Hi,

I am in the process of building a 5,200 sqft luxury home in the hudson valley (90mn North of NYC) with a 1700 sqft conditionned basement. We are looking at geothermal for efficiency reasons and also due to the high energy rating imposed by the town code. I have an independent specialist giving us an energy audit to make sure the HVAC system is sized appropriately. Just trying to sound members of this forum on the appropriate sizing of the Geo HP for my type of climate. Should I size for heating load as the contractor recommends or for the cooling load as the HERS rater recommends? He makes a point that with oversized HPs the units don't have time to cycle properly and humidity removal can be an issue in the summer (NY as a very humid summer season). We will have a back up heating system in the form of a gas furnace or boiler (we have access to NG). Any comments and thoughts are appreciated.

Thanks

If you are going to have a NG furnace, then the most economical size for the geo is smaller than usual. Ie, you want to use more auxiliary heat than you would if the aux heat was much higher cost electric.

If you are going to have a NG furnace anyway, I'd take a good look at skipping the geo and putting that money into more insulation + air source heat pumps.

For maximum comfort, consider some type of whole house dehumidification. Typical A/C (oversized or not) doesn't remove humidity when the temperature doesn't call for it to run.

It is true that in new construction the up-charge for superinsulating the house can be more cost effective than the extra tonnage on the GSHP (especially if you have to drill through granite.) And a high efficiency building envelope provides more actual comfort than any HVAC system (at any efficiency.) A code-min 5000' house could have a heat load of 60-80,000BTU/hr or if carefully designed, with very judicious & stingy window sizing & placement it could be as low as 40K.

And that's at code min. A reasonably designed with 1.5-2x code min "whole-wall" R-values could come in under 40K without living in the dark. Code min in NY is R20 or R13 + 5c.i. for stick built, which works out to about R14 whole-wall, after the thermal bridging of the framing is factored in. Code-min attics are R49. Adding R15 in rigid polyiso between the structural sheathing & siding has a material cost of about $1.50 per square foot, and cuts the heat loss from walls in half. Adding R25 to the stackup (4" of foil faced polyiso) would run about $2.50 a foot in material cost and may still be more cost-effective than the extra tonnage of GSHP. The cost of upgrading to R75 in the attic is negligible when designed into the framing from the get-go. The cost of reducing a U0.30 code-min window to U0.20 is substantial, cutting window losses by 1/3, but compared to the ~$9K/ton of geothermal cost, may still prove reasonable, and provides a great deal of comfort during cold snaps, and eliminates window condensation.

Even in a code-min house cooling loads are usually dominated by the amount of unshaded west facing windows. Sensible loads can be designed down/out using site factors, overhangs on south facing glass, and using low-gain & minimal total area on east & west facing glass. It's often of energy-use benefit to go with a higher-U but higher-gain glazing on south side, as long as there are adequate roof overhangs cut into summertime gain. This stuff can all be modeled reasonably using tools like DOE-2 /BeOpt, etc.

The financial models are not simple, and get further complicated by competing subsidies- a lot of GSHP purchasing decisions get made chasing the subsidy, and sometimes that's the right decision, other times not so much. If you have natural gas available it's really hard to make the financial argument for GSHP even WITH the subsidy, and spending the money on a truly high-efficiency building envelope may be the better investment in both comfort and operating cost. On new construction this is will almost always be the case, and often true even in retrofits located on the gas grid, but not always, since the cost of upgrading the building performance is many times greater in a retrofit situation. But if GSHP is still in the mix, a high-R envelope has an up-front cost benefit, and buys more comfort than the additional tonnage of geo, at a lower operating cost & longer lifecycle.

The Hudson Valley is the cool edge of US climate zone 5/warm edge of zone 6, depending on exactly where. (See: http://tboake.com/carbon-aia/images/climate_zones.jpg ) From a longer term financial basis there's an argument for the R30-ish whole-wall (2x code min) or R75-ish attic (1.5x code) even if you're heating with gas. Read the first chapter, and refer to the whole-assembly values in table 2, p10 of this document:

www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

Ditto everything Dana wrote.

The use case for geo is difficult to make in an area with both high electricity costs and available natural gas.

I'd go with the HERS rater's sizing advice. Properly combating latent load (fancy term for humidity) is crucial for comfort. Certainly go with at least a two stage system (or systems) for improved efficiency and better humidity control during part load conditions. Don't oversize much past the nearest whole ton, and don't be afraid to slightly undersize. Insist on a well thought out zoning scheme and that the duct system actually deliver air flow required to each room - said flows to be measured and adjusted during commissioning at least to within 10-20% of what a room-by-room Manual J compliant heating and cooling load calculation

I'm not sure what you mean by a "luxury" home, but were I spending big bucks on a big home I intended to live in for many years AND the location was anywhere near as far north as the Hudson valley, I'd be looking very hard at radiant heat.

"If you are going to have a NG furnace, then the most economical size for the geo is smaller than usual. Ie, you want to use more auxiliary heat than you would if the aux heat was much higher cost electric."

The numbers aren't that simple. Because electric auxiliary can work in concert with geo and gas forced air furnaces most often can't, depending on the circumstances gas back up may be designed for less contribution to get same operating cost.

Posted By joe.ami on 08 Aug 2013 10:19 AM
"If you are going to have a NG furnace, then the most economical size for the geo is smaller than usual. Ie, you want to use more auxiliary heat than you would if the aux heat was much higher cost electric."

The numbers aren't that simple. Because electric auxiliary can work in concert with geo and gas forced air furnaces most often can't, depending on the circumstances gas back up may be designed for less contribution to get same operating cost.

That's true- it can often be cheaper/better to run with resistance AUX heat than doing a switch-over to gas, since most of the heavy lifting is still being done by the heat pump. (This is true for better class ducted air-source heat pumps as well.)

Nearly all gas furnaces are oversized for the heat loads of high-R (or even moderate-R) houses, and way oversized for auxiliary heating apps.  In a new better-than-code-min house it's not clear if integrated gas backup ever really makes sense- adding more complexity than it's worth.

With a luxury home you are describing, a few points should be considered:

Yes, insulate the building envelope as good as you can and go the extra mile.

1) But "very judicious & stingy window sizing & placement" is usually not what owners of an large luxurious building look for. They look for high ceilings, oversized windows etc.

2) As humans need oxygen, lesser an issue in a large, high volume house. But with 2x code airtight insulation, you might be looking at the need for heat recovery ventilation systems.

3) I second Curt's suggestion for radiant, that would be not so much luxury for me, but standard.

4) If forced air, consider the 7 series variable speed unit, size it for heating, and it will have all the efficiency and dehumidification (the thermostat has a built in humistat) and an almost radiant like comfort. No short cycle issues either. No need to invest in gas backup, and to maintain a gas furnace which you now don't need anymore. Electric emergency backup (not so much supplement heat with the variable speed heatpump) does it all for very little upfront money.



But nothing would beat radiant with variable speed air handlers for A/C!

Ah yes, I forgot you asked about dehumidification. I maintain it is much ado about nothing.
2 stage or variable units running longer at lower CFM in low stage does not mean you have dehumidified more air than moving more air in higher stages faster IMHO..... and at the very least it is inconsequential in heating dominated climates.

Joe, I agree. Well, the angle I am coming from is that with an ECM fan (coming with dual and variable stage units) you can adjust the blower speed, which means you can adjust the CFM over the coil, which means you can adjust the coil temperature and therefore increase the amount of humidity you pull out. That's what I like about W-W as well, you can adjust the coil temp and therefore the amount of humidification, not via fan speed but via temperature control of the chilled water.

Posted By docjenser on 09 Aug 2013 01:14 AM
With a luxury home you are describing, a few points should be considered:

Yes, insulate the building envelope as good as you can and go the extra mile.

1) But "very judicious & stingy window sizing & placement" is usually not what owners of an large luxurious building look for. They look for high ceilings, oversized windows etc.

2) As humans need oxygen, lesser an issue in a large, high volume house. But with 2x code airtight insulation, you might be looking at the need for heat recovery ventilation systems.

3) I second Curt's suggestion for radiant, that would be not so much luxury for me, but standard.

4) If forced air, consider the 7 series variable speed unit, size it for heating, and it will have all the efficiency and dehumidification (the thermostat has a built in humistat) and an almost radiant like comfort. No short cycle issues either. No need to invest in gas backup, and to maintain a gas furnace which you now don't need anymore. Electric emergency backup (not so much supplement heat with the variable speed heatpump) does it all for very little upfront money.



But nothing would beat radiant with variable speed air handlers for A/C!

Stingy & judicious glazing is what would be required to hit under 40KBTU/hr at code min R/U. You can go quite expansive on glazing if using sub U0.20 windows (Witness the south face of a typical New England PassiveHouse!)

But even those windows will have a significant load penalty if you go expansive on an unshaded west side, and even luxury home builders/buyers would likely be willing to scale back a bit on the "Sunset View" when there is an efficiency goal they're trying to meet (as in this case), and if it's adding tonnage to the geo the true cost is going to be quite a bit more than just the cost of the ultra-performance windows.

At IRC 2012 code air tightness mechanical ventilation is required, not optional. Even at IRC 2009, it's usually a good idea- depends on where it tests out in the blower door test. But there's no question that any high performance house would need mechanical ventilation, and if you needed or wanted to run high ventilation rates, yes, HRV/ERV would be necessary to bound the heating & cooling load hits from the high ventilation rates.

IMHO mechanical ventilation capable of hitting ASHRAE 62.2 rates should be standard in ANY new home, but the actual level of ventilation should be left up to the occupants. ASHRAE 62.2 would be too drying in winter in the Hudson Valley, and add quite a bit to the latent cooling load in summer. For indoor air quality it's overkill for homes where the occupants don't smoke or use hairspray & insecticdes obsessively, etc., and dialing back rates to a fraction of the 62.2 numbers for comfort & better humidity control is reasonable. (There's little to no science behind the ASHRAE 62.2 numbers, and the recent revision upward has become a bone of some contention in the building-science world.)

Radiant floors makes sense in a code-min house, not so much in a true high-performance house.  In the code-min house it adds barefoot comfort most of the winter, but in high-R houses it primarily adds cost. The temp of the floor is only warm enough to make a signficant comfort difference for maybe 50 hours a year in an R40 whole-wall (with similar upgrades on U-factors elsewhere) house, and most of those hours are when the occupants are in bed.  A 5000' house with a 25K heat load would only hit ~6BTU/ft during the very coldest hours of the season, and the average mid-winter load would be under 3BTU/ft, which takes a floor temp less than 2F above the ambient room temp.  The heating/cooling balance point of these houses are often at outdoor temps under 50F- the heating season is both shorter and shallower than with a code-min house, and they're comfortable all the time, no matter how the heat is delivered. 

The upcharge for radiant floors just to reap the minor comfort upgrade for when you get up at 5AM to pee on the coldest day of the year is pretty substantial.  By 10 AM the same day the odds are high that the solar gain is already carrying the bulk of the rapidly diminishing load.

If you want warm feet in the bathroom, install a couple of radiant heat lamps positioned to shine on your feet and switch them on as you come in. If you stand on a rug, it will exceed the comfort from a radiant floor (often < 5F above room temp).

At this point I'm wondering if flojo is reading this thread at all, given his/her ultra-quiet response level, or maybe it's just the usual greenbuilding nerds havin' the usual conversation amongst ourselves?

On the contrary Dana1, I found all the comments quite interesting and this is pushing me to explore further. Thank you very much for posting here , this is helping me better understand the possible choices I am facing.

Posted By Dana1 on 09 Aug 2013 02:50 PM
Stingy & judicious glazing is what would be required to hit under 40KBTU/hr at code min R/U. You can go quite expansive on glazing if using sub U0.20 windows (Witness the south face of a typical New England PassiveHouse!)

But even those windows will have a significant load penalty if you go expansive on an unshaded west side, and even luxury home builders/buyers would likely be willing to scale back a bit on the "Sunset View" when there is an efficiency goal they're trying to meet (as in this case), and if it's adding tonnage to the geo the true cost is going to be quite a bit more than just the cost of the ultra-performance windows.

At IRC 2012 code air tightness mechanical ventilation is required, not optional. Even at IRC 2009, it's usually a good idea- depends on where it tests out in the blower door test. But there's no question that any high performance house would need mechanical ventilation, and if you needed or wanted to run high ventilation rates, yes, HRV/ERV would be necessary to bound the heating & cooling load hits from the high ventilation rates.

IMHO mechanical ventilation capable of hitting ASHRAE 62.2 rates should be standard in ANY new home, but the actual level of ventilation should be left up to the occupants. ASHRAE 62.2 would be too drying in winter in the Hudson Valley, and add quite a bit to the latent cooling load in summer. For indoor air quality it's overkill for homes where the occupants don't smoke or use hairspray & insecticdes obsessively, etc., and dialing back rates to a fraction of the 62.2 numbers for comfort & better humidity control is reasonable. (There's little to no science behind the ASHRAE 62.2 numbers, and the recent revision upward has become a bone of some contention in the building-science world.)

Radiant floors makes sense in a code-min house, not so much in a true high-performance house.  In the code-min house it adds barefoot comfort most of the winter, but in high-R houses it primarily adds cost. The temp of the floor is only warm enough to make a signficant comfort difference for maybe 50 hours a year in an R40 whole-wall (with similar upgrades on U-factors elsewhere) house, and most of those hours are when the occupants are in bed.  A 5000' house with a 25K heat load would only hit ~6BTU/ft during the very coldest hours of the season, and the average mid-winter load would be under 3BTU/ft, which takes a floor temp less than 2F above the ambient room temp.  The heating/cooling balance point of these houses are often at outdoor temps under 50F- the heating season is both shorter and shallower than with a code-min house, and they're comfortable all the time, no matter how the heat is delivered. 

The upcharge for radiant floors just to reap the minor comfort upgrade for when you get up at 5AM to pee on the coldest day of the year is pretty substantial.  By 10 AM the same day the odds are high that the solar gain is already carrying the bulk of the rapidly diminishing load.

Mechanical ventilation always depends for me on the way the house is used. If it is a 5000 sqf (plus basement) luxury home for 2 empty nesters, my threshold to put mechanical ventilation is is lower, but also depends on the air tightness of the home. I like to control it with a CO2 sensor, to make sure we are not bringing in too much unconditioned air, even with HRV/ERV.

Also, radiant floors do much more than feet warming. No noise, no dust blowing around, and most importantly it puts the heat where us humans usually prefer it: Within 6ft above the floors, and do not blow it directly up 20ft high under the 2 floor high great room ceiling in a 5000 sqf luxury home. A very well insulated 5000 sqf house will have about a 40-50KBTU/H heatloss in your area, that is 8-10 BTU/sqf, with an average mid winter load around 6-7 BTU/sqf.

If your load is that low, radiant install can be quit reasonable, since you don't depend on high performing systems anymore to transfer the maximum amount of BTUs at the lowest possible supply temperature. A simple staple up 16" O.C. in the joist cavity would probably do it, and you would still put out the 6-7 BTUs/sqf at 85F supply temperature.

this is helping me better understand the possible choices

To a large extent, you need to determine how you want to weight comfort (typical differences are pretty small), upfront cost, operational cost, reliability and environmental impact. There are lots of "reasonable" designs.

Posted By jonr on 10 Aug 2013 10:08 PM

this is helping me better understand the possible choices

To a large extent, you need to determine how you want to weight comfort (typical differences are pretty small), upfront cost, operational cost, reliability and environmental impact. There are lots of "reasonable" designs.

The only thing that's certain is that giving the heat plant no "weight" risks being uncomfortable in your new home. That's what people get when their counter top budget is greater than the HVAC budget.

That's because the wife 'can't' see the HVAC......:)

With all due respect to docjenser, in new construction having a heat load as high as 40-50KBTU/hr @ -5F in a 5000' home is a design choice, and not a particularly well insulated house.

The "pretty good house" standard used by some New England performance home builders is R5 (U0.20) windows, R10 slab, R20 foundation, R40 (whole-wall) walls, R75 attic it should come in 25-30K, and could be under 25K if that's a design goal.

At an average 15% glazing fraction, with 750' of U0.20 window the window losses will come in at about 11KBTU/hr (even without any type of curtains/shutters/shades).

Assuming it's 2-story without a gazillion bump-outs its likely on the order of 5000' of wall area, which at ~R40 whole-wall comes in at another 10 KBTU/hr. If you cheap out and only go R30 it's 12.5KBTU/hr, but the labor cost of the exterior foam isn't reduced- you're only saving the buck a foot or so for the insulation material itself. Weighed against $9K/ton geo, the "extra" R10 may seem expensive at ~$25K/ton, but it has a 100 year lifecycle with no maintenance costs, no operating costs, and even assuming energy costs for natural gas stay flat (unlikely) it breaks even on fuel savings on a lifecycle basis, and can be viewed as a hedge against energy price inflation. While it may be at the limit from a purely financial point of view, it's not insane if you're willing to pay a premium for performance, so for now let's leave it at R40.

So now we're up to 21K.

An R75 attic at ~2500' is another 2.5 KBTU/hr, and depending on how much above grade exposure there is on the R20 foundation, with R10 under the slab maybe another 3-5K. Call it 7.5K.

OK, that's 28.5K.

Build it air tight (under 1000 cfm/50) ventilate (but don't overventilate) with HRV, the ventilation loads are under 1K.

That's a whopping 29.5KBTU/hr.

And that's before you start peeling off for the average wattage of the 24/7 plug loads like the HRV, the Tivo, the SubZero, the wind-cooler, the freezer, the 200 gallon tropical fish tank, the 250 BTU per sleeping human, etc.. Figure on a least a 2000 BTU/hr reduction for a family of four, but probably not more than 5K. What it is at your house depends.

Peeling back by going with super-performance U0.011-0.18 windows PassiveHouse style adds substantial cost, but also add comfort. Where you stop on that front is a matter of choice (but the window choices in that performance range are limited.) Butwithout breaking the bank on super-windows, judiciously peeling back on glazing fraction where appropriate to get it down to a 12% framing fraction isn't usually all that tough either, and worth another couple thousand BTU/hr.

Modeling the design for the amount of thermal mass inside the envelope and adding some if it's coming up short can also reduce peak heating loads by another 1000BTU/hr, and average heating loads by more. (And they say that granite counters and marble staircases are mere luxury items!? :-) )

It doesn't take rocket science engineering to get the heat load of a ~5000' home into the 2-ton range without forcing you to live in a darkened hovel, and the upcharge, while substantial, is offset by the substantial reduction in cost of the mechanicals- it's not as expensive as it looks when accounted for separately.

Posted By Dana1 on 12 Aug 2013 12:15 PM
With all due respect to docjenser, in new construction having a heat load as high as 40-50KBTU/hr @ -5F in a 5000' home is a design choice, and not a particularly well insulated house.

The "pretty good house" standard used by some New England performance home builders is R5 (U0.20) windows, R10 slab, R20 foundation, R40 (whole-wall) walls, R75 attic it should come in 25-30K, and could be under 25K if that's a design goal.

At an average 15% glazing fraction, with 750' of U0.20 window the window losses will come in at about 11KBTU/hr (even without any type of curtains/shutters/shades).

Assuming it's 2-story without a gazillion bump-outs its likely on the order of 5000' of wall area, which at ~R40 whole-wall comes in at another 10 KBTU/hr. If you cheap out and only go R30 it's 12.5KBTU/hr, but the labor cost of the exterior foam isn't reduced- you're only saving the buck a foot or so for the insulation material itself. Weighed against $9K/ton geo, the "extra" R10 may seem expensive at ~$25K/ton, but it has a 100 year lifecycle with no maintenance costs, no operating costs, and even assuming energy costs for natural gas stay flat (unlikely) it breaks even on fuel savings on a lifecycle basis, and can be viewed as a hedge against energy price inflation. While it may be at the limit from a purely financial point of view, it's not insane if you're willing to pay a premium for performance, so for now let's leave it at R40.

So now we're up to 21K.

An R75 attic at ~2500' is another 2.5 KBTU/hr, and depending on how much above grade exposure there is on the R20 foundation, with R10 under the slab maybe another 3-5K. Call it 7.5K.

OK, that's 28.5K.

Build it air tight (under 1000 cfm/50) ventilate (but don't overventilate) with HRV, the ventilation loads are under 1K.

That's a whopping 29.5KBTU/hr.

And that's before you start peeling off for the average wattage of the 24/7 plug loads like the HRV, the Tivo, the SubZero, the wind-cooler, the freezer, the 200 gallon tropical fish tank, the 250 BTU per sleeping human, etc.. Figure on a least a 2000 BTU/hr reduction for a family of four, but probably not more than 5K. What it is at your house depends.

Peeling back by going with super-performance U0.011-0.18 windows PassiveHouse style adds substantial cost, but also add comfort. Where you stop on that front is a matter of choice (but the window choices in that performance range are limited.) Butwithout breaking the bank on super-windows, judiciously peeling back on glazing fraction where appropriate to get it down to a 12% framing fraction isn't usually all that tough either, and worth another couple thousand BTU/hr.

Modeling the design for the amount of thermal mass inside the envelope and adding some if it's coming up short can also reduce peak heating loads by another 1000BTU/hr, and average heating loads by more. (And they say that granite counters and marble staircases are mere luxury items!? :-) )

It doesn't take rocket science engineering to get the heat load of a ~5000' home into the 2-ton range without forcing you to live in a darkened hovel, and the upcharge, while substantial, is offset by the substantial reduction in cost of the mechanicals- it's not as expensive as it looks when accounted for separately.

With all respect to Dana1.....

First, you keep spreading rumors on the cost of geo systems. We charge about $2425K for a turn key horizontal geosystem, with HWG, 2 tanks and dual stage compressor. That is $6K/ton, not the $9K/ton you keep referring to. A 3 ton system is about $7-7.5K/ton.
Again, you are living a bit in a theoretical world. While I emphasize and agree with you that the best money one can spend is in making the envelope more energy efficient, actual life differs from theory. In the real world, people open doors and walk outside or unload the car for groceries (although nowadays you probably have an attached garage). They also leave a window or doors open accidentally (at least my kids do). And the list goes on and on. It is not always at perfect steady state as you describe it. Having lived in some german Passivhouses earlier on, with bad indoor air quality and the only emphasis on energy conservation at every price, I do enjoy the quality in my 1927 tudor house, although it is an energy hog and can never be as good as a modern new build house. As always in life, a compromise is warranted.....

Lest we forget in new construction there is a baseline cost/ton wheter the home is geo or not. I agree with the negawatts being the first line expense, but not the math employed to prove it.