Hello,
I am considering adding an ERV and Dehumidifier to my house to improve air quality. Your input would be greatly appreciated, thank you in advance for your time.
Home Layout/Size:
I live in a 4-story 3b/3.5ba townhouse, 2800 sqft livable space, ~28,000 ft3 volume, in Houston, TX (very hot & humid). Home is relatively “open” with doors only to bedrooms/closets/bathrooms. There is an open channel staircase going from 1st to 4th floor where you can see from top to bottom of house.
Existing HVAC Setup:
The house has two separate air handler units, one in the 4th floor attic-closet (services top floors 3 &4), and one in the 2nd floor utility closet (services bottom floors 1 & 2). Both air handlers are Lennox CB26UH-048 (~1650CFM each, so 7 full cycles of house air per hour if fans running at high). HVAC Supply air is ducted from air handler to ceilings of rooms and hallways. Return air is drawn in at the walls in the stairway channel areas. The Return air filters are also located at the wall of the stairway channel areas (not directly upstream of air handler). Air exits rooms via underneath the doors. Ventilation is exhaust-only via bathroom fans and range fan. House is not designed airtight; 1/2″ drywall, 2×4″ exterior studs, fiberglass insulation, OSB sheathing, vapor permeable WRB, ~30% Stucco w/ rainscreen, ~70% Cement Fiber Panels, and large Andersen windows that draft a bit — the house undoubtably “breaths”.
Problems / Flaws:
1. AC usually has to be set to <70degF in order to get comfortable humidity levels, and humidity rises very quickly after AC shuts off. AC otherwise does a good job tracking temp setpoint.
2. Frequent dustiness all over even with multiple air purifiers running 24/7, regular HVAC filter changes, and regular vacuuming. I suspect exhaust-only ventilation contributes to this.
3. Attic-closet where 4th floor HVAC unit resides is insulated from house but not from outdoors. It gets very hot in summertime.
Proposed New “Simplified” Design (see attached file):
I want to take a staged and gradual approach, first focusing on a simplified solution that can be upgraded later with more advanced controls/ductwork. This proposed new design is only for the 3rd and 4th floor system for now.
ERV Stale Air Suction: Will splice into existing 6″ main bathroom exhaust duct, to draw stale air from all bathrooms in house. This duct is simple and easy to access. I could remove the existing bathroom fans, but it’s not clear if that’s really necessary. There would be some degree of pressure loss through the fans, but when turned on they would boost flow. Any thoughts on keeping the bathroom fans in place?
ERV Stale Air Discharge: Will splice into existing 6″ main bathroom exhaust duct, pushing stale air out of the existing roof outlet. Very convenient and no new wall penetrations to make.
ERV Fresh Air Suction: From directly outside attic wall. Simple and accessible.
ERV Fresh Air Supply: To a tee’d line which tees with: (i) Dehumidifier Suction, and (ii) Duct going to/from 4th floor wetbar area. AprilAire 8100 ERV I’m considering flows up to 150CFM. The basis is to directly provide fresh air to the dehumidifier for treatment, but also not be impeded by the dehumidifier if it is off. ERV to run at all times.
Dehumidifier Suction: From the tee’d line which tees with (i) ERV Fresh Air Supply, and (ii) Duct going to/from 4th floor wetbar area. AprilAire E080 (80 pint) Dehumidifier I’m considering flows up to 185CFM. The basis is to directly treat fresh air from the ERV, and to draw the additional needed 35CFM of makeup air from the 4th floor, then also be able to run unconstrained if the ERV is down. Any thoughts on the teed line between ERV fresh supply, 4th floor general area, and Dehumidifier suction?
Dehumidifier Discharge: To a location proximal to the top floor HVAC return filter, OR, teed into HVAC return duct. The problem with the HVAC return duct is it is duct-only, there is no plenum to conveniently tie into. It would be a lot more involved to erect a plenum box.
HVAC Setting: Air handler is set to run fan for 15 minutes every hour. No plan on changing this, would like to mix this air as much as possible especially since it isn’t a highly optimized design.
Other Questions / Summary
S1. Any concerns on the ERV Fresh Discharge / 4th floor general area / Discharge Suction tee?
S2. Any concerns on having the Dehumidifier Discharge going directly to the HVAC return duct? Particularly, this tie point would be downstream of the return duct filter which is in the stairway area (not directly upstream of the air handler)
S3. Any concerns on keeping the bathroom vents intact as-is while still drawing stale air from the ERV through the bathroom vent main exhaust duct?
Q1. ERV I am planning to run 24/7, no controller, no integration, just set at a CFM and let it go. Any concerns?
Q2. Dehumidifier I am planning to have run only on its built in control loop with no integration and no external controller/sensor. It will see a mix then of up to 150 CFM from ERV, and 35 CFM from 4th floor, so I’ll likely have to set the target setpoint to a bit higher than actual house target or it will almost always be running. Any concerns here?
Q3. My house is likely a leaky one, and while the ERV will help with pressure balance the relative fugacity between ambient and inside I expect will still cause humid air to push into the house – is it possible to force the ERV to run with higher fresh flow and lower stale flow to promote slightly positive house pressure? I would prefer filtered air be pushed out rather than unfiltered air pushed in.
Thanks again for your time to read my post. Your input is very much appreciated. Please let me know if there are any areas that need clarification.
Replies
Your attachment is not showing up for me.
You might be able to have a local HVAC person modify settings or add a humidity control so that your air handler with the ac can do a better job of removing humidity.
The basic idea is to reduce air flow so that more moisture is removed by the coils.
I think it a good idea to provide some outside air to replace that removed, and an ERV will save some of that expensive, cooled air energy.
It does seem that you would do better to install some bathroom controls for the ERV, or at least refrain from use of the existing fans. ( I think the ERV pumps air in both directions, so pushing air into it might have impacts not obvious at first.)
https://images.finehomebuilding.com/app/uploads/2024/03/06124939/204376_1709747378_Ventilation-clean-r.3-flat_Optimized.pdf
Calvin thank you for posting the image.
Mike I see what you're saying about modifying the existing unit but I would prefer to have a dedicated dehumidifier to avoid using the more expensive air handler to dehumidify. I would also like to have a dedicated ERV/Dehumidifier system that I can take with me to a future home.
As far as bathroom controls for ERV I conceptually agree that would be best, but from a practical standpoint with a 4-story townhome it would be a highly involved wiring job to create that communication path.
Pushing air into the ERV is where I'm currently scratching my head. I deal with industrial heat exchange equipment pretty regularly, and there are times when we have surplus flows into exchangers - the drawback is really just efficiency in those cases. On the extreme end, a very large surplus of flow could damage the exchanger. But the flow and pressure developed from a bathroom fan I would find hard to imagine that it is enough to actually damage the exchanger in the ERV.
Adding an ERV and dehumidifier sounds like a smart move for your house, especially in Houston's hot and humid climate. Your plan for the ERV and dehumidifier setup seems well-thought-out, but I'd recommend consulting with a professional to ensure everything is correctly installed and balanced. As for your specific questions, keeping the bathroom vents intact while drawing stale air from the ERV should be fine, but it's always good to double-check with an expert.
Running the ERV 24/7 and the dehumidifier on its built-in control loop seem like reasonable approaches, but monitoring their performance is key to ensure they're effective and efficient.
Good point on the balancing aspect by a professional. In your opinion, is it worth investing in analytical equipment and doing balancing ones self? It appears to be reasonably straightforward and I think would just need a pair of manometers?