Opinions on this HVAC Design
Hi, I got an HVAC design complete as per my City requirement for permit application. I am building a two story, slab on grade house with the follow value; slab: R-32, wall: R-60, ceiling: R-90 and ACH: 0.6.
They provided the attached design with the following response below. I am concern this is over kill, especially the posted 6kW inline Thermo-Air heater since base on my heat load calculation, my house would require around 33-35k BTU. Any feedback and suggestion would be welcomed.
“Frankly, one unit for each floor is not the best way to heat. Because you will have an air heater on the HRV and your R-values are really good, it may work. The best way I think; installing small heat pumps for each bedroom, one big for the living room kitchen area and one small for the flex room. There are outside units with 5 indoor heads you can use. Also, I would install an electric baseboard heater in the laundry room and in floor heating for bathrooms.”
Thank you, Arnold
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Replies
Did you do a per room Manual J + S, T, D?
Hi Jon,
I have not yet received anything except the above attachment from the HVAC designer but I did my own to compare since I don't have a lot of confidence in many of the trades around my area for anything beyond code minimum.
Attached is the heat load I came up with which is within the range I was expecting so I think it 90%+ accurate.
Thank you,
Arnold
> air heater on the HRV
So all of the CFM listed are flowing through the HRV (ie, outside air)? Heat from the mini-split is accounted for? My initial reaction would be to use a single head ducted mini-split and a separate ventilation air system (with much lower CFM).
Hi Jon,
The CFM listed are coming through the HRV for the outside air. My wife is starting to think about going with a ducted mini-split heat pump system for the entire house using two ceiling concealed and the HRV on a separate duct system.
We initially wanted to go with ductless mini split system but it is proving to difficult especially with the heat source in each room that is required by the Ontario code.
I am open to anything that would work for our situation without it being overkill and inefficient.
Thank you,
Arnold
I guess you can heat and cool a house with an oversized HRV+coil unit but it makes no sense from energy point of view. That much fresh air would definitely make for great indoor air quality though.
Cooling and heating flow rates are much higher than what you need for ventilation, there is no point in bringing that much fresh air into the house.
The simplest is to go for a ducted mini split for heat and cooling and keep the ventilation separate.
You can usually skip the post heater for the HRV if with a higher performance unit and good register location. Go for one with mid 80 efficiency, place the fresh air supplies register up high on the wall, blowing along the ceiling and away from where people will sit. Even if you are going with a duct heater, you would be looking around 1.5kW unit max.
You can heat the house with two wall mount mini splits as shown, you would have to also install baseboard heaters in each closed room to satisfy building officials. These would not need to run much, operating cost would be very little, they need only to prove enough heat to even out room temperature.
The issue with this type of layout is cooling. The bedrooms would be too hot unless the doors are kept open. For the bedrooms at least, a ducted unit is the best way to go.
The loss numbers also seem a bit on the high side based on the size and description of the building. I would guess it would be closer or bellow 24000BTU.
Hi Akos,
Thank you for your response. I ran the number for my heat/cool load and notice an error on my part and the total heat load is now 29,200 BTU/h. I have updated the attachment in my original post.
The post inline heater was simply added to satisfy the heat source in each room that the building code requires and I was planning on going with a 1kw. I would only use it on the very cold days if the wood stove isn't enough.
We're starting to think we may need to go with a duct mini-split despite not being out top choice for cost and efficiency. I guess we dropped the ball on this and assumed two wall unit would be sufficient to comfortably distribute evenly to provide a good occupant comfort.
Does anyone have experience with ceiling unit? I am thinking that maybe we could use one ceiling mount to help distribute more evenly the heat and cool air. Here is the proposed layout.
Thanks,
Arnold
Our ceiling mount has worked well. Looks a lot nicer and seems to work just as well as the wall units. I wouldn't say it works any better, seems to be more for looks. Install is of a course a bit more difficult but it didn't make much difference in the price. I did get an external thermostat for it but even without it set points and comfort were pretty good. Our install location has it near a bunch of windows in a larger room so opted for it just to be safe. The combo of ceiling mount in the main living area and a ducted system for the bedrooms has worked very well overall.
Hi Dan,
Thank you so much for that feedback.
Arnold
If you are planning some in floor heat the best way to make the inspector happy would be to put a token amount of in floor hear in each room all controlled by the bathroom thermostat. I doubt the inspector has much code language about how much heat is required per room to give you a hard time, just show some and you could include a switch to turn it off.
Give your insulation levels, assuming no huge windows and if you tend to keep the doors open most of the time I doubt you will see much room to room temperature variation.
Why do you think the laundry room needs its own heater?
How sure are you that, heated bathroom floors are likely to get used? I understand it sound luxurious but I suspect I would turn them off when I consider that they cost 4 times as much to operate as the heat pump.
Walta
Hi Walta,
We have no intention of putting in floor heating and it was the HVAC designer who was suggesting we install them to heat either the entire second floor or just to ensuite.
The laundry/utility, again the HVAC designer indicated because of the amount of exterior wall to square footage, it would need some type of heater which he suggested a baseboard.
We are starting to lean towards doing a two zone ducted ceiling concealed unit, one each floor.
The issue is I suspect this HVAC Designer doesn't have any real experience with highly energy efficient house design and is over sizing the equipment. This is one construction area I am not very familiar and was hoping to the "pros" to help me but stuck giving it out on my own with the help of you guys on this forum. Very frustrating to say the lease.
Thank you,
Arnold
Note that closed off rooms don't get enough heat without a heat source - this isn't just a "satisfy silly code" thing. Also note that a so called "supplemental" room heat source will provide 100% of that room's heat if that room is kept as warm as the rest of the house. Summary: use a 1:1 mini-split (or two) with ducts to each room. Where room loads vary dis-proportionally, use zoning (with thermostatically controlled partial dampers).
Hi Jon,
We are starting to about doing a two zone Mitsubishi Ceiling-Concealed Style unit, one for each floor and put those two unit in utility room where we have access for servicing. We're hoping the 16" O.C. joist can be used to run the ducts and out through either the ceiling for the first floor or the walls on the second floor.
Thank you,
Arnold
We're now strongly looking at a ducted system by using two ceiling concealed units, one for each floor but have them both installed in the utility room for easy service access.
The hope is to run the ducts in the between the 16" O.C. floor joist with the diffusers coming down from the ceiling for the first floor and out the top of the walls for the second floor. I attached a very rough diagram for those who are visual.
- If this approach feasible or recommended?
- Can you run duct in a 2x4 dividing wall?
Thank you,
Arnold
Usually not a problem to run ducts along floor joists. Going for I-joist or floor trusses simplifies the runs as you can run the ducts also perpendicular to the joists. Running across I-joists or trusses is tough with hard pipe, flex is the easiest, make sure to size ducting accordingly for the extra losses.
For the 2nd floor, the simplest is to drop the ceiling in the hallway a foot and mount the unit there. From there you have pretty simple runs to all the rooms nearby. Simple runs means you can use a low static unit as well.
If you don't want to loose ceiling height, you can always bump up the overall ceiling on the 2nd floor to 9', nobody ever complains about having taller bedrooms. Another option is to order plenum trusses.
Hi Akos,
The issue is the drawing are already stamped by the engineer. Is it still possible to use ceiling concealed units and run the duct up a 2x4 wall?
Thank you,
Arnold
If your joists are dimensional lumber, the only way to run ducts perpendicular to the joists is in a bulkhead.
Looking at your plans, with the unit in the utility/loundry room, you can run across the house but you can't get to the registers you have drawn without bulkheads. One option is to run the supply trunk in a bulkhead along the unitility room, the powder room and the hallway. Form there a couple of top takeoffs can than feed each register with runs along the floor joist.
If the LVL between the living and kitchen is flush not bellow the joists, you'll have to find another location for the living room supply.
The 2nd floor is a bit more challenging, you can run 3 1/4x14 ducts inside 2x4 walls as long as you can get a feed to these from the bottom. The bigger issue is that you need a return on the 2nd floor (without a 2nd floor return cooling will suffer) which would be hard to run from the main.
A simpler option might be is to vertical mount the unit in the back of the linen closet on the 2nd floor.
Have a supply trunk in a bulkhead running along the top of the linen closet, just above but inside the bathroom and office door to feed the rest of the rooms.
The rest of the ducting can be run again in a couple of small bulkheads above the doors and top of stairwell. Generally any bulkhead above a but inside a door is not visually noticeable. If you don't want bulkheads, bump up some of your interior walls to a double 2x4 on flat and run the ducts inside it.
The best is to draw this all out on CAD, saves a lot of on-site figuring and you end up with a much cleaner and more efficient install.
Looking at the attachments, the BetterBuilt NW load tool insulation defaults are set to
" 2x4 poorly insulated"
...which doesn't correlate very well with the described
"...a two story, slab on grade house with the follow value; slab: R-32, wall: R-60, ceiling: R-90 and ACH: 0.6...."
Garbage in= garbage out.
If the better than code house description is correct the loads are likely to be less than half what the tool is spitting out.
Hi Dana,
I couldn't change the default wall assembly since their drop down menu did not work but I ended up changing the default setting manually.
You should see the Manual J the HVAC Designer sent me using Wrigthsoft Right-Suite Universal 2019 software. It is full of mistake with a heat load of 36,000 BTU/h for my house design. I am in the process of trying to get them to fix their mistake but it is proving to not be easy.
Arnold
>"...see the Manual J the HVAC Designer sent me using Wrigthsoft Right-Suite Universal 2019 software. It is full of mistake with a heat load of 36,000 BTU/h for my house design. I am in the process of trying to get them to fix their mistake but it is proving to not be easy."
HVAC designers have a strong bias to the conservative (in contravention of instructions found in the Manual-J itself), and VERY few of them are capable of calculating U-factors for non-standard higher performance assemblies such as R60 walls. The better ones will try to guess which other assembly in the tool's menu seems most comparable to them, but there really isn't anything anywhere near close to R60 walls in most tools' menus. This is the sort of thing best left to actual engineers and performance building modelers.
Hi Dana,
>"...This is the sort of thing best left to actual engineers and performance building modelers."
Thank you for your response Dana and it's what I have concluded at this point.
It's been a bit of a waste of money for me and I still don't have an accurate Manual J to submit with my building permit or to provide the HVAC contractor.
You wouldn't happen to be looking for some work by any chance Dana?
Thank you,
Arnold
Given how common errors are, standard advice needs to be to provide the load results AND all of the inputs (so one can verify them).
On other hand, a 6K overestimate on a 30K 1:1 mini-split load only has a significant effect if it forces much more expensive equipment (it typically doesn't).
Hi Jon,
The load of 30K BTU is what I got but I am not very knowledgeable in HVAC so I would not be surprise if the real load is closer to 25k as it has been suggested here but by a Passive House builder.
Thank you,
Arnold
No matter what there are built in assumptions and estimations in all models that cannot be mathematically eliminated. Good luck with "actual engineers", some are more "actual" than others and it might be hard to tell which is which since most will likely claim to be.
Thankfully the systems are somewhat flexible and this is clearly a move toward the future and away from central heat/air, a technology that, in my mind, really is 40 years behind and fading at this point.
Is it better to go with two ceiling concealed unit, one for each floor for zoning or one Multi-Position air handling unit for the entire house?
All of these would be installed in the utility room.
Thanks,
Arnold