A heat load of 50,500 BTU/hr for a 2900' house is still pretty high. That's over 17 BTU/hr per square foot of conditioned space. There are sub-code 2x4 houses of comparable size (like mine) with lower heat loads than that. A well designed higher-R house should come in under 10 BTU/ft^2.

A 6.5" EPS SIP would run about R24 for the SIP itself, maybe R25-26-ish whole-wall (maybe less depending on the amount of spline, plate, and window & door framing there is) which is a U-factor of about U-0.040. That's half the U-factor of a pretty good 2x4 wall.

If it's a polyurethane SIP it'll have an even lower U-factor (about U0.027), but high-R closed cell polyurethane delivers a heavy environmental hit from both the high polymer content, and the HFC245fa blowing agent, with a 100 year global warming potential of nearly 1000x CO2, compared to 7x for the pentane used for blowing EPS, most of which is recovered at the manufacturing plant and burned for process heat. Polyurethane SIPs are the opposite of "green building", despite their high performance.

The shape/footprint of the house can affect things quite a bit, since more complexity means more exterior surface area per square foot of conditioned space, as well as higher thermal bridging at corners from the higher corner count.

Air leakage of 3ACH/50 is currently IRC code max. A SIP house that's assembled wither air-tightness in mind should be able to come in under 2ACH/50. If the tool default is 7ACH/50, that needs to corrected, since that's not even a code-legal leakage rate. SIPs are comparatively easy to air seal next to conventional framed houses, but that's not to say the builders always take steps necessary to get there.

Fred,

I ran BEopt with a more accurate case with weather data for Detroit. I ran your SIP walls, and everything else at code minimum. I compared low solar gain windows (U=0.26, SHGC=0.31) versus high solar gain windows (U=0.29, SHGC=0.56). The results showed reasonable trends.

Low solar gain windows:
Heat energy 477.6 therm/yr
Cool energy 674 kWh/yr
Heating capacity 34.6 kBtu/hr
Cooling capacity 14.5 kBru/hr

High solar gain windows:
Heat energy 436.1 therm/yr
Cool energy 888 kWh/yr
Heating capacity 35.3 kBtu/hr
Cooling capacity 19.6 kBru/hr

These trends look reasonable to me. The absolute values appear to be in the ballpark for a relatively large house, but that will take more consideration. The high solar gain windows all around the house decrease the yearly heating requirements, but increase the cooling requirements. The peak heating load is actually higher with the high solar gain windows, since the peak load occurs before sunrise, and the U-factor is higher for the high solar gain windows. But who really cares about a 2% increase in the size of the heating plant? The increase in cooling capacity required is more significant.

From a financial standpoint, it is about a wash between these two cases. Putting high solar gain on just the south side windows, assumed to be 40% of the total window area for these two cases, and low solar gain elsewhere would likely be optimum.

BEopt should be a good tool to optimize windows as well as everything else. An exception is the insulation in the basement. Since it is a walk-out basement, you might do better to treat the basement as an above-ground first floor. Heat losses and gains will be much higher than if the basement was all below ground, and will require more than code-minimum insulation levels.

What effect does reducing infiltration from 3ACH@50 to 2ACH@50 have?

Lee's modeled ~35 KBTU/hr = ~12 BTU/hr per square foot of conditioned space on a 2900' house is a more credible number than the 17 BTU/hr per foot implied by the 50.5KBTU/hr load estimate, but still on the high side for a high-R house.

If those numbers are correct, the place could be heated & cooled with a 3 ton multi-split with a bit of margin (eg Mitsubishi MXZ-4C36NAHZ) if you wanted to. With the heads/cassettes sized correctly for the zone loads it would run nearly constantly at very low cfm, with very stable room temperatures. It would be a bit oversized for the cooling loads, but multi-splits have a typical modulating range at the low end of about 6000 BTU/hr in cooling mode, much lower than a typical 2-ton 2 stage central AC. The low minimum modulating level allows it to run very long cycles and some modulation with load despite the oversizing factor, and with much more efficient & effective zoning than a duct dampered approach. For low load rooms that are doored off you'd be using 1-2 mini-duct cassettes to get the modulation range and total capacity right, but bigger more open spaces can be served with a floor or wall coil type head.

http://www.mitsubishipro.com/media/989222/mxz_h2i_productlaunchbrochure1stprint.pdf

http://www.fujitsugeneral.com/PDF_06/2015%20Multi-Zone%20Brochure%20-%20High%20Resolution.pdf

I previously ran BEopt with all low solar gain windows and all high solar gain windows. I have added the more optimum case of high solar gain on the back, southern-facing windows and low solar gain on the other three sides. As expected, this results in a more optimum result, but only slightly. I have repeated the previous cases, adding annual energy costs and annual source energy used.

Low solar gain windows:
Heat energy 477.6 therm/yr
Cool energy 674 kWh/yr
Heating capacity 34.6 kBtu/hr
Cooling capacity 14.5 kBru/hr
Utilities $2007/yr
Source energy 176.5 MMBtu/yr

High solar gain windows:
Heat energy 436.1 therm/yr
Cool energy 888 kWh/yr
Heating capacity 35.3 kBtu/hr
Cooling capacity 19.6 kBru/hr
Utilities $2013/yr
Source energy 175.1 MMBtu/yr

High solar gain south only:
Heat energy 454.3 therm/yr
Cool energy 747 kWh/yr
Heating capacity 34.9 kBtu/hr
Cooling capacity 17.0 kBru/hr
Utilities $2000/yr
Source energy 174.9 MMBtu/yr

So, as expected, the high solar gain on the south windows only offers a sort of optimization for those particular windows selections.

I am currently exercising BEopt as it was intended, letting it optimize the natural-gas fueled hot air furnace, the air-conditioning, the insulation packages, and the size of the PV array to produce minimum source energy used and minimum energy costs. I may examine alternative heating and cooling systems after getting an idea on the insulation packages.

I let BEopt perform an optimization in which the variables included thickness of SIPs, attic insulation level, basement insulation thickness, natural gas furnace efficiency, A/C efficiency, and amount of photovoltaic power generation. The optimization was to minimize annual energy related costs, and to plot that versus source energy reduction from baseline. The increase in costs for more insulation were amortized over the lifetime of the product, or 25 years, whichever is shorter (I think.)

All cases were run with high solar gain windows on the south (back), and low solar gain elsewhere.

In some cases it ran up against a limit for the range of variables that were allowed. The results were as follows:
Attic: R-49 and R-60 were about equivalent as the best. In other words, R-60 saved energy compared to R-49, but only enough to offset the additional cost.
Walls: interestingly 5.6" EPS core SIP was computed as a better deal than the thicker cores. This was the minimum thickness included, so thinner walls should be included in next iteration.
Basement walls: R-21 fiberglass batts in 2x6 studs 24" o.c. This was thickest considered, so thicker should be included in next iteration.
Furnace: Interestingly, a 90% AFUE was judged best tradeoff, even though higher efficiencies were included within the input range.
A/C: SEER 16 and 17 A/C units were about equivalent as most cost effective choice.
Solar PV: A 10 kW system was judged to be optimum from a cost tradeoff, although that was the limit to the input range, so higher should be considered.

For those "best choices" above, the annual energy costs (including amortizing initial costs) were reduced from $2120 to $1524, and the source energy was reduced by about 73.5%. This assumes that the solar PV system is amortized over 25 years, and that the homeowner lives in the house that long, or that he can sell the house at a premium based on the life remaining in the solar system. These calculations included national averages for extra refrigerators, freezer, pool heaters, well pumps, etc., so these could also be set to "none" to reduce overall energy use, and displace a higher fraction of the source energy.

Further source energy reductions were computed up to about 82%, but too many of the variables were up against the input range limit to make those results meaningful.

posted below

Lee,
Thank you so much for the posts and the time you took to run the simulations. So solar PV systems, Is there a good thread to read up on on systems and manufactures. I think with Tesla's battery factory and improvements in technologies in general, I should at least be informed on the possibilities.

I have also put an album link that has the floorplans and elevations.

http://imgur.com/a/gmNWp

Thanks Fred

Fred,

You mentioned Tesla's battery plant. It is important to realize that the solar PV system that I ran the simulations for was a grid-tied solar PV system that has no batteries. A grid-tied system consists of solar PV panels and their racking, an inverter to convert the DC power into AC power that is synchronized (timed) to the electric grid. The grid must be there for the synchronization, as well as to absorb or provide extra electrical power as needed. Everyone that buys solar PV system figures that they would like to have battery backup, but battery-powered systems using solar PV for charging are completely different animals. Batteries are expensive and high-maintenance compared to grid-tied systems. Battery-powered systems are almost never cost-competitive unless you are in a remote location where the grid is not easily available. I visited with a friend last week in the next valley south of here, and she was looking into replacing the batteries for her system after 10 years of use at a cost of $10,000. In her case, it would cost $70,000 to run lines to tie to the grid, and off-grid systems are cost competitive.

For your area, the grid-tied systems are assumed to be available at $4.34/Watt, including panels, inverter, racking, miscellaneous parts, and installation, at least for a 10 kW sized system. To gain more knowledge for all types of solar systems, I would recommend checking out the Northern Arizona Wind & Sun Solar Forum, http://forum.solar-electric.com/. I have had zero maintenance for my grid-tied system after more than 5 years of use, so there is not much required from the homeowner. Well, I do occasionally get my roof rake and clean the snow off the panels, but only if I am in a hurry to see the solar power come back into the mix.

All of Solar City's new grid tied residential systems are battery-capable, but the economics of batteries don't work in MI at current pricing and net-metering policies. If net metering for PV goes away (as recently happened in Nevada, even for already installed systems) &/or excessive fees are attached to grid tied PV, or residential "demand charges" batteries will eventually start looking pretty good. Right now the only state where batteries are nearing no-brainer status is Hawaii.

Lee/Dana,

I had a 3rd party run a room by room manual J calc on the house plans. I was struggling finding a contractor who didn't want to put a 120K BTU furnace and 4 ton AC.

The manual J calculated 38K for heating and 22K for cooling. So I wanted to give creedence to both of your opinions and advice on here.

I have been reading up on HRVs and have a question regarding implementing one into my HVAC Or there is an all together better means to getting fresh air in. My plan was to mount the HRV in the mechanical room in the basement. I would exhaust all the stale air from the baths and laundry through the HRV and bring in fresh air back through the HRV and plumb it into the supply of the furnace. In each of the exhaust rooms there would be a booster switch to run the HRV in the higher capacity CFM mode. Would there be any issues in taking this approach?

Thanks
Fred

Lee/Dana,

I had a 3rd party run a room by room manual J calc on the house plans. I was struggling finding a contractor who didn't want to put a 120K BTU furnace and 4 ton AC.

The manual J calculated 38K for heating and 22K for cooling. So I wanted to give creedence to both of your opinions and advice on here.

I have been reading up on HRVs and have a question regarding implementing one into my HVAC Or there is an all together better means to getting fresh air in. My plan was to mount the HRV in the mechanical room in the basement. I would exhaust all the stale air from the baths and laundry through the HRV and bring in fresh air back through the HRV and plumb it into the supply of the furnace. In each of the exhaust rooms there would be a booster switch to run the HRV in the higher capacity CFM mode. Would there be any issues in taking this approach?

Thanks
Fred

Using furnace ducts as part of a HRV system means either a) leaving the furnace fan on all the time (very wasteful) or b) some pressurization/depressurization of living space when the furnace is on or off (you can't balance for both) and often c) little or no fresh air when doors are closed and the furnace is off. Best is separate HRV supply and return ducts to each closed door room (and no HRV tie-in to the furnace ducts).

Summary: multiple, independent fans sharing ducts causes balance problems.

Even ducts for the right-sized 38K heating /22K cooling loads are oversized for the HRV volumes, as are the air handlers. Separate ducts for ventilation makes sense, even if you used a common duct system for both heating and cooling. Ramping up the ventilation rate during high pollution/moisture events such as cooking or showering works just fine. Running it through oversized heating & cooling ducts just takes more power and makes it hard or impossible to balance the ventilation/exhaust flows, as jonr correctly points out.

The proposals for equipment that is ~3x oversized for heating and ~2x for cooling is unfortunately the rule rather than the exception. But now that you have a better handle on the real loads, you have a lever for pushing back. ASHRAE recommends no more than ~1.4x oversizing on the heating end, so with a 38K load even a 55KBTU/hr furnace won't be a comfort problem the way a 120K furnace might be.

My though was to save a bit of duct work and just let the HRV move the new fresh air through the furnace to the path of least resistance without the furnace fan being energized. This would most likely be the kitchen and great room. The furnace does have an ECM motor and is also able to be programmed to come on with the HRV is on boost mode (or other parameters). I thought at worst case and the house isnt getting enough fresh air I could add some cycles to the furnace blower. One HVAC contractor said I should do away with the HRV totally and just use the air handler alone to bring in fresh air.

Furnace air handlers are ridiculously oversized for just ventilation.

You can use the furnace air handler for ventilation alone without the heat recovery if you don't really care about the energy use consequences of doing it that way. In most cases it would end up over-ventilating creating wintertime dry-air comfort problems (to which the dangerous band-aid is an in-system humidifier, which typically ends up creating mold problems elsewhere in the house), and increases the heating and cooling loads.

fredjmillard-

I agree with your approach as being the only fiscally sound one for using an HRV. The cost of an HRV is marginally cost justified if you can use the existing ducting from the HVAC system. It cannot be justified if you have to run separate ducting for the HRV system. Some worry about using HVAC ducting for the HRV alone. Exactly why? In general, ducting is sized to be as small as possible and still handle the maximum airflows with minimal pumping losses in the fan system. Ducts that are oversized waste initial cost, and waste space, but certainly do not cause performance problems. Using an HRV system with ductwork sized for higher airflows will simply reduce the pressure drops and pumping losses for the HRV system.

Almost all homes that use airflow heating/cooling have HVAC duct outlets toward the outer parts of the house, and HVAC duct inlets toward the inner parts of the house. Having larger ducts improves comfort by reducing drafts from the HVAC blower. HVAC performance does not depend on having high speed jets that must penetrate somewhere in the house. If that were the case, then modulating heating and cooling systems would never work! Having lower air velocities reduces drafts in the house, and costs for fan loads pumping air in the ductwork.

If you use an outlet from the HRV into the furnace exhaust duct that is designed like a simplified air ejector, that is, with the duct from the HRV facing downstream in the larger HVAC duct, then the momentum from the HRV will cause a slight forward flow through the furnace as the two airstreams mix, or at worst, minimal reverse flow.

Concerning pressurization/depressurization caused by running the HRV and HVAC simultaneously in the same ducting, consider the following. With modern modulating HVAC systems in a well-insulated house, the ducts are sized for maximum HVAC airflow, but the HVAC rarely runs at the maximum airflows. The pressure drops in the ducting vary approximately at the square of the air mass flow rate, so a modulating furnace running at half the maximum flow rate (which is probably max flow for 80% of the time) has only 1/4 the maximum rated pressure drop. Pressure drops are so low most of the time that the HRV doesn't know the HVAC is operating. Compared to a full-time exhaust fan that many homes have, an out-of-balance HRV is certainly no worse for depressurization.

Agreed, if you pick some energy wasting practice like a full-time exhaust fan, an unbalanced HRV isn't worse than that. But isn't the point to not do energy wasteful (and potentially wall damaging and unhealthy) things?

http://info.zehnderamerica.com/blog/why-ventilation-should-be-separate-from-heating-and-cooling-systems

I have described the HRV system that I use in my house (http://www.residentialenergylaboratory.com/rel_description_am.html), and have shown through a cost analysis that even using HVAC ducts, the cost savings are barely break-even (http://www.residentialenergylaboratory.com/costs.html).

Please describe the HRV system that you are using, and the cost analysis for that system.

Lee,

Thanks for chiming in and also posting those links. Good reading and very informative.

Fred