I am building a 5000 sq ft log home.  The sq footage includes our basement which we will eventually finish but right now it is not finished.  The stairs from the 1st floor to the basement are open so we will need to heat the basement to a certain degree. 

Our contractor has calculated a 2 speed, 5 ton, 4 zone, Water Furnace envision series system. The zones are, basement, 1st floor, MB, spare bedrooms.  He is using Water Furnace software and when he plugs in the numbers he says the 5 ton system is the most efficient for our house.  He says he is plugging in 17 degrees for heat loss calculations and that if we were to go to a 6ton system it would run about $100 extra per year (operating costs) but the system can then support 12 degrees. 

We have already budgeted to drill for a 6 ton system, and we are planning on drilling, 3 bores, 300 ft each.  I am looking for some advice, I am worried about undersizing the system and not having enough heating capacity.  He says that since the house is zoned it will be rare that all 3 zones will call at once.  In addition he claims that the current system will run mostly at the lower flow rate, which is the most efficient.   This makes sense but I want to make sure we are doing the right thing.

We are building a log home with a lot of windows, you can look at the house plans, and pics at the following site:  http://www.loghome123.com

Any advice is appreciated.

Thanks,
Steve

Where are you located?

With that size of log home, I seriously question the sizing of a 5 ton system as being sufficient. This does, though, depend a lot on your location.

Are the logs "D" logs?

I am located in southeastern PA, and the logs are 8" round logs.. When you say you question a 5 ton system being sufficient, can you give me more specifics. I have been trying to figure out the size of my house in tons of air, and although I have something close for the sq ft volume, I cannot find how to convert that to tons. I have come up with about 38,000 sq ft as the volume. I took each room, LxWxH and added them to get that number.

Thanks,
Steve

I've looked at your plan on your site. What is the R-value and tightness that these logs will have?

The R value of an 8inch log is about 10, they are sealed very tight, screws, 4 beads of caulk and a gasket. In addition there is some thermal mass that helps the logs create an even higher R value, but there is no good way to rate that (from what I understand). The 2nd floor will feature a split log finish, so it is 2x6 framing with a half log on the outside. Those walls are R-19 and the roof is R-38.

Steve

What kind of insulation in the 2x6 and ceiling?

A few thoughts:

A couple remarks indicate to me that your HVAC sub is on the ball: They've calculated the load and advised you that low speed operation will occur most of the time and is more efficient. I hope he or she has asked you about your indoor design temperatures (what temps you set at your thermostat(s))

I sympathize with your instinct to oversize for safety, but I advise against it. Extra heat needed during rare extreme cold weather will (I presume) be provided by supplemental electric resistance heaters located in the blower discharge (commonly called "strip" heaters). They are indeed much more expensive to operate but shouldn't be called upon too often.

Oversizing has several distinct disadvantages:

1) Higher upfront cost for unit
2) Larger ductwork needed
3) Reduced zone design size flexibility - note that with the newer 2 speed systems (Envision being one of them), low speed is at 70% (up from 50% for older models). 70% minimum compressor output means higher minimum airflows, which in turn means each zones ducts may have to be larger than you'd like - jumping up to 6 tons would exacerbate this problem. When a single smaller zone is the only one calling, there will be a lot of air. The newer ECM fans will spin up as needed to move that extra air, up to the blower's design static limit. Small zones can get awfully windy and noisy.

4)SE PA has hot humid summers - an oversized unit makes for poor dehumidification during summer as well as reduced efficiency (in search of dryer air you may find yourself lowering thermostat settings. Oversized AC units have two settings - hot and muggy or cold and clammy.

Given the costs and disadvantages of oversizing, the $100 per year savings may be a foolish economy.

Whose your utility (I used to live near Philly) Do they have a favorable residential rate for all-electric heating? Any high efficiency rebates?

Are you going with Waterfurnace's Intellizone system for zone control? I have one and like it - LEDs on the system board show exactly what's going on - which zones are calling, unit speed, fan speed, what dampers are open, etc.

Make sure the load calc factors in your actual geothermal conditions - water temperature and flow. Tonnage ratings are nominal and set at given conditions which may differ quite a bit from your actual conditions. This is called a Manual S calculation. Waterfurnace publishes tables of capacity for all the Envision units at a wide variety of water temperatures and flows. That you are drilling for 6 tons will add a bit to the actual capacity of a 5 ton unit by giving it slightly more favorable water conditions.

For $50, last I checked, you can use Don Sleeth's HVAC-Calc to do your very own load calcs. I used both it and Elitesoft's $500 package and they were quite close in result, within 5% or so. You may find some design changes to reduce your load - for example, locating all ductwork within the conditioned space has a huge impact (15-25%) on total load. Do this by care use of soffits and / or sprayfoaming your attic under the roof sheathing. Sealing the attic this way, though expensive, confers several other advantages (cleaner more comfortable attic and stronger roof)

Finally, seriously consider the desuperheater option for preheating domestic hot water, and be sure to pipe it to its own preheat tank upstream of your regular water heater, whether tankless or traditional - a preheat tank is the only way to make proper use of desuperheating.

I've droned on long enough...

Thanks for the reply everyone. To answer everyone at once.
1. I have R19 walls and R38 roof insulation.
2. We are using the backup electric in the unit.
3. We are using the Intellizone system as you have described
4. We are doing the desuperheater for hot water.
5. My contractor uses water furnaces heat calc software.. he says the 5 ton system is enough and he said if it was his house he would use the 5 ton system. He didn't jump at the possibility of more money with me thinking about going to a 6 ton system.
6. My contractor wraps has wrapped all the duct work in the attic with insulation.

I am confused about 2 points, this comment:

SE PA has hot humid summers - an oversized unit makes for poor dehumidification during summer as well as reduced efficiency

And this comment:

a preheat tank is the only way to make proper use of desuperheating.

Can you explain that further??? I thought that pre-warmed water going to my electric hot water heater would save me money by saving me from having to heat water from 60 degrees to 120 by preheating to 80 or so?? Is that not correct?


Steve

OK, let me elaborate:

1) Unit oversizing. In a humid climate -  An oversized unit will satisfy the cooling load too quickly and cycle off leaving the room air cooler but still quite humid. People sometimes get the impression that an air conditioner is "working too hard" and therefore undersized if it runs continually during hot afternoons. Continuous operation is actually easier on equipment just as highway miles are easier on cars. Continuous operation is most efficient as it minimizes starting and stopping losses from motor startup surges and refrigerant equalization during the off cycle. Continuous operation provides most dehumidification as water is extracted from the air only when the unit runs. Good dehumidification allows occupants to be more comfortable at higher thermostat settings, which improves efficiency by reducing load and conductive heat gain (lower gradient across exterior walls, windows and doors)

2) Desuperheat / preheat. The problems with not using a preheat tank are twofold:

Checking the performance chart for a 5 ton Envision and guessing at conditions in SE PA I read a hot water capacity of 4,000 Btuh or thereabouts. Suppose you have an 80 gallon water tank and you heat 60 degree incoming water to 120 dgrees. A tankful needs around 650 Btus per degree or 40 KBtuhs total. For the preheat to provide half that would require 5 hours of operation.

Suppose you get up at 7 AM on a summer day and take a 10 minute, 20 gallon shower. A couple minutes into the shower the cold water hitting the bottom element turns it on, and 35-45 minutes later (depending on water heater element wattage) the tank is completely reheated and the element switches off. Heavy use of AC will likely not occur until the heat of early-mid afternoon, but at that time there is no hot water demand so the 4 KBtuh of available desuperheat has nowhere to go (water heater all full of 120 degree water). Even if 2-4 people take morning showers, the water heater is completely recovered within 2 hours.

The same situation applies to hot water use in evening nigh time showers, laundry and dishwashing may nearly completely empty the tank, but this use is well past the heat of the day and again, the standard storage electric water heater recovers within an hour or so.

Gas storage water heaters have higher Btuh inputs so recover even faster, worsening the problem. Tankless units render desuperheat nearly useless since desuperheat can only be recovered when water is actually flowing and the result would be a preheat of only 2-3 degrees given the slow rate of desuperheat.

The twofold problems are both rate and time of preheat production.

Some trying to sell a desuperheater without the hassle and expense of a dedicated preheat tank will say that you can reduce the bottom element thermostat setting on a standard storage water heater thus turning the lower 3/4 or so of its volume into a preheat tank. Doing so is in fact impractical as it turns an 80 gallon tank into a 20 gallon tank - first shower gets the plug of hot water at the top, but the next shower if it quickly follows the first is at or only a few degrees above the lower element setting. The person experiencing the second tepid shower will not be happy if the lower element setpoint is much below 100 degrees. Not much desuperheat can be added to 100 degree water.

During the heating season the times change but the essentials do not.

Now consider a preheat tank - cheap way to do this is a 2nd electric storage water heater not wired and piped so that cold water passes through it first. That way both thermostats on the primary tank can be set so that everybody gets a comfy shower from its full capacity, and it is refilled from the bottom by the contents of the preheat tank. Meanwhile the preheat tank stores cold water awaiting preheat from the desuperheater whenever during the day it becomes available. The colder the water sent through the desuperheater, the more heat is transferred to it. The preheat tank should be as large as space and budget allows, ideally holding a whole day's hot water use. I got a pair of 12 year 80 gallon water heaters from Home Depot for under $800 total. I have $30 digital hourmeters on the water heater elements as well as the Y1 and Y2 (low and high speed) outputs from the HVAC zone controller. I can correlate HVAC operation hours with hot water electricity consumption.

Another advantage of the double tank system is the ability to wire it (assumes availabilty of a spare pair of 30 Amp breakers) and set its thermostats for same or nearly same as primary heater, and throw the breakers only when company comes for the holidays or whatever. That will disable desuperheat / preheat for reasons discussed above, but doubles household hot water capacity for special occasions.

A similar approach connects an unwired preheat tank for desuperheat to a tankless water heater. This saves space and provides the same optimization of desuperheat, but tankless units cost more and require more electric power. This variant allows for a backup source of hot water if the tankless unit fails. Another consideration is that residential backup generators can feasibly supply a storage type electric water heater (~20-25 amps) but typically not an electric tankless water heater (60-100 amps)

I'm not a big fan of tankless electric water heaters - they've been 'greenwashed' - advertised as much more efficient than conventional storage heaters since they reduce standby losses. The facts are that storage electric water heaters are nearly all 90+% efficient, and that can be improved to 95% or so by addition of tank and pipe insulation. Tankless units:

1) Have a higher purchase price

2) Have a higher installed cost (multiple pairs of high amp circuit breakers and heavy guage cables)

3) Consume much more power when running, exacerbating utility demand problems, not at all 'Green'

4) Can't flow as much hotwater in a short time - sure it is 'endless' but tepid if multiple simultaneous users

5) Are not compatible with backup generators, and their high Watt density elements and multiple, frequently switching high amp contactors are likely not to last as long as components of storage heaters

6) Those repair parts in 5) above will be more expensive than strage heater repair parts.

Once again, I've droned on a bit too long - hope someone finds the foregoing helpful!

Interesting thread. I might note I live in Media PA which is Southeast where the subject house is being built. We are at about 4950 degree days.

I would like to note this is a green build site, and as such I am hoping a purist can be allowed to have a word. In general I agree with sizing comments, however, have an addional take on the hot water. If you are going to go with a desuperheater and the seperate tank then you might as well take the plunge for Solar Hot water now so you can use the tank with the solar heater in the summer. The tank paired with a solar heater will pay off sooner. I am not a geo expert as our engineer friend is and do not know water furnace's units well enough to know if a desuperheater can be added after the fact.

If budget is pressing you, you could get more bang for your buck from a solar hot water heater, as its pay back comes sooner and, with the savings you could later add a desuperheater's output to the solar tank? Also as regards bang for the buck, you might consider using ICFs on the basement while insulating the basement pad underneath and at the perimeter: This could reduce your heating and cooling load significantly. Then, before you pour that pad, lay a PEX serpentine in it. You can always add a heat exchanger to that second tank and do one of two things with it:

1. tie it to the pex loop in the floor.
2. tie it to a loop in your ducting and turn it on first, before turning on those "electric strips"

By doing 1, you would bank" heat in the basement floor first, or at least in a zone immediately below the stairs By doing this you might be able to use the sun to preheat the basement pad, let it rise up the staircase, and let the Geo kick in when the sun cant help.

By tying solar hot water heat exchange into something like a fan coil unit (again, I am not a geo whiz), you might be
able to blow through solar heat before blowing through those carbon and Kw hungry electric strips.

If you are building 5K square feet and log my guess is you are out away from natural gas and your electric provider is PP& L or PECO, both of whom are about to come off of rate caps in 2010. This will likely mean you will see a 30 to 40% electric rate increase on Jan 1 2011 (no matter what the corporate spin artists say). In addition, you are faced with an immediate 20% fuel adjustment cost increase on July 1 of this year.

Given this rate climate, we are advising ALL our geo clients to pair their installs with Solar TechnologiesIn order of ROI, we believe they are Hot water, Hot air, and PV. The Hot water will help offset the rising cost of grid electricity immediately. Second, we suggest they bank the monthly savings to fund Photovoltaic as the value of the electricity and renewable energy credits is rising: the only way to insulate yourself from these instability in costs is to become a producer.

Further, we invite conversation about our opposition to the inefficiency inherent in electric grid power, and, what is in our view, grid's disastrously non green status as a power source. While that Electric hot water heater our engineer friend talks about is indeed 95% efficient, the power it is so effcient at using, suffers 20-30 % loss during transmission, and another 30-50% loss when it is produced as a result of burning fossil fuels. Need we mention how many tons of carbon go into the air in the process?

I wish you luck in your effort and cannot recommend strongly enough that you investigate pairing a second renewable resource with your geo.

ok...I have been droning..best of luck!

and then add PV installs later in a coordinated, planned effort at I would be interested to know if the installer is Swinton or Brandywine Valley.

First let me thank everyone who has responded to help. I appreciate your time and efforts.

Our house is 5000 sq ft with the basement. We are not finishing the basement right now (maybe 3 yrs) but were told that it would just be cheaper to size the system for the whole house now than to have to drill and tie in another well later. So that is what we are doing.

Engineer, my contractor did say that there would be a problem with humidity with the bigger unit and it would not be as efficient, you have confirmed what he told me. I am using Ground Source Contractors out of Perkasie, PA. Can you send me a picture of your 2 water tank set up to my email address, [email protected]? That would be a great help. I would like to mention it to my plumber at geo contractor. Another question, could I use a 40 gallon HW tank for a preheat tank, does it have to be the same size as my primary tank?

Hometowngreen, I am already past pouring my basement but laying the PEX in the slab is a great idea, I wish I would have done that. I have looked into some solar options, but due to our budget it is out of the question right now. It seems that the solar option is still quite expensive. If I remember correctly, a system to offset 20-25% of my electric cost was around 16K. Maybe I am wrong and would appreciate any links or info to help enlighten me. In the future, we do plan on having radiant heat flooring in the basement, maybe we can tie that into a solar system.

Desuper needs to be ordered at time of ordering heat pump. Retrofitting would be a bear. Desuper only adds a few hundred dollars to a machine well into 4 figures. Desuper need not be plumbed or activated at time of install - just ensure circulating pump can't run dry. My unit has a switch, but I'd suggest the additional precaution of physically disconnecting its pump if plumbing it will be delayed.

I see no reason why solar and desuper couldn't coexist using the same preheat tank. Be sure solar is done correctly by an experienced installer - especially in a freezing climate. A recent issue of HomePower Magazine discussed a length the many nuances of a solar domestic hot water system.

ICF and pex for radiant heat are both good ideas. My understanding is that one should NOT circulate domestic water in any kind of space heating system as water stagnating during the off season has the chance of harboring Legonella bacteria or some such. I don't like the idea of domestic water stagnating anyway, regardless of Legonella or what ever.

I agree with the comments that the grid is, for the most part, non-green, however, feasible non-grid alternatives are not economically feasible for most folks, and then there is the issue of buying and later disposing of the battery bank needed to be totally off grid. If the upcoming changes in utility regulation will also include a stable, reliable and favorable net metering law, then by all means go for the PV, money permitting.

Desuper is a no-brainer, at least in any climate with a long cooling season, solar less so, depending on circumstances.

The other thread now on desuper plumbing provides plumbing guidance - check it out.

There are no hard and fast rules as to the size of a desuper tank - bigger is better. Ideally it holds at least a day's supply of hot water so that a hot afternoon's recovered heat is available as warm water until the following hot afternoon.

there is no way that I would use 1 big unit for a whole house with 4 zones. the unit is 2 stage, but what is the capacity split? On part load days it will be short cycling. You already know about the humidity problem associated with oversizing a/c eqipment. Especially if its not too hot, and real humid.
DX air systems are really designed for a specific air flow, like 400 cfm per ton. when zones are opening and closing, the refrigerant balance point changes and low air flow forces the expansion valve (if there is one) to shut down to prevent freezing, then the compressor shuts down making the humidity and short cycling problem worse.
I would use 1 unit for each zone, its so simple and guaranteed to work. maybe you can get the zone count down to 2 or 3.

Suit yourself.

I have 4 zones - 600 SF basement, 500 SF east bedrooms bath and laundry, 900 SF kitchen, dining, family, 900 SF top floor. All are conditioned by a single WF Envision nominal 3 ton unit. It actually moves about 30K Btuh on low and 42k Btuh on high at my waterside conditions

It works fine - home alone in the office I can just cool the top floor. Kids leave for the day so their bedroom zone is allowed to warm a bit. Basement conditioned only when occupied.

5 of us live in this all-electric (well, septic...everything) home and are on track for an electric bill of $110 or so, with dewpoint running around 70 and highs around 90 every day here in north Florida. Desuper is presently handling 60-70% of hot water load. This is our first month in the house.

I support the above conclusion with data - daily electric meter reads and with hourmeters on the HVAC compressor (both speeds) and the water heater.

The only slight disadvantage of this system is that during relatively rare small zone only calls there is a bit more than design airflow. I notice, but wife and kids don't mind.

I like the four zones and can't conceive of the complexity and hassle of 4 separate units to meet the load.

Just because DX can't handle the reduced refrigerant velocity of a multi-speed compressor is no reason to denigrate zoning and multi-stage equipment.

Geo serves an upmarket demographic that invests long term, wants to be green but in medium to large houses. I have no doubt we will see more and better zoning systems as well as variable capacity compressors. For DX to thrive in this market it'll have to find a way to provide part capacity and zoning without resorting to multiple units. Off the top of my head a reasonable way to do this might be to valve out some of the ground loops to increase refrigerant velocity in the others under part load conditions.

Well, thanks engineer. I am at least a little relieved that there is a chance that sorleski (and family) won't really hate this 4 ton, 4 zone system they plan to put in their new house. A beautiful one, I might add, I checked out their web site.
Until now, I had believed that it was impossible to 4-zone a DX system and have the occupants like it. At least everyone that I had met up till now in a multi-zone DX system has hated it. This is assuming that we extrapolate your 1 month positive experience in this house out to 20 years.
But, let me note a few differences between their house and yours.
You are in florida- Your system is cooling dominated. the sorleski house is in Pa., its heating dominated. They are as opposed as south and north.
the south system is 2900 sqft, the north system is 5000 sqft
the south system is 3 Tons, the north system is 5 tons.
the south system wells are in sand with a high warm water table, the north wells will probably be in rock with cold deep water.

I guess that the only thing in common is the 2 spd, 4 zone system.

I don't have DX. According to the Earthlinked site their DX won't support a 2 speed compressor owing to refrigerant velocity concerns. My understanding of zoning is that a single speed compressor system supports only two zones, certainly not 4. But I certainly don't have all the answers.

I'm curious to hear specifics of zoning complaints - certainly they are vulnerable to mis-design by the usual hacks in the HVAC biz. Part of the problem is setting customer expectations...no you can't run heat in one and cool another...no you can't cool a 2nd floor zone to 75 and hold the first floor zone at 80 if they are connected by an open foyer...ain't gonna happen.

Zoning allows some variation in area temperatures, provides limited economy by reducing conditioning of unoccupied zones, and may allow a slight undersizing of a system in recognition of load diversity - West facing zones can get a greater percentage (or all) of a system's cooling capacity in late afternoon while east zones get little or none, their peak having past by late morning. That last point may not apply to heating. Judiciously applied, zoning helps with system sizing issues and fixes the oft-heard complaints of rooms which are perpetually too hot or cold

I designed my zones based on out-of-date Waterfurnace docs stating that part load was half load. The newest 2 speed units (not just Waterfurnace but any using the Copeland Ultratech scroll compressor) run at 70% in part load. That in turn raises the minimum CFM, slightly overblowing my two small zones. What saved me was dehumidification mode, (which I planned to use anyway), which cuts airflows in cooling mode by an additional 15%.

When I got word of the higher part load capacity I made provision for a barometric dump zone damper in a stairwell near a big return - if needed it'll act a bit like a bypass damper and duct. I don't think I'll need that but I won't know until our excuse for a heating season arrives around Christmastime here in Jacksonville

I hear of systems in the pipeline which will turn down a compressor like a lightbulb on a dimmer, as well as allow heating one zone while cooling another; sending heat to domestic water if needed rather than rejecting it to air or ground (not just desuperheat)

OOPS!

BillN - based on your post at the oil heat guy's thread I now think that by DX you mean Direct Expansion, as found in all air-air and water-air systems. By DX I mean Direct Exchange, the geo variant where copper refrigerant tubing is directly buried.

So, by your terms I do have a DX system....gotta watch these abbreviations!