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HVAC contractors do not know how to design for this home....

Posted by lzerarc (My Page) on
Tue, Feb 21, 12 at 10:15

I have been round and round with various HVAC guys. Its getting to be quite frustrating. Most are letting their "typical" and "tried and true" mind set cloud over reality and the change in the construction industry. Most are basically ignoring changes and advancements in insulation and building shell tightness. I have done my own calcs in my own programs, have had local utilities do calcs, and my energy assessor/Energy Star/HERS rater also do calcs.

Things are all over the place without much common ground. When I question it, the typical response is "we have never seen something like this, so were not quite sure how to approach it"...

Here is a run down of the home:
Climate zone 6, approx 6900 HDD, design temp -14.
Homes is a 1600 sqft ranch style with full basement. Daylit rooms.
2" XPS r10 underslab, Superior wall r15 basement walls with additional r-19 batts. Main floor walls are r40, 100% thermally broken double stud walls with ZIP sheathing for exterior air barrier and air tight drywall for interior air barrier. Home will be blower door tested with a goal of <.1 air changes @ 50 pac. There is about 9% glass on the south face, triple pane u .20 with SHGC of .44. No glass on east or west. About 8% on the north with u of .19.
Roof is vented truss, r60.

Heat loads have been right around 19k BTU including the basement. Cooling around 13k. Other load calcs have been in the low 20ks. HVAC guys have been everywhere from low 20s to 45k! Lots of research will show super insulated homes using hvac as simply as a single mini split heat pump. However I think we want to go with a traditional furnace/hp or ac setup, 2 stage, 2 ton.

Does anyone on here have any experience, thoughts or suggestions on something like this?


Follow-Up Postings:

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RE: HVAC contractors do not know how to design for this home....

one of the reasons you hire an energy rater is to guide
you through the process.

as a rater I have worked with several hvac companies
and provide several companies to contact.
in my experience it is the rater who provides the homeowner
with like minded trades people.

I would go back to the rater and find out who they have sucessfully worked with in the past.

in my area the utility company has an energy efficient
'design one' guide. maybe yours would also.
has utility provider worked hvac compaines
that have sucessfully installed low tonnage high
efficiency units on other efficient homes they were involved with?
this may be the battle.

could the builder provide companies that he has sucessfully
worked with on efficient homes?

heat pump association?

the 'answer' to your situation for too many
hvac companies, is to oversize with vs/2stage that runs
most of time in low speed.
higher cost to the homeowner..but when temps are severe
the unit has extra capicity is there a reason for oversizing?

best of luck


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RE: HVAC contractors do not know how to design for this home....

It seems like you intend to install a forced air system as opposed to some sort of radiant. From afar it looks likes you should be OK with your choice of a 2-ton 2-speed HP and matching variable speed air handler or furnace.

Any winter shortfall in heating is easily handled with staged backup heat strips when using an electric fan coil, although I understand your desire for proper sizing. However, cooling must be sized right from the outset in order to get proper run times for humidity removal.

Has an HRV or ERV been added to this equation?

SR


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RE: HVAC contractors do not know how to design for this home....

Yes, and HRV would be installed. I am battling between having it installed to run through the ductwork or direct piped to the space to be more efficient.

ER- that is the problem. The utilities companies have not seen something like this in this area, and do not really know how to approach it. Same with hvac guys. I have contacted many good ones, and have a couple on board with figuring it out, but I also do not want to be the guinea pig either.

While I would like to get a HP, there is also a $5k rebate if I use gas as my primary heating source on this type of a house.


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RE: HVAC contractors do not know how to design for this home....

and your energy rater has never worked
with a/c companies in your area?

we are in different locations, erv rather
than hrv.
we put a 12x12 filter back grill located
under porch or patio. ducted to return air
with barometric damper.
only operates at fixed rate when house
calls for fresh air. air is cleaned
measured and dehumidified before it
enters living space.

with access to natural gas, I'd look at
two stage high efficiency gas heat with
mid seer (15 to 17) seer a/c.

have you tried posting at hvac-talk.com?
they have aop section and lots of like minded
hvac people there. some may be in your area.

just a thought.

best of luck


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RE: HVAC contractors do not know how to design for this home....

GOOD LUCK COOLING YOUR HOUSE WITH 13000 BTUS


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RE: HVAC contractors do not know how to design for this home....

AS long as your house is tight, lots of insulation the best windows.
Then if the load calc was double checked and called for 1.5 tons then I would install it.
Most contractors dont have the balls to down size.Im one of the few who down size parcticly every replacement.


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RE: HVAC contractors do not know how to design for this home....

I think that getting a good heat load calculation for a superinsulated house is outside the experience of most HVAC contractors and perhaps most energy raters as well. Getting a good number for such a house takes a lot of time to get right, more than most are willing to invest in it. Nearly everyone doing such a calculation use canned software, such as HVAC-Calc or Wrightsoft, and without very detailed information about the structure and the willingness to enter it all have to rely on values in drop-down menus and assumptions about the structure. Most of the time these assumptions are adequate for conventional construction, but that isn't enough for a superinsulated house.

Two years ago I was going through the same thing for our new house (superinsulated, Zone 6, 7500 HDD). I did my calculations with a simple spreadsheet. That way I knew what the "behind the scenes" calculations were and could make the model as detailed as I had to, without making guesses as to what a package was doing. I used the model to tweak some aspects of the outer shell, such as window selection.

A new well would be needed for the house anyway, so a GSHP made a lot of sense; no extra drilling just for the heat pump would be needed if the depth expected for just house water was enough for a heat pump if the load was small enough.

At the time I was looking for an installer, three heat calcs were done by others, and all gave numbers within a few thousand BTU/hr of about 55,000. Since my own calculations at the time showed about half that, I asked for details of the calcs the others did. I was given detail for just one set, so I could do side-by-side comparison of the numbers.

What I found was that two key assumptions had been made as to air leakage. An assumed (but nonexistent) fireplace and a much higher hourly air leakage rate accounted for about 80% of the difference between my total load and that done by the other. Selection of numbers from the bottom of drop-down menus for such things as wall R value were off considerably also, accounting for the rest of the difference.

After making final design changes, I got my total load down to about 22,000 BTU/hr. The house has a footprint of about 2,000 sqft and is on two levels, the lower of which is half walkout (the house is on a hill). This was comfortably below the capacity of the selcted mfg's tables for a two-ton unit, so I specified that. The three other sets of calcs called for a five-ton unit, although their calcs for the final design might have been low enough that their recommendations could have been for a four-ton unit with reliance on electric strips for the peak of the load. I didn't ask for a recalculation, as I had no confidence in their ability to get it right for such a house.

While this winter has not been severe, with very few nights down around zero, the two-ton unit has been keeping the house at setpoint quite easily, in just first stage, and not running all the time either. My choice of size was spot on. Last summer, the occasional use of cooling mode kept the house cool easily, also in just first stage. The energy model I built showed that the two-ton unit would be twice what was needed in the worst conditions.

To rely on your own heat load calculations, you must be very sure of your numbers, especially those with significant impact on the bottom line. Leave out nothing; a lot of small numbers add up and may be quite important for a small system. However, if you are confident of your numbers and can't compare line by line with calculations done by others, you have to go with your own. For the house you described, it does sound like the two-ton size is correct, and you can go with it. Just make sure the ground connection is properly sized.


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RE: HVAC contractors do not know how to design for this home....

CUSTOMER;Hey why is the air coming out of my duct not as cold as I think it should be?
CONTRACTOR: oh because I put a 1.5 ton system in your 2000 SF house because the heat calc called for that.
CUST> Yes I know but it just dosn't seem to blow down my neck in the summertime like I like it to.
CONT.; I know that is the way it is designed to work.
CUST; Goes on website HVACwhatever.com- My contractor undersized my unit I don't think it is working right cause I can't feel it blowing down my neck when it's 98 deg. outside, can I sue him to upgrade to a larger unit or is it too late? He guaranteed his work and I told him that no one could properly install an ac system in my house because I researched it and I'm an engineer and I know everything, Is there any good lawyers in the house? we coulod team up and really get me a good system and a chunk of change.


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RE: HVAC contractors do not know how to design for this home....

contractor: I know you only have a ton and a half load
but I can't- wont- don't know how to do that. so instead
of the proper sized unit, we are going to install a
4 ton unit and set the fan speed on low.

client: what?

contractor: and you know on the 4th of july or new years eve when you have 40 people over, the unit will be able to
keep up. it has the capacity.

client: so this is still $$AABBCC price?

contractor: oh no..its $2,000 more.
and if we increase from legal minimum of 13 seer
add another $800

client: so what about the load calc? the duct
sizing & design?

contractor: don't need them..this is what we have
been doing for xxx years.
if you want 2 tons..I'll put it in, but won't warranty
it. let that company that did those calcs warranty it.

client: ok ok...what about zoning the unit?

contractor: we can do that. one unit for living kitchen den,and another for bedrooms. that way if one unit goes
out, you still have a warm/cool place to go.

contractor @ heat pump association: yeah made another
customer double size of system..and sold some high efficiency equipment..
and not one system..but two!
hey anyone know how to stage
heat strips or install a programable t-stat??
and what the heck is a fresh air intake??

I've heard it before. as always there are
two sides to every story.

education only helps if you pay attention.

OP why not contact the rep of the mfg of the system you
want to use? I've had clients that worked with
trane, goodman, lennox. the co rep did calcs
and recommended a local company to install.

best of luck


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RE: HVAC contractors do not know how to design for this home....

If you think you can cool a house with a ton of a/c go for it, I gauratee the dog days of summer they will be complaining about not enough cooling in my house. Of course if they are willing to pay for zoning, staged cooling and proper duct work they will be better off and I could see this scenario working if load calc is correct. These types of customers would put up a red light for me , because, if the system does not perform just right they will be complaining constantly and you would have to move a tech into the guest bedroom to babysit the system. If a contractor wants to go to bat against a client who has all the answers then more power to him> make sure you wear you shoe booties and do not track the driveway with that oil leak in your truck cause you will be buying that to.


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RE: HVAC contractors do not know how to design for this home....

It is a very tricky situation getting the RIGHT amount of air going through the home. With all the a/c people you have been working with surely things should be narrowed down. It seems you KNOW what you want to do - I'd say go for it, but do not overlook the advice of the professionals. SOMETIMES we do have something to contribute.


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RE: HVAC contractors do not know how to design for this home....

Very well said Chapmanair (southern cali)? you are right, I stand corrected. I can be overbearing at times.


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RE: HVAC contractors do not know how to design for this home....

thanks for those who have contributed something worthwhile. Heatsinker is a prime example of some of the hvac guys I have been dealing with that deny the numbers are correct. One word for you: Passivhaus.
While this is not going to be one, it is also designed to use the concepts. But then again, my 13k cooling load would make them cringe!!! That number is way too high still!

I have narrowed it down to a single company who seems to be getting it, and actually enjoying it. Our numbers are now meshing now that they understand the design and the effects the design has on the equipment.


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RE: HVAC contractors do not know how to design for this home....

Heatseeker not sinker- I would love to visit your house when you call for not enough cool when it's 100 degrees, but since you got all the numbers correct you are the smart one I concede. You are too smart to be my customer I will tell you that much. I have exclaimed the scenario engineers or want to be engineers are the worst people to deal with trust me.


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RE: HVAC contractors do not know how to design for this home....

I would be curious in seeing if you have a good grasp on the numbers I posted originally and the effects they truly have on a home, along with a program you use that actually takes into account some of these things, such as thermal bridging and extremely low infiltration rates...cause ours does. Some of the hvac guys I talked to their programs maxed out at r29 insulation in the walls! Not to mention their "infiltration numbers were, well not numbers at all, but rater "loose, medium, and tight". (tight being 2 ach@50 pac, still way too loose).
Its not a fight between who is smarter. Its a matter of new ways of doing things. Tried and true and gut feelings are all fine and dandy on typical code homes, but not here. It gets people out of their comfort zones and actually size things correct, heaven forbid.
My HERS rater and ES rater came back with similar numbers now. I guess we are all crazy and the home will really, really suck. (just like the PH homes in VT, IL, MI, and other places that use a single point cooling such as a mini split with great results....)


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RE: HVAC contractors do not know how to design for this home....

glad that you have had sucess in finding a
forward thinking company to work with.

over the years I've realized that the rule of
thumb companies make business for the problem
solvers.

hopefully you will share the process with
everyone.

best of luck.


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RE: HVAC contractors do not know how to design for this home....

I'm not sure what heat seeker is thinking. With no E and W windows, a house your size at code level insulation would probably do fine with 2 tons in NC ... as long as ductwork was in conditioned space.

Now on the heating side - are you just using electric strips when it gets really cold? Obviously a 2 ton rated HP won't produce that at zero degrees. I'm not suggesting you go bigger but you will be in backup often - or at least at design temp. I wasn't sure if you were doing a Carrier greenspeed but even that can't get near rated output at -14. Sounds like Carrier is recommending going oversize in colder climates (makes sense).


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RE: HVAC contractors do not know how to design for this home....

I know it Dosen't get that hot in nc I was just being a pita, the 13000 btu kind of thru me, to see what the reactions would be, please forgive.


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RE: HVAC contractors do not know how to design for this home....

"design temp -14."

Nice for heat, what about cooling load?

Cooling loads are actually tougher since solar gain can easily become a large factor with a lot of south windows.

It sounds like you need someone who is more experienced than 'running code' on a laptop.
It might be time for an actual licensed ASHRAE pro (a real engineer) to take a look.

I have never used 'canned' software for load calcs, but have my own software that I know and trust, and can alter as needed.

I have even used actual heating data from energy bills to compute loses against the weather that occurred at the time to determine measured building losses.

The measurement has been performed, all you have to do is back out the loss numbers.


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