balance point for duel fuel system

[docholiday]

You will be hard pressed to find a place in the US where the HP part (above balance point) of a duel fuel costs more to operate than a gas only system. Once you get below the balance point, the fuel is generally less expensive than straight electric heat.

[Lorne]

Yes I will save money, under a hundred based upon average bin.

[lksmith]

Rog09,

How are you going to change the droop from the 2 degree setting to 1 degree of droop? I didn't see that as an option on the IAQ.

Larry Smith

[rog09]

lksmith

There is no way to reduce the 2 degree droop. It can simply be turned on or off and 'configured' to do 2 degree droop by itself or have 2 degree droop and the upstage timer. Typically, if the upstage timer is setup, the upstage timer would be the threshold, not the 2 degree droop (assuming you are near the capacity balance point, which would be why you would have droop in the first place)

[Lorne]

Originally posted by dan sw fl

Originally posted by Lorne
We will (in IL)no longer have an all electric rate everyone pays 10.5 KW

this has made a dual option not cost effective unless we have a mild winter. I just installed mine last year. #$%^

3412 ____ BTU/ kW
3.0 ______ COP
10236 ____ BTU /Heat Pump

100000 _____ BTU / Therm
9.77 ______ kW / Therm
0.105 ____ $ /kW
$1.03 _____ $ / 100,000 BTU
____
100000 ____ Therm Gas
0.9 ______ Efficiency
____
$1.20 ____ Therm Cost
$1.33 ____ Effective Cost

COP should average > 3.0 for > 25'F,
so electric would be cheaper
when the rate is < $0.14/kW
and gas is $ 1.20 / therm

So what is the economical balance point for 2.9 cop at 25 degrees versus $.83 therm

[rog09]

plot your COP's on a graph...find the equation of the line (not exact as COP's are not a linear line, but close enough). Set that equation equal to Cost of Gas/Cost of Elec and it will give you your economic balance point (or just read it from the graph to approximate).

Based on your #'s above, $.83/therm for gas would mean the economic balance point would be above 25...

[dan sw fl]

Originally posted by Lorne

Originally posted by dan sw fl

Originally posted by Lorne
We will (in IL)no longer have an all electric rate everyone pays 10.5 KW

this has made a dual option not cost effective unless we have a mild winter. I just installed mine last year. #$%^

3412 ____ BTU/ kW
3.0 ______ COP
10236 ____ BTU /Heat Pump

100000 _____ BTU / Therm
9.77 ______ kW / Therm
0.105 ____ $ /kW
$1.03 _____ $ / 100,000 BTU

So what is the economical balance point for 2.9 cop at 25 degrees versus $0.83 / therm

Efficiency 85%
Cost per Therm = $ 0.98

About 29'F may be the economic balance point,
C.O.P. at 17'F, 35'F and 47'F and
burner efficiency were not provided.

For example, at x'F heat pump cost would be:
$ 0.105/ kW
'F _ C.O.P. _ $/ Therm
47 .. 4.40 $0.699 100000 Btu/ Therm
46 4.33 $0.712 3412 BTU/ kW
45 4.25 $0.724 29.308 kW/ Therm
44 4.18 $0.737
43 4.10 $0.751
42 4.03 $0.765
41 3.95 $0.779
40 3.88 $0.794
39 3.80 $0.810
38 3.73 $0.826
37 3.65 $0.843
36 3.58 $0.861
35 .. 3.50 $0.879
34 3.44 $0.895
33 3.38 $0.910
32 3.32 $0.927
31 3.26 $0.944
30 3.20 $0.962
29 3.14 $0.980
28 3.08 $0.999
27 3.02 $1.019
26 2.96 $1.040
25 .. 2.90 $1.061
24 2.88 $1.070
23 2.85 $1.080
22 2.83 $1.089
21 2.80 $1.099
20 2.78 $1.109
19 2.75 $1.119
18 2.73 $1.129
17 .. 2.70 $1.140

So, switchover in the range of 27' to 30'F
if the efficiency and C.O.P. of the example
are close to your situation.

[hp_dude32]

In a lot of these posts the energy prices seem a bit low...


You should always include any delivery or system costs in the prices for electricity, or gas, as the base CCF or kWh rate is misleading. Delivery and other fixed fees can be up to 40%-50% of your gas/electricity bill.

These costs are not proportional for electricity or gas, so each needs to be calculated separately.

[brjones]

When figuring my cost per KWH I just take the total bill and divide it by the total KWH's used. That way it includes all the taxes, fees, demand charge, and all the other misc charges. The total is the only one that counts!

Last winter it averaged about 3.2 cents per KWH total. Now you see why I never want the HP locked out and even removed the gas furnaces and replaced them with FE AH's.

Propane has been 1.50 per gallon or more and elect 3.2 cents KWH. Easy choice in my case. We have had a rate increase since. It is suppose to be only 3-4% but it also included a cost of fuel clause which makes it somewhat variable. I am still very happy with this system.

[hp_dude32]

3.2 cents, all inclusive, per kWh is outstanding!

What state/area are you in that offers power so cheap??


At 2000kWh per month, we pay about 11 cents per kWh, all inclusive. (the rate will drop slightly if you use more than that, but not substantially...)

What are your area gas costs?

[brjones]

The wheat state!

Everyone should always check to see if there are any rate breaks for going total elec or if other favorable rate structures are available.

I am on a demand rate which meters the peak demand during a 30 min period each month.

Since the rate increase this year my summer demand only runs about 11-12 KVA and my total cost per KWH averaged about 6.6 cents. The demand charge is a lot higher during the summer months. Last winter the demand ran about 20-22 and I was using well over 3000 KWH's.

I have not followed nat gas prices but I know they are still high and I hate the way the gas price can double or triple in no time. I had a dual/fuel system at my last house and within 2 years of installing it the cost of gas went sky high.

Electicity will go up but at least when it does it is somewhat predictable or at least it has been and with the regulators overseeing it I doubt it will make any drastic changes.

[oldsouth]

I am installing a dual-fuel system to replace my aging pulse gas furnace. The heat pump is a 2-speed 5 ton with a HSPF of 8.4 and a seer of 14.0. The furnace is 95% efficient. Electricity cost .0952/KWH and natural gas cost 1.47/ccf. Under those conditions what temperature would be the point where the gas unit should run more efficiently? I failed to mention I am also setting it up to run in a zone type system with an EWC control UZC4. The control can act as the dual fuel control or let the thermostats do it. I will be using Honeywell Vision Pro 8000 series thermostats.

[dan sw fl]

I am installing a dual-fuel system to replace my aging pulse gas furnace.

The heat pump is a 2-speed 5 ton with a HSPF of 8.4
and a seer of 14.0. The furnace is 95% efficient.
Electricity cost .0952/KWH and natural gas cost 1.47/ccf.

Under those conditions what temperature would be the point where the gas unit should run more efficiently?

Gas 1.47 * 0.95 = $1.40 per Therm ( 100,000 BTU)

Economic balance point is ~ -3' F
which does not take into account defrost cycle.

kw = 3412 BTU, Therm = 29.31 kW, Rate * $0.0.95 / kW
$2.78 for 100,000 BTU electric

Effective Cost = Therm Cost / C.O.P.
C.O.P. for a typical 2-stage heat pump
might be about 2.6 at 17'F

'F __ C.O.P. __ $/ Therm
47 3.60 0.77
46 3.53 0.79
45 3.46 0.80
44 3.39 0.82
43 3.32 0.84
42 3.25 0.86
41 3.18 0.88
40 3.11 0.90
39 3.04 0.92
38 2.97 0.94
37 2.90 0.96
36 2.89 0.97
35 2.87 0.97
34 2.86 0.98
33 2.84 0.98
32 2.83 0.99
31 2.81 0.99
30 2.80 1.00
29 2.78 1.00
28 2.77 1.01
27 2.75 1.01
26 2.74 1.02
25 2.72 1.02
24 2.71 1.03
23 2.69 1.04
22 2.68 1.04
21 2.66 1.05
20 2.65 1.05
19 2.63 1.06
18 2.62 1.06
17 2.60 1.07
16 2.57 1.08
15 2.54 1.10
14 2.51 1.11
13 2.48 1.12
12 2.45 1.14
11 2.42 1.15
10 2.39 1.16
9 2.36 1.18
8 2.33 1.19
7 2.30 1.21
6 2.27 1.23
5 2.24 1.24
4 2.21 1.26
3 2.18 1.28
2 2.15 1.30
1 2.12 1.31
0 2.09 1.33
-1 2.06 1.35
-2 2.03 1.37
-3 2.00 1.39
-4 1.97 1.41
-5 1.94 1.44

[oldsouth]

Wow! Thanks for the information Dan. Since I live in Louisiana and it hardly ever gets below 20°F, I should never use the gas furnace when the heat pump is keeping up with the heating. I guess it will just be trial and error as to what outside temperature the heat pump will have trouble keeping up. I know it is not really recomended but I eventually plan to try to heat and cool my house with one 5 ton system. I have a 2 story house with 2204 sq ft per floor with a 3rd story playroom with 306 sq ft. Ceilings are 12' on 1st floor and 10' on second and 13' in playroom with insulated windows around the sides. House is very well insulated. Presently I have a 5 ton 2-speed unit with pulse furnace per floor and a 2 ton unit in the playroom. The 2-5 ton units have NEVER operated on high speed and really don't run very much on low before they kick off. My 1st floor unit bit the dust and what I will do next week is replace it with what I previously described and zone it to the playroom also. So I will be running two 5 ton units, one new and one old. As soon as the 2nd floor unit goes out, I plan to zone the whole house to the one 5 ton and try it for a while. If it is too much for it to handle, I can just go back and replace the 2nd floor unit. All units are in the attic and are easily zoned.

[jkish]

Gas 1.47 * 0.95 = $1.40 per Therm ( 100,000 BTU)

Shouldn't the effective cost of the gas be $1.47 / .95 or $1.55 per Therm?

You have to burn more gas to get a Therm of output since the the furnace is less than 100% efficient.

[dan sw fl]

Since I live in Louisiana and it hardly ever gets below 20°F, I should never use the gas furnace when the heat pump is keeping up with the heating.

I guess it will just be trial and error as to what outside temperature the heat pump will have trouble keeping up.

I know it is not really recomended but I eventually plan to try to heat and cool my house with one 5 ton system. I have a 2 story house with 2204 sq ft per floor with a 3rd story playroom with 306 sq ft.

Ceilings are 12' on 1st floor and 10' on second and 13' in playroom with insulated windows around the sides. House is very well insulated.

... , I plan to zone the whole house to the one 5 ton and try it for a while.

One 5-ton should be more than adequate for the 2-story house you describe.
Why not just go with the 2 zone system and new 5-ton heat pump Now?

< 50 hours at less than 27'F for Alexandria LA,
so a furnace may be just an adder that is NOT Useful.

[oldsouth]

Because I just replaced the compressor in my 2nd floor system last summer. Thought I would get all I can out of it before it goes to leaking like the 1st floor eq. This year the inside coils started leaking in my 1st floor equipment so I must replace it now. The 3rd floor compressor cycles a lot and is making some funny noises so that is why I will zone it now with 1st floor eq.

Thought that I needed more than just a heat pump on heating. The high ceilings really hurt in cold weather. Would be more trouble to run wires for backup heat for heat pump as opposed to gas. The gas is there now. The difference in initial price doesn't bother me. I am new to heat pumps and they will have to prove themselves to me.

What I am still unsure of is whether to let the thermostat control the heating or allow the zone control to do it. The thermostat is a 2 cool and 3 heat stat. I will have 4 possible heat stages. Heat pump has high and low and the furnace has high and low. So I think it would be best to see at what outside temp the heat pump has to struggle to keep up and have the zone control switch over to gas at that point.

[oldsouth]

Dan, I took your advice and hooked every zone up to the single new unit. It cooled the entire house all summer. It would go to second stage when the outside temp got above 90&#176; but would still catch up and cycle off at over 100&#176;. I am very pleased. Well now we are getting into colder weather and the heat came on for the first time yesterday. I have one question, can you wire the 1st stage of gas heat (really 3rd stage overall) to come on when the heat pump goes into defrost cycle? The thermostats are the Vision Pro IAQ -- 3 of them, wired to an EWC zone controller. The way it is currently set up is that 1st stage heat comes on then goes to the next stages as the thermostat calls for it. EXCEPT -- the EWC goes to 2nd stage heat after 15 minutes of continous operation. It will not go to the gas heat unless the thermostat calls for it or the outside temperature goes below the temp that I set it to change over. After it goes to gas heat the same 15 minute period applies to ramp up to 2nd stage gas or 4th overall. Verl slick! I would just like to heat the defrost cycle air and not let it blow cold. But I do not want to alter any other settings.

[docholiday]

I wouldnt tie the gas in during defrosts. Where you live you'd be hard pressed to see defrost cycles last over 3 or 4 minutes for the most part.

[oldsouth]

So they are not a certain timed event? The unit uses defrost only as long as needed?

I remember on the outside unit, there is a dip switch that is set to 30 minutes between defrost cycles.