To Duct or Not to Duct an Air-Source Heat Pump
Building a home in Oakville, Ontario. Trying to build a Pretty Good House. That consists of basement, main floor, second floor, double stud wall, ICF basement, triple pain windows, etc and is roughly 2700 sqf. Currently, house is designed my an engineer with fully ducted single VRF unit in the basement to heat and cool the house. However, this appears to require a lot of duct work and seems overly complicated. A few potential alternatives are as follows.
1-multi-split with refrigerant lines going to individual rooms/ ceiling or wall units.
Pros; lot of zones/ control, no duct work.
Cons; cost potential as I would need purchase so many ceiling or wall distribution units (not sure though). Louder, as ceiling units have less fan speeds than the wall units.
2-multi-split with ducts. One of the main floor + basement and a second unit for the 2nd floor. (or one for each floor)
Pros; Concealed look, no wall or ceiling units, I more quiet as the unit is generally further away from individual room exit points.
Questions:
1- Is the duct work in the attic outside the building envelope or do you bring it inside and have to design bulk heads etc?
2- Where do you house the house these ducted units in the house?
I feel like I can’t get a good answer from my designer (Passive house designer), HVAC installer, or ASHP companies. Looking for some advice.
IN addition, I need an ERV that does at least 200 CF. What is the best ERV for this area/ building zone? Venmare?
Thanks
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I can’t answer your duct question, but I did notice a potential ERV oversizing.
You said that you need an ERV that does at least 200CF. Assuming 200 CFM here.
I call question to the size of your ERV as a good rule of size is found using the go-to formula: 7.5cfm/person + 3cfm/sq ft or area.
Since you’ve given us 2700 sq ft area this brings that side of the equation to 81. Adding a couple of people living in the house would only bump the requirement up a few CFM. For example, two residents brings the requirement to 96CFM.
In other words a 200 CFM ERV at your size of house could supply fresh air for about 15-16 individuals dwelling in your residence.
Hi Drew.
Please see the attached file that was provided to me from our HVAC engineer to assume I needed a 200 cfm unit. I would love it if I could use a smaller unit if possible. Another note we are not installing any bathroom fans or a range hood fan. Want to keep penetrations to the outside minimal.
There's not really such a thing as oversized ERV, as long as it has multiple fan speeds. You can't have too much ventilation headroom.
Having monitored C02 levels in my house, I dispute any claims of 7.5cfm per person. I know that's the prevailing wisdom, and I don't know the methodology, but what I can say for certain is that with 100cfm, 6 adults will cause runaway CO2 levels; over 1000ppm within a couple of hours. 15 people and 200cfm is going to be really bad.
PGH and ductwork outside the building envelope don't work together, very bad idea for energy efficiency. When designing from scratch is it easy to hide the bulkheads in hallway ceilings or thicker interior partition walls. There are also plenum trusses you can order that will have a raceway for ducting.
For any low load house you never ever want the one head in a room type of setup. A lot has been written on this forum on why that is bad. You want a properly designed ducted setup.
If you want to minimize ducting, for a larger 2 story house, the simplest would to have an air handler in the basement for the basemen and main floor and separate air handler for the 2nd floor.
The 2nd unit could be one of the slim ducted units like this:
https://ashp.neep.org/#!/product/31984
This could be in a small utility closet in the 2nd floor or inside the dropped ceiling of the hallway.
Generally for these systems it is best to have a dedicated outdoor unit for each indoor unit, they have much better modulation range and efficiency. This also gives you a backup in case one unit fails. The parts cost is about the same, so there is very little benefit with going with a multi split.
Make sure to plan how to duct your ERV. Generally the best is to have dedicated ducting for the unit supplying fresh air to all the rooms. A bit of careful design with mini splits can let you share the supply ducting since the fan on the air handler always runs. You would still need dedicated ducting for the stale air pickups.
If you have two air handlers, I would go with two smaller ERVs.
Great advice. The only part I doubt is combining ducts--I can see how that could work but given how short the minisplit ducts are, the savings isn't that much, so I'd be inclined to just avoid potential for problems and keep them separate.
Hi Akos, would you be so kind and post the treads regarding "For any low load house you never ever want the one head in a room type of setup"
Thanks
https://www.greenbuildingadvisor.com/question/mini-split-btu-vs-electric-baseboard-btu
General issue is the smallest branch box multi split unit is usually 4 ton, which is probably 2x the heat load of your house, so it will cycle a lot on any day outside of polar vortex.
Smallest head to that you can get is 6k, which is 3x to 5x oversized for a bedroom load creating comfort and moisture removal issues.
Heads in bedrooms also still make some noise, rattling fan (happens when they get dirty), condensate pump cycling or even diverter vanes moving. None of it is loud but not something you can easily ignore.
Most multi split indoor heads don't modulate. They are either on at rated capacity or off. This can create issues with humidity removal, which is a big problem in low load efficient construction.
Go with ducted whenever possible. It offers a lot of future proofing as well. There are many negatives to the ‘head in every room’ as others have mentioned but one thing that doesn’t get mentioned much is that pretty much everything needed to make these units operate (refer lines, communication wires, condensate lines etc) will end up getting buried in the walls. With how fast equipment controls and Freons change it is very likely that when it comes time to replace your equipment the stuff in the walls will be outdated or under/over sized. Having ducts in place makes it much easier, with many more options, down the road. Carefully laid out webbed trusses with plenum spots are usually effective to keep ducts out of the attic. If the trusses line up correctly we have always been able to make an install work without putting anything in the attic. When the trusses are offset or get installed in an alternating pattern it can be tricky. Round pipe works well with these trusses. In my house which is a two story house with basement, PGH about 2800 SF above grade with my family came back at just under 100CFM for ventilation.
Go ducted to eliminate cold corners. It's hard to match the comfort of properly placed/aimed diffusers with a point source head, and your load profile will match the equipment selection better and probably allow you to use more efficient 1:1 indoor/outdoor equipment that has a much higher HSPF and actually modulates. But I would recommend adding a dehumidifier to your plan if you don't already.
That ventilation design called for 188cfm boost but more like 70cfm low speed. I would recommend the Renewaire EV Premium M or L. It's ECM has dual speeds and goes up to 225/280cfm depending on the model.
Since your low speed setpoint is below 100cfm I suspect M would be sufficient. The L has a high max flowrate and has a larger core, so it's more efficient at higher flow rates, but at lower flows I'm not sure the added cost is worth it.
https://www.renewaire.com/erv/ev-premium-m/
https://www.renewaire.com/erv/ev-premium-l/
Your supplier can model the performance based on your equipment selection and design selection and OA conditions. This beats trying to interpolate generic HVI test data. I've attached a screenshot of the report they created for me.
They can model the ECM fans according to your ductwork flow/ESP.
I'm about to install a EV Premium M in my house in Southern Ontario, so I'm a little biased but Renewaire offers good data, and nice controls that are easy to interface third party components too. Fan speed can be set by 0-10v signal if you want or internal adjustment dial, and boost/high speed can be triggered by anything with a dry contact. Since I do commercial building controls for my day job, I will probably interface a CO2 sensor and possible some humidity sensors in the bathrooms along with manual boost switches.
The Vanee Gold Series and Panasonic Intelli Balance were also on my list, but I think Renewaire has a better product for the dollar spent if you need more air than the Panasonic can provide. If you're within the efficient range of the Panasonic it's a good choice too and offers boost on the newer models. I like that it tries to maintain constant flow. Make sure you have electronic balancing with ECM fans. This beats playing with dampers to balance.
Hi Josh,
Who are the HVAC contractors you dealt with? Thanks
Josh,
Thanks for the reply. (Side note it would be great if you could message members directly. Does anyone know if you can do this?) Who are your contractors for your HVAC system?
After much debate we are thinking of installing a whole home/ multi position forced air source heat pump. I am not sure what brand to use. I prefer to have one of the big asian brands (Daikin, Fujitsu, Mitsubishi, Panasonic, LG etc). The thought of having to tear our ceiling apart to replace a ducted air source indoor unit in the future does not appeal to me. Further, we are not wanting to install this in the attic. We would have preferred to have it within the building envelope. In addition, we would have to have changed our truss design and interior specs would need to be altered. All of those things start to add up when people are charging you $125 to make these changes. Having one large unit in the basement although less efficient seems a lot wise and simpler for us. We are aware that this style of air source heat pump is less energy efficient.
Unfortunately, it is so challenging to get information from these companies as they all want you to speak with dealers. To make maters worse when I go to these brands websites and locate a dealer they often are unaware of these products and their specs. I have literally contacted 10 dealers for this job and have received next to no information. This is insane to me as this would seem like a nice job if I was in this business.
I created another post appealing to those in the know to inform me of brands/units that are designed for the cold. NEEP.org was suggested. I was able to speak with Fujitsu and they have a new product (2 months old) that is good to -4f (not sure this is good enough). Daikin is similar good to -4f. It appears that Mitsubishi is the only one I can see providing reliable heat in the more extreme cold via their Hyper Heat models.
https://www.greenbuildingadvisor.com/question/best-cold-weather-multi-position-whole-home-heat-pump
We regularly install both the mid static Fujitsu and the multi position Mitsubishi. In my climate zone we usually incorporate some kind of backup electric resistance heat. If you need back up heat the Mitsubishi makes this a lot easier than theFujitsu. The Fujitsu can be used with a themolec duct heater and some third party controls. Given the choice and if space allows we usually prefer the Mitsubishi system. I think the Mitsubishi units are rated down to -13 if the wind baffles are used
I'm down the road in KW.
Stay away from the multisplit approach.
I have a single ducted Mitsubishi PVA, and it works well. Just make sure it's sized appropriately.
If the contractors know ducts, it'll be easy easier to do ducts.
Steve,
Ive been waiting for you to reply. I'm basically, going to be installing a similar system to yours. You got me scared after reading all the issues you had with your zoning system. How is everything working now? Any tips or things to avoid if I go with Mitsubishi?
Definitely don't go with the Air Zone system. It is no longer supported by Mitsubishi.
The Mitsubishi system itself is excellent, and provides incredibly confirm while being almost entirely silent.
I would use insulated ductwork in the basement if I were to do it again, and not use the zoning system.
After sorting out the zoning system, its working very well, and we largely get to ignore it.
Steve Grinwis
Can you elaborate on using insulated ductwork in the basement as well as not using a zoning system?
If we were to install a Mitsubishi Zuba or its equivalent we were hoping to have three zones branches off the indoor unit (mechanical baffles I beleive).
Thanks for your help
The HVAC design for the house has a lot of ductwork in the basement, so even with the zoning, the basement can go over heated or over cooled. I think insulated ductwork would help with that.
The zooming system we use is AirZone. We've had a pile of issues with it. I cannot recommend it. I'd tend to want to use manual dampers that you adjust seasonally.
We have a Mitsubishi ductless mini-split set-up in our Passive House. We have a head on the wall in our kitchen/family room area, along with one in each of our bedrooms (15k Btu, 9k Btu, and 6k Btu respectively). We've been very happy with it so far.
We've not experienced any of the modulation issues noted above. When they're first turned on they ramp up to meet temp and then appear to modulate. We've had no issues with either heating or cooling in terms of maintaining comfortable temps. But I also haven't been closely monitoring this either. Also, the bedrooms aren't that small, at 16x14 and 16x12, with WIC's.
Also worth noting, our 15k Btu unit does all of our heating, apart from when days get down to zero or below when we'll turn on the other 2 heads.
In summer, on the other hand, we enjoy having all 3 heads on for the hottest days of summer. I grew up without AC, so I frankly relish this.
Overall, we're using just over 4,000 kWh to heat/cool our home on an annual basis. Even with modulation issues, how significant could this be in energy use terms? I wonder if anyone has quantified this with research. It would be interesting to know.
The consensus seems to be growing that a 1:1 set-up is ideal:
https://www.greenbuildingadvisor.com/article/goldman-on-minisplits
I agree with Akos that 1:1 does help you should an individual unit fail, or if it just has issues.
Based on what I've read, Passive House projects are using a ductless head in a main floor living area (assumes open floor plan), and then ducting to individual rooms upstairs.
If you have space upstairs for a ducted unit, it would limit duct runs to the second floor.
The heads on the walls don't bother us in aesthetic terms. If they're 7-8' off the floor they're not really in sight lines for the most part. They become background after first couple of weeks. I think it's easy to forget that there's almost always some evidence of HVAC systems visually, e.g. supply/return registers around windows/doors and on walls/ceilings, or baseboard units.
https://kimchiandkraut.net/2019/03/06/ductless-mini-split-for-passive-house/
We don't have any active heating/ac in our basement. In winter, temps typically stay above 61º F (entirely dependent on outdoor temps), which works for us. I'm guessing even a 6k or 9k Btu unit would be enough to maintain 68-70º down there (depending on total sf). Temps in summer are within 5º of our main floor.
We have a Zehnder ERV unit, but there are any number of other options. Just as important as performance/efficiency is your level of confidence that your HVAC installer will service/repair the unit should issues arise.
https://kimchiandkraut.net/2019/02/21/hvac-for-passive-house/
Great blog, Eric!
Thank you, Will!