HVAC Muilt-zone Unit conversion to VAV with reheat - savings

[gooutandgetem]

Does anyone have a good source that covers the retrofit options to make a Multi-zone unit (hot deck and cold deck sytems that mixes the two air streams to condition a zone)more efficeint? There must be some good info out there but I have not found it yet. I have seen the hot deck disabled and VAV boxes installe (with reaheat).

 

[walkes]

Let me know if you find something. All I have done in the past is the VAV with reheat option.

 

[gooutandgetem]

I will let you know. The challenging part is calculating energy savings. If the system does not have a lot of simultaneous heating and coolng requirements the savings may not be all that great. It may be a good case for a Carrier HAP or Trane Trace energy model.

 

[MTES93]

Yes, as you mention, disable the hot deck and install vav box/reheat coils. Depending on your climate you may need to look at mixed air temperatures at minimum supply air CFM. As per current codes/standards (and just good engineering practice) you need to maintain vent air thru-out the full range of the supply fan speed. Measurement and control of this outside air is up to you. You don't necessarily need to use an airflow station. You may need to install a preheat coil or possibly modify the hot deck coil (not sure how) to provide the necessary preheat.

Look at fan motor and make sure it is suitable for use w/ VFD. Check fan curve and make sure you will operate over a good range. Some of these old dog ahu's may have a forward curved fan which has some trouble spots at different RPMS...

One of the T.C. Contractors I work with has decoupled the hot & cold deck dampers (typically tied to one actuator) and installed individual actuators on each deck. He then controlled the hot/cold deck dampers independently and allowed the zone air volume to vary. In essence he 'built' a pressure dependent VAV system which according to them worked reasonably well at a significantly lower cost than the VAV-reheat option. Static pressure control was a bit tricky as you are reading it right off the fan upstream of the modulating dampers and coils (I believe is where they put it).

Yes modeling.. Trane Trace, Carrier among others should give you a reasonable estimation of the energy savings. That coupled w/ an accurate cost estimate will give you a ROI or present worth analysis if you so choose.

 

[Drazen]

when you have system in place for some time, real-life information is much more valuable than design assumptions, for a simple reason that you do not have to assume anything if you can measure it.

what do you need to measure is clever part. you should make some model of current system, to present yourself how it works year-round, than seek for data.

there are many question as regards to model: for instance how ventilation is modulated (if at all), is dehumidification of even humidification part of system, is there some differentiation between perimeter and internal zones etc.

than climatic conditions are certainly crucial - you would need to seek for data about magnitude and frequency of boiler use, regular heating should be separated from summer reheat for purpose of analysis. type of occupancy should certainly be tracked in some historical data. i assume that occupancy is quite variable if such initial system is used.

the only real rule is that you cannot give prescription without good examination (similar to doctors), there is certainly no magic formula.

if client insists that you give him something (...quickly!!), good engineering practice would be to explain to him that you need careful analysis, first, maybe some tracking period as well.

 

[urgross]

Have you considered, if duct sizing allows, going to dual fan, dual duct? The University Of Nebraska, Omaha has some good LCCA documents on modeling, as well as Texas A&M. The Canadian NASA has a good wite-up as well.