Hi - Long time reader, first time poster... I have a two year old six ton Synergy3D unit. We started a major home renovation this past fall. Up until the time where the new addition was tied into our we existing home, structure, HVAC ducts etc., our system performed beautifully. After the tie in was complete, our system struggles to maintain 72 degree and the Aux heat runs nearly non-stop. Details: -Original home 2700 sq ft. Addition added 1200 sq ft for a total of roughly 3900 sq ft. -WF unit is in middle of existing home basement. Two plenums run off the system - one to the left, one to the right. (Think of a T design). - The duct work in the addition was tied into the top right of the T ductwork. All the math and load calcs show that there is sufficient capacity in the tonnage, ducts and CARs. - Cold Air returns are not tied in and active yet - only the ducts are. -Thermostat config on Honeywell Vision Pro show the following: --0176: Heat or Heat Pump Aux Stages: 1 --0240: Stage 1 Heat Cycle Rate: 3 --0250: Stage 2 HCR CPH: 4 --0260: Stage 3 HCR CPH: 4 Long story short - we have all new Andersen 400 series windows, new insulation, new doors etc etc. Yet our house can't sustain 72 degrees when it drops below 30 outside. Prior to the addition, and with our old drafty windows, doors and minimal insulation in place - we had no problem. Some of the installers have made the crack that the duct run to the addition is a long haul. I show 85 degree coming from the farthest ducts. Help!

Six tons may be marginal with the added square footage.Was a heat load calculation performed.Post a diagram of your duct system, include length of run, duct size, number of grilles, and size of registers.Pictures are always helpful.

6 tons should not be marginal at 30 degree outdoor temps. In Portland Oregon we have 5,000 sq foot homes on 5 ton systems and our design temp is 24 outside and they do fine. Since the house was updated with new windows, insulation etc it should be almost new built.

Second... Why a synergy3d unit? What water are you heating since you are just talking about air?
Is your ductwork fully sealed with mastic? Is your ductwork at least R8 insulation? Have you had any home performance air sealing on the older side of the house?

Heat load cal was perform on multiple occasions - new install and for building permit process. I have drawn the layout but the system limits the upload file size to 100kb. I also have the summary of the load calcs. Please message me and I will send them directly.

We chose the Synergy unit as we told the original salesperson that we wanted to share the system with a future addition to our house. What we asked him was it possible to share the loop field with another compressor / system, and he pointed us to the Synergy. Long story short and two years later, we understand what the Synergy was developed for and it wasn't for what we thought it was for. So I keep the idea of either radiant floors etc in the back of my head as an option. We had our hot water heater looped into the system so the pump circulates the HWH water through the synergy unit. All of the new ductwork is fully sealed, blanketed and has tape where it is rigid alum. THe flex ducts in the attic came pre insulated with R8. I have taped all of the seems that I have found and have access to over the past year or so. FWIW: My rough calculations suggest that the HVAC company attempted to just connect the new duct into the existing tail end of my current duct system. (The sales person had estimated the need to remove 25' of existing duct work, enlarge it and then build the new duct work of this increased duct work. Based on the math - they are over 400 CFM short of what they need to feed the existing system and the new ducts/vents simultaneously. My sense is the installer supervisor saw all of the work involved and decided to try and cheat the system by just extending the tail of what we had. The plan calls for the install of zone dampers on the system with the addition of a 4 zone controller. That doesn't resolve the fact that there is simply too few CFM to share with all of the feeds / vents off the main duct.

Posted By jwp on 19 Jan 2013 07:26 PM
We had our hot water heater looped into the system so the pump circulates the HWH water through the synergy unit.

Tell us more about this? While correct ductwork sizing is important it is not, unless off to the extreme, usually not the reason why there are not enough BTUs delivered to your space. If you are moving less air through, that air will become hotter. The overall BTUs are less, but usually not more than 5-10%.