[dbbarron]

After many posts and views on this forum, I have now used Hvac calc software to model my house, then I measured airflow actually delivered to each room and I compared the results hoping to fine tune my approach to fixing the house either with mechanical systems or otherwise (envelope, windows, etc.).

In short, 2 story plus basement new england, 2200 sq ft main floor, 1200 sq ft. upstairs bedrooms, 1400 sq ft finished in basement. Wide open spaces and open from upstairs to down. Single Carrier Infinity 100kBtu, 5ton system in corner of basement about as far away from upstairs points as possible. Manual J indicates system sufficient in size, but airflow to upstairs is the practical problem.

Ductwork difficult to get to (for boosters) and not possible to easily retrofit.

Practically, upstairs was always hot/cold. Added booster fan for MBR, now that room is much better, but son and daughter far bedrooms still too much variation.

Was thinking of central system for all upstairs or mini splits in each room upstairs so I could zone for efficiency. Contractors who sell both have recommended both. One did a rough manual J but did not measure cfms delivered to each room. However, due to open nature of house, not sure central system can effectively zone off upstairs. Mini-splits could easily zone bedrooms with doors closed.

See attached spreadsheet, now clarifying everything and seeming to indicate that the only meaningful problem remains my son and daughters room with only 1/3 the needed CFM being actually delivered. But for the MBR booster fan added last year, the MBR would be even worse than son and daughtor's rooms.

So - the questions of what I should do to now 'fix' the system assuming a mechanical solution (per Manual J, simple fixes like new windows in the problem bedrooms would not even come close to fixing the overall problem).

First - assuming I can live with treating the whole house as one zone (except for basement - separate mini splits down there), it appears the only real problem is the two far bedrooms.

1)Use space heaters in winter. Use portable AC in summer. (last winter, we used a 1kw space heater in my son's room and it was perfect - amazingly enough, this seems to calculate out to exactly the deficit we are experiencing on the colder days per the Manual J and actual CFM delivered calculations)

2)Mini-splits in two far bedrooms, treat whole house as one zone and live with it for a year. Might be all we ever need.

3)Mini-splits for all bedrooms. Definitely can zone used spaces, but perhaps not really needed.

4)Central system for all upstairs. Not sure this will really enable efficient zoning due to open floorplan - maybe overkill.

Assume from a cost perspective 4 mini-splits and central system are about the same cost. Assume 2 mini-splits (or a 2 zone) is actually less than half the cost (due to tax credit and energy company rebates) of 4 minis.

Spreadsheet attached.

Thoughts?

 

[coolmen]

Simply put, If you do not have the properly sized duct going to the second floor(supply/return) To deliver the required airflow then, You should think about adding system in attic. Its nice to see your on the right track with the measurements,manual J ect. doing static pressure test of duct work will tell you how bad the duct is.

 

[emarston]

I had the same kind of issue with my upstairs. We went to 2 systems (new one in the attic with all no ducting and returns in all rooms). Work was just finished yesterday, but every room in the house (including my finished basement) is awesome. Temperatures are very consistent throughout. It took a week for the attic system install then another week to seal off old ducting, add supply drops and install the 2nd system. I had the same thoughts initially as you but the wife and I decided to bite the bullet and just pay to make it right. Our contractor was excellent and worked with us every step of the way. It's expensive, but now the duct system is great and we can expect many years of comfort in our home.

 

[dash]

If there's a "space" where a duct can be run from the indoor unit,to the second floor attic,it can provide additional supply air ,or if original supply ducts are in the attic,it can be tied into those to get more supply air.

If there's no way for an additional duct,adding turning vanes and better fittings,to the ducts near the indoor unit,can help or solve the issue.Someone with the knowledge and experience,can tell if this would solve it.

Adding additional insulation to the attic and any kneewalls,will reduce the amount of air needed to cool the second floor.

 

[dbbarron]

Unfortunately, there is no contiguous attic in the upstairs, or even attic at all for some rooms, that is the real problem; no easy way to get a new trunk upstairs, no easy way to install a central system without boxing in some ductwork in several rooms, no easy way to add multiple returns (a central return at the second story high point is simple, but not optimal. Only thing that would be easy are mini splits and since I really only have two problem rooms, this seems like the right way to go.

 

[dash]

No way to do any of this?:

If there's no way for an additional duct,adding turning vanes and better fittings,to the ducts near the indoor unit,can help or solve the issue.Someone with the knowledge and experience,can tell if this would solve it.

 

[skippedover]

Sounds like things were screwed up from the start and unless you're willing to open the walls (gut the house) and do it right, absent an attic over the entire 2nd floor to be treated, I'd recommend the mini-splits as the ideal solution. Of course, if your existing system is properly sized including those rooms and it's just the distribution system that's the problem, then clearly your existing system will be oversized with the introduction of additional heating and cooling. But at least the bedrooms will be comfortable.

 

[dbbarron]

I'm convinced at this point that there is no viable retrofit option.

The question I was originally curious about is based on the attached (original post) spreadsheet if adding mini splits to the two far bedroom that have the greatest deficit of airflow should fix the issues assuming we run the entire house on a common setpoint (since all other rooms seem to have sufficient airflow to match the Manual J model). Or, am I missing something fundamental (I know hard to say without seeing the house) in my analytics?


I do have the separate upstairs system option (system in one of the attics, several duct runs breaking through walls and placed against walls where they will need to be boxed in. Basic framing, drywall, plaster and paint - no big deal and the aesthetic acceptable as we envision it.

Alternatively, four mini split indoor units.

db

 

[dash]

If the basement is unfinished you might be able to solve the issue with duct changes there.If unfinished pics of equipment and ducts would help determine.

Since you are convinced mini-split is the way to go,be aware that the forced air system will be calling for heat or cool,based on demand at the therostat location,and the minis there stat locaton. Minis could be staisfied and still get air from the forced air system.

 

[dbbarron]

Basement is finished. I've been told entry into the main trunk could be improved, but not to the extent needed to fix issues. Bottom line is there are too many ducts branching off an intermediate trunk in basement and not enough flow or static pressure to get enough air all the way upstairs (6" upstairs ducts branch off this basement intermediate trunk, run 10-20' in basement, turn upwards, go up to second floor and then exit)

If we installed mini-s or new central system upstairs, we would have to close the upstairs ducts from the old system to insure we don't overheat or cool when the downstairs system turns on if the upstairs is already satisfied. This would be the case with mini's or a conventional upstairs system. The minis would be a little less risky as they would operate room by room versus the setpoint.

 

[Shophound]

You might also consider multi-split, which are essentially mini-splits that are all tied to one outdoor unit. Saves a bit on electrical and lineset costs, and they can come as heat pumps so they'll make heat in winter. Some can even heat and cool simultaneously...not that you might need that feature.

 

[dbbarron]

I have considered both the multi-zone mini-splits (one outdoor unit) and the multi-splits (have not found a contractor that sells them yet). In either case, I am considering using a single wall control to drive the same setpoint to the multiple units in the various rooms according to a standard schedule so they all act as one upstairs "zone".

db

 

[udarrell]

Way To Go...

I have considered both the multi-zone mini-splits (one outdoor unit) and the multi-splits (have not found a contractor that sells them yet). In either case, I am considering using a single wall control to drive the same setpoint to the multiple units in the various rooms according to a standard schedule so they all act as one upstairs "zone". db

It is great to see home owners doing research & looking for options that will solve their H-VAC problems... op: - Darrell

 

[dbbarron]

I have had three contractors in the house and although the solutions they all suggest will all likely work, I have been disappointed in the lack of due diligence and science they apply to the task.

Of the three, only 1 even looked at the existing system and actually conducted a static pressure test and did a manual J calculation (although I don't think his results were correct - made wrong assumptions and a trainee operating the program).

The other two just went straight to suggesting an upstairs system of various size.

It would be nice if I could hire someone to do a proper evaluation on the whole house, blower door test, proper manual J, etc.; but how do you know who to trust???

db

 

[Shophound]

Of the three, only 1 even looked at the existing system and actually conducted a static pressure test and did a manual J calculation (although I don't think his results were correct - made wrong assumptions and a trainee operating the program).

What made you distrust his results - aside from the trainee? Was he present with the trainee?

 

[dbbarron]

He seemed unsure of the program - asked many questions of the more experienced person, but was mostly on his own with the PC. He showed me the floorplan he drew and it was not terribly accurate.

Moreover, Although my house is 1200Sqft upstairs and 2200 Sqft Downstairs, the upstairs rooms are almost all on exterior walls with high cathedral ceilings, skylights, more windows than downstairs, etc. Downstairs is more than half interior space with conditioned basement and second story above it.

Despite all this, he ended up with 6.1Tons of heat load total with 1.2 up and 4.8 down. When I model the house myself using HVAC calc, I get about an even split in up/down with 4.5T total and it makes sense when I examine the individual components in the manual J analysis - I just can't see how the contractor's totals add up.

db

 

[DavidNJ]

How did you measure the airflow?

 

[dbbarron]

How did you measure the airflow?

Measured each supply air velocity with handheld anemometer, summed total velocity and then took fractional component and multiplied by total system vfm to get cfm per supply.

This method has many assumptions, but likely gives a reasonably accurate result in my system.

 

[DavidNJ]

What was the velocity profile across the duct?

 

[dbbarron]

What was the velocity profile across the duct?

I disregarded velocity profile as all supplies were identical geometry and duct size and were all measured at the central velocity peak. Since I used a ratiometric calculation, velocity profile should not be relevant.