Using Manual J+Actual airflow measurements to fix house

dbbarron · May 27, 2010

shophound said:
I would put the volume of your house close to 35,000 cubic feet, assuming ten foot ceiling heights per floor. Could be less. Regardless, a considerable amount of air to turn over.

Reviewing your data superficially it appears you're starved for return air.

Not only is the return path undersized, but most of the return air is from near floor level on the first floor. I think my system just keeps recooling the same cool air, although it heats well! This is why I'm thinking running the ceiling fans will help. See pic of the return air ducting a few posts below.

dbbarron · May 28, 2010

shophound said:
I would put the volume of your house close to 35,000 cubic feet, assuming ten foot ceiling heights per floor. Could be less. Regardless, a considerable amount of air to turn over.

Reviewing your data superficially it appears you're starved for return air.

First experiment tonight that produced some promise!

Based on shophounds comments and my gut feeling that I was just recooling the same cool air over and over (Tstat would reduce 1F per hour, no more), I circulated air in my downstairs level by turning on two ceiling fans (sort of broken that's why not used before) to move the hot air layer down to be returned and cooled and also aimed a box fan upwards in my entry to move cool air up to the second level catwalk area (a very hot area) - I could make this air movement permanent by installing a ceiling fan instead of entry light fixture (need to find a nice one as it is our entry).

This now circulates the air vertically in about 3/4 of the open plan high ceiling first level.

All of a sudden, the Tstat started climbing and the system turned on - and is cycling at 10 minute intervals, cooling and heating back up at about 0.1F per minute each way. I actually get the feeling the system is cooling warm air!

Interestingly enough, the upstairs bedrooms are also dropping in temperature (they extend away from the first floor area in a perpendicular direction). I will note the progress as the evening moves on and where everything equalizes. Possibly the large hot stagnant air mass at the top of the foyer and entry to second floor BR hallway was forming some form of barrier to getting the heat out of the upstairs.....

And is it possible the temperature drop between return and supply aid is now greater? (about 25F, used to be 20F - not sure on this one)

HOWEVER, today was cool - need to observe in this configuration for a few hot days...

db

Shophound · May 28, 2010

Cooling two levels with just one system is difficult on the best of days. Where I work we have a high ceiling art gallery with a mezzanine level. The only way both the bottom floor and the mezzanine level stay fairly even in temperature is for the blower to run constantly and the availability to cool is always present and variable according to demand. But it was designed that way, whereas most residential systems are not...they are either on or off, and for humid climates, running the blower between cooling cycles can raise indoor humidity levels.

So what you've done is create air movement independent of your cooling system, and that is why you're seeing improvement in performance and comfort. The ceiling fans on the bottom floor will stir cool air up off the floor, which has to be replaced by warmer air that is higher up. Since most thermostats are about five feet off the floor, stirring up the air around it will change what it senses.

If you have ceiling fans upstairs I would try running those as well, since you have supply air that enters each room at baseboard level. You could try running those in reverse so they push air against the ceiling, forcing the cooler air near the floor to rise up and replace it.

dbbarron · May 28, 2010

The increased cycling continued last night, but as the Tstat area temperature approached the 70deg set point, the rate of cooling slowed from 6deg/hr to 4deg per hour - about 4, 1 degree cycles per hour. Nonetheless, the system continued cycling and cooling quickly, unlike the long, almost ineffective (1F/Hr) runs I had before.

Despite all this, at the end of the evening and this morning (after the system had been off for a few hours - but circ fan on low), I achieved about a 2-3 degree gradient between downstairs central area and upstairs. This is better than before, but still a problem for me and this was a quite cool day - this weekend will bring better weather to experiment and I will leave the configuration.

One more thought - one thing I've noticed is that the later afternoon heat load on the MBR and other upstairs areas is far far greater than downstairs; thus, when the setpoint lowers in the late afternoon, the system can often not 'catch up' upstairs, we start with a +10 degree or more differential to downstairs.

To remedy this large afternoon gradient, my tstat can use a remote temperature sensor and will operate in a mode where it will average the local and remote reading. If I place the remote sensor in the MBR upstairs and averaged the reading with the downstairs Tstat sensor, this should 'catch' the afternoon runaway and essentially keep the up and downstairs on either end of the 2-4 degree differential. Any thoughts?

Lastly, as the house appears to naturally statify - if I installed a second system just upstairs, would the downstairs naturally end up 2-4 degrees cooler, effectively overcooling the downstairs and ending up with the same result as I have now, just with additional cooling capacity? Would reducing upstairs circulation and using mini-splits in the BRs better isolate the zones and remedy the aforementioned natural stratification problem?

db

dbbarron · May 29, 2010

More testing last night; Although system definintely cycles more normally and clearly cools first floor better and catwalk area is cool, MBR adjacent to catwalk remains warm and does not cool down at the same rate as downstairs, so it remains hot. One can walk from the cool catwalk area into the MBR and feel the heat immediately. There is decent supply into that room, but very little return. There is a 6" return high in the room that will hold tissue paper against it, but does not even read on my air velocity meter. Thinking there is a return issue.

I was going to try a booster fan in the return duct. Thoughts on whether this might be effective? Any thoughts on whether (as a test as they are locally available) the axial inexpensive 'inductor' brand fans will do any good or will I need more static as with a centrifugal fan (Fantech FR-150)?

db

dan sw fl · May 29, 2010

The real performance is ... ?

dbbarron said:
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.).

. Mini-splits could easily zone bedrooms with doors closed.

the only meaningful problem remains my son and daughters room with only 1/3 the needed CFM being actually delivered.

Spreadsheet attached.

Thoughts?

The spreadsheet states that you measured the Velocity, not the airflow.

How do you know the total air flow from the air handler?
Have you measured the static pressures and reviewed the blower performance curve?

dan sw fl · May 29, 2010

What are sizes and locations of skylights?

dbbarron said:
Second story gets late day sun load due to western ext wall, west tilt cathedral ceilings with skylights.

6am, 62, 72.7,72.9, 73.0,73.0

Cathedral celings are a killer.

Skylights ARE A KILLER ! !!
:ban:

dash · May 29, 2010

"feel the heat immediately. There is decent supply into that room, but very little return. There is a 6" return high in the room that will hold tissue paper against it, but does not even read on my air velocity meter. Thinking there is a return issue."

If it doesn't cool with all the doors open,it's lack of supply,though you could still need more return.

dbbarron · May 29, 2010

Dash: "If it doesn't cool with all the doors open,it's lack of supply,though you could still need more return."

It cools, but slowly with the door open. Also, there is a ceiling fan running, so the high and low air mixes and any air returning out the door is not a cold floor layer, but mixed. Thus, you are likely correct, additional return will not help much. A box fan exhausting into the cool hallway from the MBR does cause the room to cool much faster.

Dan: "Cathedral celings are a killer....Skylights ARE A KILLER ! !!"

The MBR and MBath have both high ceilings and three large skylights! According to the Manual J, the heat gain is about 13kBTU for this space. One duct with boosted airflow that is good, others are anemic.

(and CFM measurements based on ratio of total velocities as all registers from same duct size and have same profile - I had a pro measure system CFM and it jives with fan curves).

hvacrmedic · May 29, 2010

dbbarron said:
Dash: "If it doesn't cool with all the doors open,it's lack of supply,though you could still need more return."

It cools, but slowly with the door open. Also, there is a ceiling fan running, so the high and low air mixes and any air returning out the door is not a cold floor layer, but mixed. Thus, you are likely correct, additional return will not help much. A box fan exhausting into the cool hallway from the MBR does cause the room to cool much faster.

Dan: "Cathedral celings are a killer....Skylights ARE A KILLER ! !!"

The MBR and MBath have both high ceilings and three large skylights! According to the Manual J, the heat gain is about 13kBTU for this space. One duct with boosted airflow that is good, others are anemic.

(and CFM measurements based on ratio of total velocities as all registers from same duct size and have same profile - I had a pro measure system CFM and it jives with fan curves).

You're spinning your wheels. A multistory dwelling needs zoning. Either separate systems for top and bottom or a "good" zone system(s). The stratification that you're experiencing isn't due to hot air rising as assumed, or even to heat rising; it's due to the greater heat infiltration through higher portions of the structure. In your case the general direction of heat flow is down, not up. The sun shines down on the house, heating the roof. That heat is transmitted downward through the ceilings. The lower floor is conversely cooled by the ground. There is a large external temperature gradient across the structure from top to bottom. The air next to the ceiling isn't warmer than the air next to the floor due to warm air rising, rather it's due to the fact that the air next to the ceiling is in thermal contact with the ceiling, which is warmer than the floor. Heat flows from warmer to cooler, so heat is migrating downward through the house, following the temperature gradient. In addition to heat flow by convection, there is also heat flow by conduction and by radiation, lest we forget that fact.

Because of solar gain and/or relative constancy of ground temperature, the top floor is in general going to have a higher heat gain in the summer per square foot of floor space than the bottom floor. This does not reverse exactly during the heating season because the ground under the bottom floor is not going to be heated by the sun. In fact, the ground is going to stay at about the same temperature year round and the sun is still going to be providing heat to the roof. The direction of the gradient may reverse on very cold days, but on milder days the solar gain can prevent a reversal of the temperature gradient. As a result, when you finally get the system balanced and providing even cooling, then come winter it will not be balanced for even heating. The upstairs will run too warm. OTOH, if you have even heating, then come summer the upstairs will (again) be too warm, which is exactly what you've described. IOW, with your existing system the upstairs will need more air in the summer and less air in the winter. Without some sort of air volume control, or separate systems, you're out of luck. No amount of envelope or duct improvement or is going to resolve this issue completely.

There is also heat infilration through walls, windows, etc. but I didn't want to overcomplicate the issue. I just wanted to illustrate that your existing system is incompatable with it's application.
_______

Given the situation that you've described with your house design and duct system, minisplits would be a very good option. You could also go with PTHP units (through the wall heat pumps). Cap off ducts to the upper floor and resize the downstairs unit accordingly.

BTW, I was impressed with your delivered air calculation method. However, you didn't take into account duct heat loss/gain. Calculations are a good thing, but when huge factors are omitted then the calculations can become invalid. For long runs you can easily loose a significant percentage of the capacity delivered to the duct. I've seen duct heat losses/gains over 100%, meaning that they were heating the room rather than cooling it, and cooling it rather than heating it. Forget calculations of duct heat loss/gain on existing systems using duct wrap R-values. The R value won’t necessarily be as great as you think it is, and you'll have to guesstimate too many of the parameters. It's much simpler, and immensely more accurate, to actually measure the duct heat loss/gain.

Measure the air temp exiting the supply vent and compare that to the air temp leaving the evaporator coil. As a rule of thumb, for every 1° degree difference (in cooling mode), you have lost 5% of the sensible capacity that was delivered to the supply duct. Example: A 5° rise from evap outlet to supply vent would correspond to a loss in sensible cooling capacity of approximately 25%. For losses/gains at design conditions you'd need to measure them under design conditions.

dbbarron · May 29, 2010

HVACMEDIC:

Thank you for the thoughtful and well formulated response. This does appear to explain my situation and I am sold on some form of additional system upstairs. It agrees with the recommendation from the contractor I trusted most (seemed most scientific/analytical in approach) who recommended a downsizing of the downstairs compressor to 3 ton from 5 (the air handler and coil are compatible) and the addition of a new central system upstairs (Rheem 3 ton 2 stage). Due to lack of contiguous attics and lack of places to run ducts, will need to box in some ducts to get from here to there, but its only drywall and plaster!

I also considered mini splits, but the overall cost is significantly higher than a conventional system (perhaps 40%), aesthetically difficult, and will not directly condition some high heat gain areas (catwalk, long hallway, and BR bath (4k BTU load on its own)). The only risk I see with a central system is that the heat load is so different in different upstairs rooms that it will itself be out of balance, but I'm speculating that will not be the case.

PTHPs or even thru wall air conditioners/heat pumps are clearly another option, but same issues as mini splits. Even portable AC/Heat pump, but central systems are just cleaner from an end result standpoint.

Would appreciate any thoughts on the new central system vs. other options..Am I missing any risks/advantages?

db

DavidNJ · May 29, 2010

Which brand/model anemometer did you use?

dbbarron · May 30, 2010

DavidNJ said:
Which brand/model anemometer did you use?

Lacrosse ea-3010.

DavidNJ · May 30, 2010

Thanks.

dbbarron · Jun 3, 2010

After much horsing around, I am now fairly confident I will go with a separate upstairs conventional system and will have to box in some duct runs due to the lack of available and contiguous attic space. For reference, I attach a few pics. One of the catwalk area and two floorplans. The downstairs plan has the existing HVAC system in blue (basement air handler and main trunks shown - all other ducts runs are 6" runs then oval duct up interior and ext walls, even all the way to upstairs BRs which explains airflow problems. Upstairs plan shows proposed system in RED in attic with trunks. There is a limited attic space over the catwalk where the system will be located. All the BRs have high ceilings and no attic above.

All comments welcome.

DavidNJ · Jun 3, 2010

dbbarron said:
HVACMEDIC:

Thank you for the thoughtful and well formulated response. This does appear to explain my situation and I am sold on some form of additional system upstairs. It agrees with the recommendation from the contractor I trusted most (seemed most scientific/analytical in approach) who recommended a downsizing of the downstairs compressor to 3 ton from 5 (the air handler and coil are compatible) and the addition of a new central system upstairs (Rheem 3 ton 2 stage). Due to lack of contiguous attics and lack of places to run ducts, will need to box in some ducts to get from here to there, but its only drywall and plaster!

I also considered mini splits, but the overall cost is significantly higher than a conventional system (perhaps 40%), aesthetically difficult, and will not directly condition some high heat gain areas (catwalk, long hallway, and BR bath (4k BTU load on its own)). The only risk I see with a central system is that the heat load is so different in different upstairs rooms that it will itself be out of balance, but I'm speculating that will not be the case.

PTHPs or even thru wall air conditioners/heat pumps are clearly another option, but same issues as mini splits. Even portable AC/Heat pump, but central systems are just cleaner from an end result standpoint.

Would appreciate any thoughts on the new central system vs. other options..Am I missing any risks/advantages?

db

Are you adding a furnace upstairs and downsizing the downstairs one. Your layout and area are not that different from ours.

dbbarron · Jun 3, 2010

DavidNJ said:
Are you adding a furnace upstairs and downsizing the downstairs one. Your layout and area are not that different from ours.

Upstairs will be a 2 stage 3 ton heat pump. Ideally downstairs should be reduce from 5 ton AC to 3 ton, 2 stage. Gas furnace downstairs. Upstairs heat viewed as supplemental.

dbbarron · Jul 11, 2010

Upstairs separate system and ducts installed. 3 ton, 2 stage heat pump, rheem, sticker says 16.75 seer. Entire upstairs comfortable and even temp, not even pressure tested or balanced yet. Good decision. Will be interesting to see interaction with downstairs zone.

genduct · Jul 11, 2010

Any real improvement is going to be challenging. How do you think you're going to get those pipes to the minisplits? Next thing is someone is going to suggest a high velocity system and the truth is you are going to need to do some GC work to fix the problem. That means a scaled floor plan that will take advantages of room corners and closets to route pipes or ducts or whatever you come up with. Don't know what you current system looks like or how it might be integrated into the "Final Solution" Best advice at this point is there is no silver bullet. With the 3 large and diverse spaces, I'm surprised that you didn't think zoning like you did with the minis
PS the spread sheet suggests the children's rooms are the ones that suffer the most. What's the problem? As parents isn't it our job to torture the children?

dbbarron · Jul 11, 2010

genduct said:
Any real improvement is going to be challenging. How do you think you're going to get those pipes to the minisplits? Next thing is someone is going to suggest a high velocity system and the truth is you are going to need to do some GC work to fix the problem. That means a scaled floor plan that will take advantages of room corners and closets to route pipes or ducts or whatever you come up with. Don't know what you current system looks like or how it might be integrated into the "Final Solution" Best advice at this point is there is no silver bullet. With the 3 large and diverse spaces, I'm surprised that you didn't think zoning like you did with the minis
PS the spread sheet suggests the children's rooms are the ones that suffer the most. What's the problem? As parents isn't it our job to torture the children?

Not sure I was clear in my prior post. New system for second floor is installed, went with conventional system, and yes, there will be some boxing in needed. Everthimg working great.

Using Manual J+Actual airflow measurements to fix house

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