Calculating the balance point

[wango]

I had a duel fuel system put in my house over the summer reusing my HP and putting in a furnace. My installer is coming back at the end of the month to fire up the furnace. He didn’t want to do it in Mid August when it was 95 degrees here in MD. What I was wondering is how to calculate the balance point?

[BaldLoonie]

On graph paper, draw 1 line showing heat loss of the house and another line showing output of the heat pump. Since heat loss will increase as it gets colder and heat pump output will decrease, there will be a point where they intersect. That's your balance point.

But most guess :(

[Jultzya]

By reading your post, you seem to be a HO.

I would call the company back and have them set-up the unit correctly. Unless your just asking.

If you are just asking, well then Loonie answered your question. ;)

[junkhound]

For the average HO, if you have some math skills it is instructive to know how and why your heating dollars vary. If not math inclined, just ask your tech.

If you are math inclined,request your installer provide the spec data sheet for your HP or get it off the net, then you can make up a table as follows. Use the installers manual J calcs for the needed BTU. If your installer did not do a manual J or similar for you, you can calculate the needed BTU/hr by calculating from the average temp for a winter month and usage on a metered montly oil bill or metered nat gas bill or your HP bill (using the spec data table), then ratio to the table numbers. First balance point is heating capability, in the example table case the HP cannot heat the house below about 12F (the need BTU/hr is greater than the HP capability) and also a 2nd economic balance point - say you pay $.90 a therm with a 90% eff gas furn, your breakpoint for $$ is where the HP therm costs more than nat gas, or about 18F. You can up the size of the HP to lower the heat capability balance point, but you are paying more and may not be happy with humidity during cooling season if oversized.

degF need BTU/hr BTU/hr $therm
3Ton COP HP

-5 21000 14000 1.7 $1.21
0 19200 14700 1.75 $1.17
5 18270 15700 1.85 $1.11
10 16830 16500 1.9 $1.08
15 15400 17600 2 $1.03
20 14000 18900 2.1 $0.98
25 12500 20600 2.3 $0.89

[scouts22]

I'm a slightly confused homeowner on calculating the balance point. I recently had installed a Trane XL16i Heat Pump, XV80 Two-Stage Variable Speed Gas Furnace and Trane XL802 stat (same as Honeywell VisionPro 8000).

I am in the Kansas City area and the electric company, KCP&L, recommends 30 degrees. I guess they benefit with a lower balance setting. My installer recommended and set 40, saying that's what most customers seem to like. That seems too high and far from the 30 from the electric folks. I also called the Trane distributor. He told me he personally uses 30. I'm inclined to use 30 but am curious if anybody has any other thoughts. The HP (up to 10.55 HSPF) puts out 48,000 BTU and the gas furnace 52,000/79,000. Insulation is probably average for a 20 year old house. Average of last two year's temps:

Month AvgTemp AvgHi AvgLo

Nov.......43.........51......34
Dec......33..........41......23
Jan.......27.........35......19
Feb......40..........51......30
Mar......54..........60......40

Gas will be about $12/Mcf come December. Electric is about $.04/kWh in the winter for heat pump owners.

Also, the indoor stat does have an outdoor sensor but I think the balance point is run from the outdoor stat. Is it preferable to run the balance point from the indoor stat? Thanks.

[deme]

Heat pump Carrier 38YXA. 2 ton nominal capacity working with natural gas furnace. Vancouver B.C.

TEMP.... HEAT REQUIRED BTU/H...... 38YXA OUTPUT BTU/H
30*F--------- 23,503----------------------- 18,000
35*F--------- 20,407----------------------- 19,500
40*F--------- 17,318----------------------- 20,500

Set balance point at a conservative 40*F (4*C)

Annual heating degree-days Celcius Vancouver B.C. below 18*C = 2926 degree-days Celcius
Heating degree-days Celcius below 40*F (4*C) = 252 degree-days

Percent heating energy supplied by heat pump (above balance point) = (2926 -252)/2926 = 91.29%

[Edited by deme on 10-10-2005 at 12:54 PM]

[amickracing]

scouts22, personally I think 40 is WAY high, 30 likely is even high.

Just an idea to throw out there for y'all to consider and hopefully not flame me too much. When you are calculating the heat loss some of what you have to do is guessing. Also if you do it per the manual J, it's going to be a big higher than the actual loss (the cya factor).

What I've always wanted to do is just run the heat pump all of the time and keep track of the house's temp. Once you notice that the house isn't warming up any more (by looking at a thermometer or the stat), then you know the approximate balance point. Since efficiency varies with how it's installed, heat loss factors sometimes become more of a guess than an actual fact, and could help those math challenged.

Once you found the point that way maybe add 5 deg or so to that temp and use that for the cut off point.

Even if you do calculate everything out, doing the above idea might be fun just to see how close it really is.

[docholiday]

Originally posted by scouts22
I'm a slightly confused homeowner on calculating the balance point. I recently had installed a Trane XL16i Heat Pump, XV80 Two-Stage Variable Speed Gas Furnace and Trane XL802 stat (same as Honeywell VisionPro 8000).

I am in the Kansas City area and the electric company, KCP&L, recommends 30 degrees. I guess they benefit with a lower balance setting. My installer recommended and set 40, saying that's what most customers seem to like. That seems too high and far from the 30 from the electric folks. I also called the Trane distributor. He told me he personally uses 30. I'm inclined to use 30 but am curious if anybody has any other thoughts. The HP (up to 10.55 HSPF) puts out 48,000 BTU and the gas furnace 52,000/79,000. Insulation is probably average for a 20 year old house. Average of last two year's temps:

Month AvgTemp AvgHi AvgLo

Nov.......43.........51......34
Dec......33..........41......23
Jan.......27.........35......19
Feb......40..........51......30
Mar......54..........60......40

Gas will be about $12/Mcf come December. Electric is about $.04/kWh in the winter for heat pump owners.

Also, the indoor stat does have an outdoor sensor but I think the balance point is run from the outdoor stat. Is it preferable to run the balance point from the indoor stat? Thanks.






I woulda went straight HP.

Really, why mess with an outdoor stat, let second stage call the shots.

[scouts22]

Thanks to all that replied. The installer did not do a J calc but I will call and ask for one.

I agree with "amickracing" about basically doing trial and error till finding the sweet spot. Unfortunately, I can't keep calling the installer back to change it every week till I find the right point. I'm fairly handy and wouldn't mind doing it myself but the installer said it was not a homeowner adjustment. I couldn't find anything in the furnace or HP install manuals about this.

I did not understand "junkhound's" calculations, possibly due to this web page crunching tables so they are incomprehensible (need to put dots or dashes between data).

I did understand "deme's" calculations but don't know my BTUs required. Again, I'll check with the installer. My house is a 2-story with about 3,000 sq ft and average insulation for a 20 year old with builders grade single pane and storm windows. Getting new windows next spring.

[deme]

Originally posted by amickracing
scouts22, personally I think 40 is WAY high, 30 likely is even high.

Just an idea to throw out there for y'all to consider and hopefully not flame me too much. When you are calculating the heat loss some of what you have to do is guessing. Also if you do it per the manual J, it's going to be a big higher than the actual loss (the cya factor).

What I've always wanted to do is just run the heat pump all of the time and keep track of the house's temp. Once you notice that the house isn't warming up any more (by looking at a thermometer or the stat), then you know the approximate balance point. Since efficiency varies with how it's installed, heat loss factors sometimes become more of a guess than an actual fact, and could help those math challenged.

Once you found the point that way maybe add 5 deg or so to that temp and use that for the cut off point.

Even if you do calculate everything out, doing the above idea might be fun just to see how close it really is.

I had the opportunity to do just that a couple of days ago.
We had foggy weather with 100% relative humidity just above 35*F.

I dropped the balance point temperature from 40*F to 35*F and jacked up the house temp. thermostat setting by 2*F to see the reaction. the heat pump operated for about 1 hour 15 minutes, give or take a couple of minutes, and became completely covered with frost. After defrosting (set at 90 minutes) there was less frost buildup during normal operation.

I'm finding that with heat sources in the house, not accounted for in HVAC-Calc, 35*F balance point seems to be ok in the day time. It may be too low at night however night temperature is set back from 68*F to 64*F and heating requirements are reduced.

The house thermostat is a Carrier programable dual fuel thermostat which handles night setbacks well (so far).

The heat pump balance point is set at the Carrier programmable dual fuel thermostat but is not given in the "owner's manual" however is given in the installation manual along with other settings such as Anticipator adjustment, clean filter adjustment, Room temperature offset adjustment, etc...

[lyork]

I too live in the Kansas City area, 2800 s.f. house and a Lennox HP26 4 ton heat pump. 30 degrees is the general concensus for the cut-off on the heat pump in our area. My house was built in 1981, however I have had new windows installed. When you start talking balance point for my house, it is appearing to be around 20 degrees. That being said we have to start conceding things to use that as a cut off temp. Don't set back the stat at night more than 1 degree, (I don't set back at all once the temps aren't going to rise above 40 during the day). Consider the amount of activity in the house in regards to lights, applianes, hot showers running, they all make a small, yet significant impact on the ability to maintain the temp in the house. At 20 degrees outside and stat at 70 inside, the heat pump is going to run almost constantly to maintain the inside temperature. I have one advantage over your install though, My Robertshaw 9720I stat has adjustable balance points on it, it also has an adjustable delay before calling for second stage heat (gas furnance) that is on a timer set at 45 minutes. So if the stat isn't satisfied after 45 minutes, it locks out the heat pump and kicks on the furnance. Since your balance points need to be set by your installer, I would consider having him set it in the 25-30 degree range. This is just my opinion though.

[subdewd]

I keep lowering mine. Started at 40 dropped to 35,30
now @ 25 in st. louis. 2 story 8"cement block with 4"
brick. New windows and 8" blown in attic. Was down to
20 last night so gas finally kicked in. With all the thermal
mass the HP still only runs for 15 0r 20 min. Tstat set 68
days 65 nights.

[SBuckJ]

Here's a somewhat silly question for all the professionals. If you install a dual fuel system, other than the variable speed, is there ANY benefit to the two stage unit. Seems to me that the heat pump would basically be the first stage if the system is designed properly and the first stage would operate minimally if at all.
SRJ

[deme]

In a previous post I mentioned that I reduced the balance point setting from 40*F to 35*F for experimentation.

Since then, I calculated the additional saving realized by going to 35*. It amounts to about 60 bucks per year.

Not worth the effort; going back to 40*F!

[dan sw fl]

Originally posted by subdewd
I keep lowering mine.
Started at 40 dropped to 35, 30 now @ 25 in st. louis.

2 story 8" cement block with 4" brick.
New windows and 8" blown in attic.
Was down to 20 last night so gas finally kicked in.
With all the thermal mass the HP still only runs for 15 0r 20 min.
Tstat set 68 days 65 nights.

I agree with DocHoliday.
.......

My opinion ( at his point):
Set balance point to 22'F
(using 10 kW [ 34,120 BTUh ] electric strip heater
as Aux Heat)
with LOW, LOW, LOW, LOW KC P&L rates$

$12/ MCF = $ 0.04/kW as Straight Electric

TRANE XL16i model 4TWX 6048
O.A temp_ C.O.P._ Capacity
47'F ____ 3.82 ___ 45,000 BTUh
35'F ____ 3.08 ___ 33,156 BTUh
17'F ____ 2.76 ___ 27,600 BTUh

Even at 22'F with heat pump running and AUX Heat electric strip you are running much more Economical than nat. gas.

Let us know how you adjust your set-up.

ABSOLUTELY GREAT Equipment Selection!

[tedkidd]

On graph paper, draw 1 line showing heat loss of the house and another line showing output of the heat pump. Since heat loss will increase as it gets colder and heat pump output will decrease, there will be a point where they intersect. That's your balance point.

But most guess :(

Nice.

I'd say your thermal balance point is when Mom calls on a 9f evening saying: although the house is maintaining 70f, the air at the registers feels cold.

(you get your butt to mom's and program the lockout to 35. You thank the installer oversight as you learned a valuable nugget about mom's house, and hybrid heat in general.)

[beenthere]

Ted. This is an almost 6 year old thread.

[tedkidd]

Lol, i was referred to it somehow today...

[seatonheating]

Try this guys.

I realize this is an old thread, but feel free to post it elsewhere for all to see:


Dang, can't post an excel spreadsheet. Just email me if you want to see it :).