units to VAV, with chilled water cooling, and created a chilled water loop. No change to the heating system configurations. In this model the cooling loads on each AHU increased by about 50%, and the total annual heating energy increased by a factor of 5. I have perused the detailed reports looking for causes and can find nothing. Zone load reports are identical, but the system design parameters show dramatically higher cooling requirements for the chilled water architecture. Zone temperatures are fine in both models.
Any thoughts out there?
Without looking at anything you have in the model, I have a couple items to point out. Firstly, a large heating energy increase is to be expected with a VAV AHU system using terminal box heating. Those type of units are not very efficient when it comes to heating. Cooling on the other hand is a different story. The only thing that pops out of my head is that possibly you have the chilled water set point at 40 or something like that... That can have a big energy effect... Also, are the AHU's set at variable flow? There may be too many questions to ask about the cooling side of things...
Peter,
Why would you characterize a VAV system with terminal reheats as inefficient in heating? I can understand more heating energy being expended during cooling season if the system is not controlled properly, but in heating mode the only real inefficiency should be minor losses in the distribution piping to the reheats, and this should be at least partially offset by lower temperature losses in the duct trunks. I should also note that there will be a significant outdoor air percentage in these systems due to health care ventilation requirements. Regardless, the packaged DX model I used for comparison included hot water reheats, as I have a couple of units serving multiple zones and required reheat in the non-controlling zone to maintain temperature. Changing the system type to from packaged DX to VAV caused dramatic increases in both heating and cooling loads. The chiller setpoint was at the default of 44F; the air handlers were set to variable flow.
Is it possible to attach eQuest files here?
Matt, you are correct that the heating delivery system is very efficient for a VAV with reheat. However, think about what temperature the VAV delta T is. It is probably around 50 degrees, since the AHU supply is 55 degrees. That seems to be causing the problem with reheat, also one of the reasons why I am not a fan of those systems, unless you have a large interior load where heating will be used very rarely.
To the cooling, I can say less about it, as I am new to the Equest side of things I cannot direct you as efficiently. But from a general standpoint I have a couple questions that may shed some light...
a) Did the zone flow ratio get adjusted so the VAV can modulate? If not this may be the problem here, as the zone will be dumping cold air, and during the winter, it will be heating all the air, not just some like normal.
b) How many zones are running off a single AHU? I know from working with other modeling software, a VAV AHU needed to serve at least 3-4 thermal zones to get reasonable numbers.
c) Is the chiller sized properly to the building, maybe it is undersized for the load that is in the building? This may be related to Equest, more than I can speak for.
I hope this helps!