Duct insulation and electrical rates question?

[Larry2]

Some folks may be weary of my postings concerning my failing air conditioning/heat pump system. I think my immediate issue might be solved by replacing the fan motor cap with a correct value part.

I have even more design questions that are just beyond my reach as a EE in the radio frequency field. I installed quite a few systems in the late 60's, charged them and went on my way back then, but I didn't do any load calculations on those. Thas was done by others. I guess I am lazy and should research and learn this stuff myself since I am the one in need answers, but anyway, I am asking for more help on my questions. Those more experienced can help focus my attention on the calculations for planning future direction. Since the questions are somewhat different from my failing unit, maybe I can get new help in this area.

My upper floor system has never kept up with summer heat gain. In fact, in September while outdoor temperatures moderate but solar gain increases with a lower sun angle, this unit has always failed to keep up with it's peak load. My first floor has 150 or so square feet of low-E glass facing West. My second floor has slightly more Western facing glass. I had my original contractor exchange the original 3 ton system for a 3.5 ton system. Without my consent or following our contract, without notice etc., he downsized the second floor unit to 3 tons. I think 3.5 tons was his original "seat of the pants" quess. He never showed me any load calculations. I don't think he calculated anything beyond a walk through the house.

This increase to 3.5 tons provided moderate improvement. Three and one half tons cooling was contracted performance. I don't believe he ever did a proper manual-J heat gain calculation on my home, even though provided him with my architectural drawings. I don't think he considered first floor heat gain on the second floor . My home has loads of Western facing glass at both levels. The rest of major glass area faces South. Some of this was by intent since heating is our major season here. Mostly it was by esthetics and I was the architect to blame for excess glass. It is "low E" glass, but the glass vendor installed the lessor of the available efficency glasses available at that time.

The home is otherwise well insulated 6" frame and brick construction. The calculated R value of the composite framing and insulation is 19.8R. The Brick is currently 112F. The attic is currently 97F, I have no patience to measure it in the current heat, so could be higher in spots. At the thermostat it measures 74.5 and this is the coldest point on the second floor. The unit has run all day without cycling and these measurements are as the sun load is decreasing. The attic installed duct work appears to be inadequately insulated from the start, as I mentioned in a different thread. I was amazed to see 15-20% cooling loss through the longer duct.

From late morning, until dark is when the second level proves to be a problem. It always has. My first complaints about the sytems unable to "satisfy the thermostat" caused my contractor to alter the thermostat level to make the thermostat shut off earlier rather than to improving cooling. Later, when I discovered the sytem he installed was only 3 tons, not the 3.5 tons under contract, I had him swap out the system for the contracted equipment. At that time, the system still would not keep up with outdoor temps of 78 F or lower when the a lower angle sun caused high solar gain through the windows. So, such is the background on my questions, problems and concern.

The home has a two story open design with a foyer that is open two stories. Heat from the first floor quickly rises through the foyer and family rooms to the second level. All of the first floor is an open design, so heat gained there promptly rises to the second floor. I have a lot of Western facing glass at both levels. Most glass is Southern and Western facing and is of considerable area, maybe 10% of the building walls. Solar gain, even through low-E glass is considerable. For cooling, the second level system has always proven inadequate. The second level system is a Lennox HP22-461-3P condensor while the blower/coil is a CB18-511-2P which folks tell me can provude 4 tons cooling at 1600 + CFM air flow. I can't say I ever received 3.5 tons cooling from this system, the rated capacity. I've not meaured unti now. It has never caught up with afternoon loading during summer, even on early September days when sun load was the important issue, rather than heat gain through the 2X6 construction brick faced walls. This home has a great deal of West-facing, though "low-emissivity" glass. When the sun hits the glass, the unit mostly cuts on early morning until the sun goes down late afternoon.

My questions are about fitting the system to provide efficent and adequate cooling. It seems like the outside system is well worn and will need replacement. (I await comments on that question) I need to know if the second floor system ever provided the spec cooling and I need a larger system, or if I only need only correct worn out gear.


Windows are mostly 32" wide and 72" tall.

Sat dishes are how I reach my friends here on the fridge net. :D

Thanks, all!

[frank]

Hi Larry

Nice House :)

Do your units operate on fresh air input or do they just recycle the air through the house?

[Larry2]

I have three heat pump systems. There is a 3 ton system for downstairs, a 3-1/2 ton system upstairs and a 2 ton system for the bedroom that is over the garage. All the systems recirculate indoor air. Fresh air is only available through air leak through the windows. There is adequate fresh air leaking in.

The center 1/3 of the house is open two stories so the second floor unit is heavily loaded on Summer days with the solar gain and heat rising from the first floor.

If I were designing the house today, I would add passive solar features, such as roof overhangs and a front portico to shade the South and West facing windows in summer.

[Andy]

I have three heat pump systems. There is a 3 ton system for downstairs, a 3-1/2 ton system upstairs and a 2 ton system for the bedroom that is over the garage. All the systems recirculate indoor air. Fresh air is only available through air leak through the windows. There is adequate fresh air leaking in.

The center 1/3 of the house is open two stories so the second floor unit is heavily loaded on Summer days with the solar gain and heat rising from the first floor.

If I were designing the house today, I would add passive solar features, such as roof overhangs and a front portico to shade the South and West facing windows in summer.

Larry your A/C is undersized;) in the UK there is requirements from building control that fresh air be brought into the building, varies from area to area, probably about 10% of flow rate:), if you were to consider any great amount of fresh air it would double the size of the system.

Proper calcs would be a good start, but better would be a look at a similar house in your area, that would give you a more accurate reading than any calculations.:)

Kind Regards Andy:)

[Andy]

I have three heat pump systems. There is a 3 ton system for downstairs, a 3-1/2 ton system upstairs and a 2 ton system for the bedroom that is over the garage. All the systems recirculate indoor air. Fresh air is only available through air leak through the windows. There is adequate fresh air leaking in.

The center 1/3 of the house is open two stories so the second floor unit is heavily loaded on Summer days with the solar gain and heat rising from the first floor.

If I were designing the house today, I would add passive solar features, such as roof overhangs and a front portico to shade the South and West facing windows in summer.

What size is your house in sq feet:) I will give you an estimated load, ball park for your cooling duty.:)

Kind Regards Andy:)

[Larry2]

Andy,

I've searched online for an excel template for this estimate. I'm sure someone has one free excel sheet worked out that will do the manual J. I could follow a procedure from a textbook and get a number, but I don't think it will account for the heat that rises from the first floor up to the second floor late afternoon. The center of the house is open both stories. The connecting rooms don't have any doors. Heat collected on the South and West sides of the house rises to the second floor level promptly. I am guessing that a 4 ton unit is marginal or shy for conditioning a twenty degree drop. I'll be interested to hear your estimate.

if you look on tables, Baltimore MD or Washington DC would be nearby for load figures.

I have 1950 square feet on each floor.

The basement is mostly below grade and there is no insulation between levels. It has no ducts and is unconditioned space.

The walls are constructed with 2"x6" wood platform construction framing, wrapped in 3/4" styrofoam board and wrapped with tyvek. I think as an assembly it calculated to perform as R20 as built. The wooden windows are glass double pane, low emissive glass. The attic is insulated with R30 fiberglass.

The first floor figures out as

North 422 sq feet wall, 21 sq feet glass
East 600 sq feet wall, 150 sq feet glass
South 484 sq feet wall, 86 sq feet glass
West 600 sq feet wall, 116 sq feet glass

The second floor figures out as

North 352 feet walls, 21 sq feet glass
East 448 sq feet walls, 115 sq feet glass
South 352 sq feet walls, 63 sq feet glass
West 448 sq feet walls, 160 sq feet glass

The North walls are shared with unconditioned garage space on level one.

[Andy]

Andy,

I've searched online for an excel template for this estimate. I'm sure someone has one free excel sheet worked out that will do the manual J. I could follow a procedure from a textbook and get a number, but I don't think it will account for the heat that rises from the first floor up to the second floor late afternoon. The center of the house is open both stories. The connecting rooms don't have any doors. Heat collected on the South and West sides of the house rises to the second floor level promptly. I am guessing that a 4 ton unit is marginal or shy for conditioning a twenty degree drop. I'll be interested to hear your estimate.

if you look on tables, Baltimore MD or Washington DC would be nearby for load figures.

I have 1950 square feet on each floor.

The basement is mostly below grade and there is no insulation between levels. It has no ducts and is unconditioned space.

The walls are constructed with 2"x6" wood platform construction framing, wrapped in 3/4" styrofoam board and wrapped with tyvek. I think as an assembly it calculated to perform as R20 as built. The wooden windows are glass double pane, low emissive glass. The attic is insulated with R30 fiberglass.

The first floor figures out as

North 422 sq feet wall, 21 sq feet glass
East 600 sq feet wall, 150 sq feet glass
South 484 sq feet wall, 86 sq feet glass
West 600 sq feet wall, 116 sq feet glass

The second floor figures out as

North 352 feet walls, 21 sq feet glass
East 448 sq feet walls, 115 sq feet glass
South 352 sq feet walls, 63 sq feet glass
West 448 sq feet walls, 160 sq feet glass

The North walls are shared with unconditioned garage space on level one.

Larry:)

I have a spreadsheet from the IOR in England, I will check it and see is it what you need first.

My back ground is industrial refrigeration, but recently I have been asked to design large water based AC systems, so all my systems are kind of industrial:D

I make you load check figure to be 11.4 tons, I would expect your load to come out near this:)

To calculate this I used the figure of 100watts/m sq

Kind Regards Andy:)

[Brian_UK]

Larry, accepting that you have a 'heat chimney' effect inside your house would it be possible/feasible to provide some form of air outlet at roof level.

This would allow the 'hot' air to rise through the structure creating an updraft and prevent hot static air forming. This is/was used in the middle east/far east as a means of keeping building cool by natural means.

I appreciate that it would fight your a/c systems but it alleviate some requirement for cooling if the high temperature can escape from the building.

Feel free to shoot me down over this ;)

[Larry2]

Brian, I can certainly try that by opening a few upper windows a bit and see what happens. Maybe some sort of fan and duct to return warm air back to the lower level would help too, so the lower system can do more of the work.

Andy, that seems to be a lot of cooling for a home. Maybe I am shy 4-5 tons because of the glass. Is your estimate that high because commercial applications demand more performance? Shops here in the US seem to cool down to 70F when its 100F outside. For homes we're content to with a bit less cooling on the worst days.

[Andy]

Brian, I can certainly try that by opening a few upper windows a bit and see what happens. Maybe some sort of fan and duct to return warm air back to the lower level would help too, so the lower system can do more of the work.

Andy, that seems to be a lot of cooling for a home. Maybe I am shy 4-5 tons because of the glass. Is your estimate that high because commercial applications demand more performance? Shops here in the US seem to cool down to 70F when its 100F outside. For homes we're content to with a bit less cooling on the worst days.

No you are 4 ton shy because that is what it would take to cool the rooms to a point where they will of cycle on the warmest of days.

To me the units you have are not the required capacity.

What about spot cooling the glass area with a split A/C to make up the differance:)

Kind Regards Andy:)

[Larry2]

Andy, that would explain why my system's can't keep up on hot days. As far as I know, the largest residential unit available for the single phase power available is 5 tons each.

I've always had trouble with summer comfort on the second floor, even when the unit was new. The contractor installed 36,500 btu/hr for floor one and 28,000 btu/hr for the second floor. He installed the wrong gear on the second level compared with the contracted equipment. His contract was for 36,500 but/hr on both floors. The load calculations were his own. I think he walked through and did it by seat of the pants. Maybe a wet finger in the air to guess wind? It was "sized" for cooling load and the heat made up with resistance. January and February my electrical demand is astonishing! I consume around 38 KWH per year with this equipment including all electrical use.

When I complained the second level unit could not keep up with demand, even on autumn days when outdoor temperatures moderated, he tinkered the thermostat needle to tell me I was comfortable and he altered the thermostat level so it could shut off sooner. When I discovered the second level equipment was smaller than the contracted size, I persuaded him (my lawyer-brother was present for that meeting) to install one ton larger upstairs. The new gear has still proven inadequate through the years. Now as it ages, it is worse. Your calculations seem to show why.

I'm reading the duct insulation wrap label as 1-1/2" fiberglass with foil wrap. The duct is considerable in length and surface area. It is in unconditioned space. I measured last week around two degrees F heat gain across the longer run, for about 20% cooling lost. If I properly insulate the attic ducting and install 5 tons of cooling upstairs do you think I would be close enough to my requirements that the next home owner won't be disappointed?

This is enough equipment change that perhaps I need to look into a geothermal source for my heat pump. I am convinced we will see a doubling of electrical rates here in the next five years. On this side of the pond, we have told utility companies, through regulations, how much they can charge. That is all changing and soon we shall pay what the market will bear.

[Andy]

Andy, that would explain why my system's can't keep up on hot days. As far as I know, the largest residential unit available for the single phase power available is 5 tons each.

I've always had trouble with summer comfort on the second floor, even when the unit was new. The contractor installed 36,500 btu/hr for floor one and 28,000 btu/hr for the second floor. He installed the wrong gear on the second level compared with the contracted equipment. His contract was for 36,500 but/hr on both floors. The load calculations were his own. I think he walked through and did it by seat of the pants. Maybe a wet finger in the air to guess wind? It was "sized" for cooling load and the heat made up with resistance. January and February my electrical demand is astonishing! I consume around 38 KWH per year with this equipment including all electrical use.

When I complained the second level unit could not keep up with demand, even on autumn days when outdoor temperatures moderated, he tinkered the thermostat needle to tell me I was comfortable and he altered the thermostat level so it could shut off sooner. When I discovered the second level equipment was smaller than the contracted size, I persuaded him (my lawyer-brother was present for that meeting) to install one ton larger upstairs. The new gear has still proven inadequate through the years. Now as it ages, it is worse. Your calculations seem to show why.

I'm reading the duct insulation wrap label as 1-1/2" fiberglass with foil wrap. The duct is considerable in length and surface area. It is in unconditioned space. I measured last week around two degrees F heat gain across the longer run, for about 20% cooling lost. If I properly insulate the attic ducting and install 5 tons of cooling upstairs do you think I would be close enough to my requirements that the next home owner won't be disappointed?

This is enough equipment change that perhaps I need to look into a geothermal source for my heat pump. I am convinced we will see a doubling of electrical rates here in the next five years. On this side of the pond, we have told utility companies, through regulations, how much they can charge. That is all changing and soon we shall pay what the market will bear.


Hi Larry:)

Single phase would be a problem. Three phase compressors are more effecient:) and you wll get a greater choice of units.

Modern inverter technology would be a bonus for you, single phase input and no stsrting current (which currently limits your unit size)

Inverters also allow load matching, in the warm summer the unit runs flat out and in the Autumn the unit backs off running at it's most effecient:)

Geothermal has a very high installation cost, but will give you very economic cooling, wasting the heat to ground at 12 deg c not air at 30 deg c.

Larry what way do you heat your house, gas would be the cheapest, geothermal only works out if you are buying the electric off peak at an agreed cheap price

Kind Regards Andy:)

[Abby Normal]

Larry your A/C is undersized;) in the UK there is requirements from building control that fresh air be brought into the building, varies from area to area, probably about 10% of flow rate:), if you were to consider any great amount of fresh air it would double the size of the system.

Proper calcs would be a good start, but better would be a look at a similar house in your area, that would give you a more accurate reading than any calculations.:)

Kind Regards Andy:)

Proper calcs by a pro is a great idea Mr. Andy.

Sometimes mechanical ventilation takes natural infiltration out of the equation and will not really increase the load that much.

Example a system that merely draws in outside air when it operates into the return air system would tend to pressurize the home with conditioned air. Air would tend to want to escape the home.

A ventilation system that works with point exhaust would make the home negative with respect to ambient pressure and the infiltration would actually be make up air. It could typically exceed and over power natural infiltration.

Ending a long winded post, sometimes you only have to allow for mechaincal ventilation and can neglect infiltration.

Balanced ventilation schemes like an HRV would still be prone to infiltration.

[Abby Normal]

Andy, that would explain why my system's can't keep up on hot days. As far as I know, the largest residential unit available for the single phase power available is 5 tons each.



Multiple units get you past 5 tons

[Andy]

Proper calcs by a pro is a great idea Mr. Andy.

Sometimes mechanical ventilation takes natural infiltration out of the equation and will not really increase the load that much.

Example a system that merely draws in outside air when it operates into the return air system would tend to pressurize the home with conditioned air. Air would tend to want to escape the home.

A ventilation system that works with point exhaust would make the home negative with respect to ambient pressure and the infiltration would actually be make up air. It could typically exceed and over power natural infiltration.

Ending a long winded post, sometimes you only have to allow for mechaincal ventilation and can neglect infiltration.

Balanced ventilation schemes like an HRV would still be prone to infiltration.

Hi Abby:)

not that often I'm called Mr anything. Andy will be fine:D

Larrys house is brick, as such infiltration should be less than pure stick built with timber cladding:)

In the Uk building control have the homes at a point where the infiltration is very low, too low if you ask me;)

Infiltration calculations would be very hard to carryout, with an allowance in the calculations made based on judgement. This is a major load, but one which has to be estimated, there is no duct to bring the air in that we can say this is our fresh air.

If we had freah air intake, which was greater than our extract we would have a pressurised house, and as such our infiltration would be comming through a duct and could be calculated. The only thing we wouldn't know was the loses of conditioned air:confused:

Who said it was easy:D

Kind Regards Andy:)

[Abby Normal]

Could be face brick, meaning there is still a framed wall behind. An expensive siding in reality.

Some extremely tight homes in Canada, tight to the point where you need air exchange or your winter time RH gets too high and window condensation is problematic. Humidifiers are becoming dinosaurs.

Typically in Canada, infiltration is treated as so many air changes per hour, a bit of a classification system that factors in construction quality, building height (stack effect), and building exposure (wind effect). As an example, a home on the coast with open water would be a little more prone to wind driven infiltration than a sheltered rural home surrounded by forest.

The rates are based on a lot of blower door testing.

Stack effect is driven by temperature difference and height, recessed ceiling light fixtures are quite the natural chimney vents.

So you can have a balanced ventilation system such as an HRV with an exhaust fan and a fresh air intake fan, balanced with heat recovery and ventilation makes the home more or less neutral with respect to ambient air pressure so there is still infiltration.

Or you could have a pressurization/depressurization scheme which would 'overpower' natural infiltration, therefore heat gains/losses for ventilation would take natural infiltration out of the equation.

Down in the Caribbean, it is customary to call men Mr. "Firstnane" and the woman Miss "Firstname". Like Miss Ellie from Dallas

[frank]

Down in the Caribbean, it is customary to call men Mr. "Firstnane" and the woman Miss "Firstname". Like Miss Ellie from Dallas

Aahhhh - manners - I remember them. :)

Such a pity the younger generation are not taught them. :(

[Larry2]

Abby is correct. The house contruction is "veneer brick". It is an expensive siding, but in the overall cost of things, it added $30K or 10% to the structure cost at that time. That is calculated by subtracting the cost of an alternative siding material. It will never need maintenance in my lifetime and it adds many time's its cost to the value.

The walls are 2x6 framed construction. Over that is 3/8" plywood for "shear" strength and 3/4" styrofoam board to block heat conduction through the frame connection between interior walls and exterior surface. The entire house was then wrapped in tyvek fabric which blocks air drafts while allowing moisture to escape. The brick is in front with a 1" air space and tied to the frame through steel brick ties. It's a popular way to build well insulated walls. I would say infiltration through the walls in negligable.

Infiltration is going to be through the windows and the attic located mechanicals. These are double glass wooden assemblies that press tightly into a plastic sash and are properly, more or less, gasketed to block air exchange between inside and outside. Even with these measures, the house is not as tight as I expected it to be. I think a blower door test would give the best answer.

A major blunder on my part was not planning for locating the mechanicals within the conditioned space. I was the architect, contractor, electrician and finish carpenter for the house. So final blame for the blunder is mine. I allowed the HVAC contractor to locate the air handler and all second floor ducting in the unconditioned attic. He assured me this would be insulated and efficient. I was quite tied up with contracting details and building the home and took this detail for granted. I should have revised the plans, sacrificed some interior space and put the second floor mechanicals in the basement.

In winter, the attic is at outdoor temperatures because it is ventilated. The ducts cold-soak until they are filled with exterior ambient temperature air. Then the system switches on for heat demand and empties the entire duct volume of 0*C or worse air from the ceilings, right over my bed. This continues until the ambient air contained is emptied from the ducts and the heat pump coils finally warm.

Last month, I measured almost 20% heat gain across the longest duct path during summer cooling. That was quite unexpected by me. Temperature differences inside to outside are even greater in winter. My very next step will be to better enclose these ducts with insulation. I'll will also search for infiltration spots. There has to be spots that need to be taped up. On the warmest days, one can smell the odor of the hot attic wood, so I know attic air is entering the system somewhere into the ducts.

[Abby Normal]

Cold soak sounds like when the system is off the air inside the ducts cools off. Then when the fan starts on an initial heat call the cold air gets purged into the conditioned space.

Constant fan in winter would eliminate this shot of significantly cooled air. With the fan off, probably some natural convection as well, a constant slow cool draft falling from supply and return grilles.

Duct sealing is as important as proper insulation in unconditioned space. A leaky supply duct is a double edged sword. It functions as a heated or cooled exhaust, it waste the energy you just spent conditioning the air PLUS, it depressurizes the home and causes increased infiltration of outdoor air into the space.