HVAC – Zoning
sunil_j
| Posted in General Questions on
Hi,
~4100 sf new construction outside of Houston, and planning for a normal split system design. One 3-ton and one 4-ton is what the HVAC contractor recommends. We’d like 3 zones: 1) for open social areas living/dining/kitch on first floor, 2) for 3 bedrooms including master on first floor, and 3) for 700 sf upstairs that has a bedroom and office.
I’m getting mixed advice from the contractors on zoning. One says to have the larger unit support zones 1 and 2 and is dead set against zoning across floors, and the other is recommending the larger unit support zones 2 and 3.
Does this matter? TIA
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Replies
First step is to have an independent engineer run a proper manual J on your place. Since you can can easily cool a similar sized 100 year all brick non-insulated house with around a 4 ton unit (in slightly colder summer temperature but not far off), I highly doubt a new construction needs 7 ton of cooling.
The sizing has very much a feel of a 500sqft/ton rule of thumb:
https://energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
Once you have that, you can figure out what size unit you need and how much zoning makes sense. Generally you want to group zones with similar cooling load together and maybe add a zone for bedrooms to keep them cooler. For example if you have a great room and study with larger west facing windows, you want these on the same zone as they both have very large cooling load in the afternoons.
There is nothing wrong with zoning across floors but generally it is much simpler to have an air handler per floor.
One last item is to make sure that there are no ducts in your attic. This is a huge energy penalty and can easily add about 1 ton of cooling load (and cost to run that extra cooling) plus create a lot of comfort issues. You want all ducting and air handler inside your conditioned envelope.
Thank you, Akos.
The installers say they will do a Manual J later. I do worry about this, as I have read that article you cited and it does feel like a rule of thumb approach that will leave me with oversized units. Maybe this is a sequencing thing, as they say I have to choose them as the installer before they will do a Manual J. Seems like I'll have to do this no matter what, so I might see if I can pay for this separately and provide it as an input to the quoting process.
All of the ducts are within the thermal envelope.
It is very easy to have a manual J give the results you are looking for. I wouldn't trust an HVAC tech to do an accurate one. It is very easy to tilt the scales by using the wrong design temperature, incorrect wall insulation and clear windows instead of coated low E units.
What you want to do is find a hvac designer whose job is to come up with an accurate calculation and have them run the Manual J. They will also be able to advise on the best way to zone your place.
Designing in a ducted dehumidifier and an ERV is also a good thing up front.
Anybody who says they have already decided on the equipment size and will do an manual J later has just undermined any credibility they might have had. That's sounds like the punch line of a joke making fun of sketchy HVAC contractors.
Do you have a room-by-room Manual J? The units should be sized to the needs of the zones they serve. HVAC units don't like to run at much lower airflow than their design flow, so if one air handler serves two zones and they are different in size that may not work.
I've done Zoning in many different ways.
After 25 years of experience the only way I would do it now as with variable speed equipment. I use Trane comfortlink variable speed which measure static pressure per Zone and modulates the fan and compressor accordingly the number of zones calling. Zero wasted energy. If you zoned a 4-ton system and had to bypass 1/3 ( dump the Supply right back into the return) to maintain airflow, the wasted energy would pay for the upgrade in a couple years, and provide the ultimate comfort with temperature and humidity control.
" I use Trane comfortlink variable speed which measure static pressure per Zone and modulates the fan and compressor accordingly the number of zones calling."
+1. I have a Trane variable speed zoning system installed in my house, and it works incredibly well.
Thanks Aun. Good to hear the positive feedback on Trane as we are also considering them.
Thank you Tinman77.
I'm exploring a ducted heat pump system, but at the moment our plan is to use the Trane XV18 which has a variable speed function. Is this the kind of system you were referencing?
I have the XV18 outdoor unit matched with the TAM9 indoor unit and the ComfortLink II zoning system. I have 4 zones -- 3 on second floor, 1 on third floor. System can easily maintain each zone to within 1/2 degree of setpoint, even if the setpoints in each zone differ by 4 or 5 degrees. For example, master bedroom set to 73F and third floor attic set to 78 F. No problem.
Edit: Oh, one other comment. During ice storms this past winter, the ODU fan blades got some ice on them, causing some significant vibration of the unit, so I switched the thermostat to emergency heat 3 or 4 times. For that reason, if you live somewhere that has ice storms in the winter, I would consider the XV20 instead of the XV18. The XV20 has a more protective "top" on the ODU that might prevent freezing rain from causing problems the way that it does on "open top" ODUs like the XV18.
Edit 2: Now I see the OP says you are in Houston. So probably no need to worry about ice storms for you.