Fun and Games with HVAC calculations
I have 1600 sq ft house. Two furnaces where installed because, at the time, that is the only way I could get anyone to do proper heating in the 1st floor.
The 1st floor is slab on grade for 70% of the perimeter and about 4 ft underground for the remaining . And lots of glass on the ground level side (lake view).
Most of the installers would just say that they will install a couple of registers in the ceiling for the “basement”. Could not convince then that it was not a basement and it needed full conditioning.
Finally found some one that said that they would install 2 units and ducts under the slab.
So I have a total of 186,000 BTU of furances for a 1600 sq ft house. If they are 70% eff that gives me 130,000 BTU of hot air.
By noticing long long they run I was guess that the lower unit was 3 time the size needed than needed and the upper was 2 times.
I am hoping to combine the two system with one unit and dampeners.
Since I had problems with the existing installation and I have a less than standard system I wanted some data on what I needed.
I got a HVAC calculator.
The HO version is $49. Plus, in my case 49 cents from the CC for a “forgien” transaction (Canada). HO version is the same as the pro, but limited to 2 months and the Registered User (company name for pro) is fixed and printed on all reports.
Found out that the house, in it’s current state except no skylights as they are getting removed during the reroofing, only needs 56k BTU. So I was right in my guess of 2 and 3 times oversize. I played with the numbers and found that I with the current furances it is good down to -75F. Design temps for this area are +6.
The AC system is only on the 2nd floor unit. But seems to do a fair job of keeping the 1st floor under control.
It is currently 2.5 tons. But the calculation show only 2 tons are needed.
That one I am not sure of as the old one seemed to be about right. But it was probably not operating correctly most of the time as I had to get it topped off each year. Everyone that looked at it said that could ID the leak. But there was a better than even chance that it would end up worse.
The software is easy to use. It is “simple” in both a good and bad way.
It is a plain text format and is fast and clean. But in most areas you can only select from drop down list of options. For example For walls they have all kinds of options, but when you get framed, sips, ICF, log, and brick/block, but no poured concrete. And for framed you can selected different R values in the cavity and also different insulated sheathing. But you can’t spec 16 or 24″ spacing.
It is accurate enough for sizing. But might not be if you are trying to meet some Energy Star or Leeds certifications. But I don’t know what those require.
You start with the design conditions (local weather and how you want the interior). Then define zones, typically floors. But could be wings of the house, etc. Those and room names have defaults, but you can change them t o anything.
Then for each zone you start by adding rooms. Then each room you add floors, ceilings (both only if they is unconditioned spaced below/above). Then outside walls and for each wall windows and doors.
I was able to quickly play “what if” by making improvement.
And it has simple reports by component (walls, windows, ceilings, etc) and by zone and floors.
It also has a detailed report showing the gains and losses by zone, t hen by room, then by each component.
William the Geezer, the sequel to Billy the Kid – Shoe
Replies
We've got a "split entry" (bottom floor about 4' below grade), maybe 1900 sq ft, not too many windows. 1976 2x4 construction -- unremarkable at best (though we've done a lot of caulking and sealing since it was built). Bottom floor heat ducts are under slab (Transite pipe). Originally built with two 75K furnaces (60K output), side-by-side, one upflow and the other downflow. Replaced with one 65K, 96% furnace & dampers, and that is plenty to keep us as toasty as we want to be in MN winters.
The only thing I miss about having two furnaces is the redundancy -- with the old lashup there was no need to really worry that a furnace might go out while we were out of town, etc.
Where are you located again, Bill? I forget (a lot). Your profile only gives a street.
BTW Thems are dampers, not dampners.
Yeah, whoever sold you the original dual systems sold you a bill of goods. Is he driving a BMW, now?
Cooling loads depend a LOT on orientation and exposure to sun and windows, etc. a 2.5 ton for a 2 ton calc load sounds about right. Depends on the accuracy of the calc. If there is slop in the calc, then you are about right. Heating load can be similar ... if there is a lot of slop in the calc, the furnace should be larger. Biggest unknown is e.g. air leakage rate... most of the time a #### shoot at best ... unless you have a blower door test to back it up. But there are many other vairables, too, depending on your software ... and with software that doesn't distinguish between 16 and 24 oc, it's not too precise (although it may be accurate ... there is a difference).
Why would you install ducts under existing slab? That seems like a lot of expense/work.
I am in the Kansas City area."BTW Thems are dampers, not dampners."In a different lifetime I used to do software design for controlling dampner systems uses offset printing presses."Yeah, whoever sold you the original dual systems sold you a bill of goods. Is he driving a BMW, now?""Why would you install ducts under existing slab? That seems like a lot of expense/work."No, I had to beg for it. This was 30 years ago and radiant was not that common and certainly not in my market.But when everyone else wanted to put a couple of vents in the ceiling for NON-"BASEMENT" with 3 32 sq ft windows and two doors. I knew that it would be freezing in their during the winter.Other than the unit short cycling the system worked fairly well. And only running the AC on the 2nd floor as also worked out well. It kept the 1st floor reasonable. And I live on a lake and in and out during the summer. And didn't want the first floor so cold that it would shock the body going in and out. That is what I found in some other houses.The slab was not existing. This was new construction.If I was building the house now I would check into a hybrid system. Hot water source feedign radiant in the 1st floor slab and a hydro air for the 2nd floor. And AC on the second floor, but with a couple of registers "stuck in the ceiling" that can be opened to direct a little more the cooling to the first floor. It does not really need much.Don't know if that would work or not, but I would want to start their."Cooling loads depend a LOT on orientation and exposure to sun and windows,"Yes, you can orientate walls (and thus the windows in them) to one of 8 compass points. And for windows you tell it the external shading and any internal blinds/draps. Also the type of glass, if it is low e, and if it has heat film."it's not too precise (although it may be accurate ... there is a difference)."Yes, I know the difference. And AFAIK it is accurate enough for the application, but would probably not work for super tight/super insulated homes because it does not offer enough options for the really find points."Biggest unknown is e.g. air leakage rate... most of the time a #### shoot at best ..."Yes, I know. This is one of the points where it does not offer enough granularity. But then I don't have any numbers to better put in.It offers average, poor, and tight construction. The house was "tight" for the time, but some details where not done right. So I used average.Two literal holes are the combustion air let and the space around the exhaust flue. I hope to go with high eff units and close off both of those. And future plans to seal some other areas. But I doubt that I could get it as tight as carefully detailed new construction. I did some "what if" calculation with some changes that I want to make. For infiltration I ran it again with tight construction and them split the different.Doing that decrease both the heating and cooling loads by about 20%. .
William the Geezer, the sequel to Billy the Kid - Shoe
Yeah, we found with our house that the downstairs didn't need AC. When the system was reworked (single furnace with dampers), though, we went ahead and had AC enabled for downstairs, since all it took was a heating/AC thermostat downstairs, vs the heating only unit.In your case, with lots of windows, there may be merit to giving the windowed area it's own thermostat. The controller we have can do three zones.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Well the lower level is open. One corner has a utility/bathroom space. Other than that there it is open and the stair come down, with the cool air, in the middle of one wall. You are right, it probably could use something better if the lower level was divided into rooms..
William the Geezer, the sequel to Billy the Kid - Shoe
You can still zone it, even if the area is open.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
How to do you zone an open area?.Anyway the duct work would make it hard and expensive..
William the Geezer, the sequel to Billy the Kid - Shoe
You think factory floors aren't zoned?
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
It would require extra ductwork, though -- you'd have to add separate ducts to the windowed area. In some cases it might be possible with only minor surgery, but more likely you'd have to tear up ceiling or floor, or get creative with some sort of "industrial-look" exposed ductwork.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Yeah ... the industrial look. Quality spiral duct neatly installed, hung, and painted. It CAN work (certainly not saying it would in this application).
They zone open areas in commercial applications all the time. Areas near a curtain window wall is one zone and the area interior to that is another (e.g. 20-30 ft in), for example.
It sounds like you have a split-level? Two floors anyway. The windows that face the lake--what is their orientation?
These are important Q's because your house may have more than its share of incompatible spaces. For instance, if the upper floor has a lot of west-facing windows, there may times when the room(s) with such windows will need cooling while the lower floor spaces with maybe only N-facing windows will need heat, or at least no cooling.
The problem you're facing is common with forced-air systems, and it's worse when you have only one duct system and one t'stat. Your idea of zoning the system with (I assume) more than one t'stat will help a lot with comfort, but the original design with 2 systems was probably a good idea--too bad they over-sized the units like they did. I think you would be happier with 2 small units that are properly sized than with one large unit, even if it is zoned with dampers.
Here's why. Let's say that to meet the 56k heat load, you install an 80k, 80% efficient gas furnace--that would result in 64k output, or about 14% oversize. So far, so good.
The unknown that could create problems is duct sizing. If a zone that needs only 20k of heating is the only one open to airflow, the heat exchanger could overheat if the ducts on that zone are not large enough to handle to blower's airflow, which, in the case of an 80k furnace would be about 1200 cfm (3-ton blower), or even as much as 1600 cfm (4-ton blower).
Under-sized duct might not allow anywhere near that much airflow, the result being overheating the heat exchanger. True, there would probably be a bypass damper to let more air circulate thru the furnace, but the bypassed air would already be heated, so the temp. rise problem would not be solved. [BTU output divided by cfm divided by 1.1 = temp rise.] With 64000 BTU and 1200 cfm, your temp rise would be 48 degrees above ambient, which is in-range for most gas furnaces. If you had only 600 cfm passing thru the one zone damper, the rise would be 97 degrees--far above the upper range.
Cooling problems can be even worse with too-low airflow: freezing the coil or excess superheat going back to the compressor.
Doing a reasonably accurate load calc is an excellent and necessary start, but it's only the first step. Next is to properly size and select equipment and then comes the all-important step of duct sizing. Since you already have ducts installed, you will have to be aware of how their sizes and restrictions will affect airflow.
Without knowing more about your situation, my best guess is that you would get the greatest comfort and best efficiency out of two separate systems. The cost of trying to unite them and install dampers and controls would probably be about the same as buying two units instead of one.
"It sounds like you have a split-level? Two floors anyway. The windows that face the lake--what is their orientation?These are important Q's because your house may have more than its share of incompatible spaces. For instance, if the upper floor has a lot of west-facing windows, there may times when the room(s) with such windows will need cooling while the lower floor spaces with maybe only N-facing windows will need heat, or at least no cooling."Both floors have most glass on the west and a fair amount on the south. And little on the north and east.But the first floor west windows are under a 2nd floor deck. Also whole west side has heavy tree shading, but as the instructions suggested I did than as unshaded because trees can die. In fact in a couple of weeks I can going to do a significant amount of trimming on them which will allow in a little more light, but I doubt that much."Here's why. Let's say that to meet the 56k heat load, you install an 80k, 80% efficient gas furnace--that would result in 64k output, or about 14% oversize."Don't forget I currently have a total of 186k furnaces. 86K upstairs and 100k downstairs. Even if I got 100% of a 80k unit it should be able to handle it.I want to go with high eff unit so that I can get rid of the combustion air inlet that also adds a lot to the infiltration. I am not sure that between the openable windows, the fireplace chimney, and the outside wall corners that there is even enough for one set of exhaust. Much less 3 (that includes a direct vent WH so I am thinking about changing to electric).If it is not practical to go with HE furnace then I will probably go with 2 small ones. If there are ones that are small enough available.I DEFINITELY don't need run heat and cooling at the same time. My climate control for 3-4 months of year is called opening and closing windows and running a whole house fan.And currently there only AC on the 2nd floor unit, but that does keep the lower level OK, but not great. So directly 10-20% of the cooling air down might be nice. But I suspect that just have the lower level returns (both high and low) will drawn in just enough hot air off the ceiling to make the desired difference.I an going to check to see if retro fit is practical and if it is I want to try with manual dampers that I switch from summer to winter.My main concern is the difference in thermal mass between the two levels..
William the Geezer, the sequel to Billy the Kid - Shoe
I DEFINITELY don't need run heat and cooling at the same time. My climate control for 3-4 months of year is called opening and closing windows and running a whole house fan.
Right. I did not mean to say that the incompatible spaces will mean that you might do both heating and cooling at the same time. Just that there will be times when the upper floor will need cooling, but the basement would not. If there's just one t'stat, and it's on the upper floor, you can get uncomfortably cool basement rooms. Or, in winter, the upper floor might be plenty warm, furnace shuts off, but basement rooms are still too cool. I mention this because the 2 systems that you now have are a good way to address the incompatibility issue--better than a zoned system, IMO.
If it is not practical to go with HE furnace then I will probably go with 2 small ones. If there are ones that are small enough available.
Most mfrs make 40k units with 2-ton blowers. Carrier/Bryant/Payne makes a 40k with a 3-ton blower, which might be nice for the system with A/C. These are HE units, as well.
If I go with the single system with manual dampers I plan on have a switch and control it from the upstairs thermostat for cooling. And I thing the downstairs for heat.I think that will work."Carrier/Bryant/Payne makes a 40k with a 3-ton blower, which might be nice for the system with A/C. These are HE units, as well."Again, the real problem is finding space with the needed clearances with one set of terminations, 2 will most likely be impossible. Also with a single system I might go with a dual fuel setup (heat pump with gas furnace).With the dual systems it won't be worth it to have the heat pump just on the 2nd floor..
William the Geezer, the sequel to Billy the Kid - Shoe
One more option:
There are some nice dual-fire gas furnaces now with variable speed blowers. One of them would be more compatible for use with a zoned system.
In sizing such a furnace, don't size the low fire to the load, size the high fire to the load, because the load (56k) is the 99% temp. (or 99.6%, depending the program you used.) This means that,except for 1% or less of the time, the furnace will be over-sized for the load. So, again, you might go for an 80k high fire with 50k or 60k low fire.
Incidentally, every load calc program I've seen ends up over-estimating the load because of the variables that make it impossible to nail down the load exactly. So, they always err on the side of excessive loss or gain, which is not to say they are wrong, but just that you need not fear that the program will short-change you on sizing.
70% seems really low these days for efficiency. Direct vent units are in the 90s.
I'm amazed that for 1600 SF your heating load is 186 MBTU ... I'm heating nearly 6000 SF (hydronically) with a 155 MBTU boiler - and it does the domestic HW passively too.
Jeff
Look at my message again. I know I don't NEED 186k. That is why I got the calculaltor. I only need 56k, but 186k is what I have.And 70% is a guess for 30 YO equipment. At the time the only H.E. option where units with a motorized damper on the fuel linked with spark ignition system. I opted to instead have then in a separate space with outside combustion air. But it turned out not to be sealed so that did not help any..
William the Geezer, the sequel to Billy the Kid - Shoe