Heat Pump vs Wood burning...

[Hankjones]

I hope to add to this body of knowledge. Seems like a few folks on this forum like numbers, graphs and charts...

Basically I tested out a Heat Pump vs Heating with wood in a 1200 sqft split level in VA. House is well insulated and a Lopi Revere is in the family room downstairs. I keep the thermostat at 70F. I've got marginal wood, some dry poplar 15% moisture and some hickory and red oak 20-25% moisture. Picked today to test because it was overcast and around 30F throughout the day from 9am to 7pm it was 28 to 30F according to weather.com and 30-32F according to my own measurements. Did the HP test first from 9am to 2pm, heat pump has been running all night, so the whole house was pretty much the same temperature.

Heat Pump 9am to 2pm
1) heat Pump cycled on and off 7 times. When it was on, it used 4kWh, when it was off the house used about .7kWh
2) From 9am to 2pm, I used 15kWh, all rooms were around 69 to 70F
3) Outside temp according to my measurements was between 31F and 32F
4) Electricity cost: $1.65 ($.11/kWh)

Wood Heating from 2pm to 7pm
1) started from pretty much a cold stove, 3 pieces of hickory/red oak at 20-25% moisture, a handful of chopped poplar to get things going.
2) Turned off Heat Pump and just heated with insert
3) Ended up using 2 loads of wood, approx 2.3cuft of mixed wood.
4) At 7pm stovetop was around 475F, and seemed to be locked in for another hour or so. Probably could have heated the house for another 2hrs
5) If I burned 24/7 I would probably have less wood usage
6) If I had really dry wood, I probably would have used less wood.
7) The room with the insert ranged from 70-75F, other rooms ranged from 67-69F.
8) Electrical was around .7kWh during the time, ended up measuring, 4kWh usage
9) Total Cost: Electricity:$.44 Wood: probably around 5 splits would have done the trick, if they were dry maybe less.

So.... For me... are 5 splits of wood worth $1.21 (not including all the equipment, hand splitting, and scrounging/stacking etc...)? …. So far...........



yes.... wife is happy.






Kitchen is on the ground level
Lodge is where the insert is.
Laundry 3 is a heat probe attached to ducts

 

[Troutchaser]

That's interesting Hank. We also have a heat pump and haven't turned it on since early Dec.
Now I'm wondering how the electric bill still topped $225 last period.

 

[pen]

Sweet graphs!! Woot for those!

I don't have a heat pump but for me, I've burnt about 2 cords of wood so far this winter. I didn't have to purchase them but had I the cost would have been 300.

So far this winter I have used 1 gallon of oil to heat my house (I turned the boiler on 2x for 20 minutes to make sure it still worked)

I do have 1, 6ft electric radiator that I will turn on if it is extremely cold. With that running to help keep the house at 66-70 it costs roughly a dollar a day. I bet I have spent maybe 10-15 dollars on electric to run that thus far.

If I had to buy oil to do the same heating, I bet it would have cost me upwards of 700 - 1000 dollars so far this winter and my basement family room wouldn't have had heat.

I can't imagine being w/out wood.

pen

 

[nola mike]

Interesting. Like you said, starting with a cold stove probably hurt. But if you used 2.3 cu ft, that's $2.70 worth of wood @ $150/cord. Also, at that rate you'd go through 2.6 cords/month, which seems excessive. I've gone through maybe 1/2-2/3 cord with wood being the primary source since the end of December (I'm curious as to what my gas bill will be).

 

[cmnash]

Hank,

Does your price per kwh include the distribution cost ? Here in Connecticut, our real total kwh cost is approx. 19 cents (generation plus dist. plus fees,etc.),
2nd highest in the nation.

Those geo thermal heat pumps love the amps too much; mine spiked my electric bill big time.

3 years ago I crowded on 36 solar panels on my south facing barn roof.

Now I'm at peace with it all. I generated 38 kwh yesterday under full sun.

 

[Hankjones]

That's interesting Hank. We also have a heat pump and haven't turned it on since early Dec.
Now I'm wondering how the electric bill still topped $225 last period.

When I was burning a few days straight, my heat pump would turn on to go into defrost once in awhile. A hvac guy said it had to do that to make sure some liquid wouldn't freeze. It drove me nuts for awhile since I was heating with all wood and I still had some electricity kick in for the heat pump even though it wasn't heating the house.

 

[Hankjones]

Interesting. Like you said, starting with a cold stove probably hurt. But if you used 2.3 cu ft, that's $2.70 worth of wood @ $150/cord. Also, at that rate you'd go through 2.6 cords/month, which seems excessive. I've gone through maybe 1/2-2/3 cord with wood being the primary source since the end of December (I'm curious as to what my gas bill will be).

I think my main problem may have been the wood, it was a bit hard to start. When I used some well seasoned wood, they just took off. The other thing was I probably could have gotten another two hours of heat from that load.

When you're heating, when do you reload? I'm reloading when the stovetop goes down to 400f to try to keep it around 400 to 650.

 

[DBoon]

That’s interesting Hank. We also have a heat pump and haven’t turned it on since early Dec.
Now I’m wondering how the electric bill still topped $225 last period.

Perhaps what you mean is "We have a heat pump and I haven't turned it on since early Dec." Who knows what the wife and kids are doing when you are not around...

 

[maverick06]

$0.11/kwh... lucky, ours is about $0.164/kwh. Its expensive. (That includes all taxes/generation/transmission/etc).

Very neat data, How did you measure amp draw of the heatpump? I cant seem to get mine to work with my kill-a-watt meter

 

[begreen]

I will have to check ours. Electric bill for Jan was $111 with fairly frequent heat pump usage.

 

[henkmeuzelaar]

Hank,

Great graphs!!

Can you describe your monitoring set-up, please?

Henk

 

[Hankjones]

I used virtual weather station and Oregon scientific sensors, something like this package


http://www.ambientweather.com/wmorscwiprwe.html

 

[daleeper]

Interesting test. I would be interested in seeing it run with the wood stove in the morning, and the heat pump in the second shift to see what happens. I am guessing the heat pump would win in that scenario, as the warmer air would help it out, and less heat demand in the afternoon with a house that is warmed by afternoon/evening activities.

 

[Lanningjw]

In the Minnesota cold weather the Air heat pump has not caught on. We had a one installed with a 95% NG furnace. The heat pump worked great, it switches oner at 25 degrees to the NG furnace.

After it snowed over 60 inches the heat pump was full of snow and would not work on days when it got over 25 degrees. I poured hot water on it to melt the snow, it works now.

 

[fdegree]

When I was burning a few days straight, my heat pump would turn on to go into defrost once in awhile. A hvac guy said it had to do that to make sure some liquid wouldn't freeze. It drove me nuts for awhile since I was heating with all wood and I still had some electricity kick in for the heat pump even though it wasn't heating the house.

There are folks on here that are more knowledgeable than me when it comes to heat pumps, but this does not sound right to me. If the heat pump has not been running, there is nothing that will freeze...the defrost cycle prevents moisture, that has formed on the outside of the coil while it is running, from freezing...it does not prevent anything on the inside of the system from freezing. The only thing on the inside of the system is refrigerant (a.k.a. freon) and oil, neither of which will freeze under normal conditions.

The defrost cycle should only come on after the heat pump has been running for a set period of time...30, 60 or 90 minutes of cumulative run time...which ever it happens to be set for. If the heat pump has not been running, it could not have reached that predetermined run time and should never have gone into a defrost mode.

Either it is running without you realizing it, or there is something wrong...at least that's the way I understand it

 

[WhitePine]

There are folks on here that are more knowledgeable than me when it comes to heat pumps, but this does not sound right to me. If the heat pump has not been running, there is nothing that will freeze...the defrost cycle prevents moisture, that has formed on the outside of the coil while it is running, from freezing...it does not prevent anything on the inside of the system from freezing. The only thing on the inside of the system is refrigerant (a.k.a. freon) and oil, neither of which will freeze under normal conditions.

To my knowledge, no heat pump of mine has ever gone into defrost on of its own accord when it hasn't been running. I also see no reason for it.

Minor quibble on the explanation, the defrost cycle does not normally prevent moisture from freezing. Instead, it thaws moisture that has already frozen. That's why its called defrosting.

The defrost cycle should only come on after the heat pump has been running for a set period of time...30, 60 or 90 minutes of cumulative run time...which ever it happens to be set for. If the heat pump has not been running, it could not have reached that predetermined run time and should never have gone into a defrost mode.

Basic heat pumps tend to have timed defrost cycles. More expensive units sense the need to defrost and switch to defrost mode only when needed (demand defrost). [/quote]

 

[DanCorcoran]

Is it possible that the heat pump that needs defrosting is a geothermal unit, with a liquid that circulates? My purely electric heat pumps never defrost themselves.

 

[begreen]

All heat pumps are purely electric. Just different heat exchangers. Something does seem amuck though. Or this is a really big barn being heated.

 

[Hankjones]

Rats... I thought something was wrong, that's why I called the hvac guy, now I'm starting to think I need to look into it again. The problem is I have to wait to next winter. Anyone know if a heat pump from the 1980's would work differently?

 

[WhitePine]

Rats... I thought something was wrong, that's why I called the hvac guy, now I'm starting to think I need to look into it again. The problem is I have to wait to next winter. Anyone know if a heat pump from the 1980's would work differently?

My guess is you have either timed defrost and an incorrectly wired heat pump, or a stuck Snap Disc or Thermodisc control putting power to the unit when it shouldn't be there. Those things fail quite often, sometimes sticking closed.

However, it's possible that the factory design is to blame, and the engineers just weren't thinking too far down the road. A really good HVAC guy could either fix or rewire it for you, or at least be able to give you a lucid explanation of why it is the way it is and why it needs to stay that way.

Any 80s heat pump has lived well beyond its design lifespan. It's probably due for replacement anyway. Any new unit would be much more efficient. Have you considered replacing it?

 

[cottonwoodsteve]

We have a Bard PH24 made in 1980. When it is OFF it is OFF. It does not go into defrost or nothing runs on its own whims.
Technically it is slightly ON since you have a 220V to 24 volt transformer that keeps your thermostat ready to respond.
When it'a running, at temperatures from 40 degrees or lower, you outside coil gets so cold it will condense and freeze the moisture out of the air. This builds up ice on the outside coil and blocks the air flow through it. When it is running at colder temps, a temperature sensor clipped to the coil will tell the system to activate the defrost cycle every hour. The inside fan shuts off. The heat pump runs backward and warms up the outside coil to melt the ice. Then after 10 minutes it goes back to normal.

To keep the oldie running I reccommend replacing the fan and compressor control relays. They are called contactors. A fancy name for a big relay. They are about $18 each if you can do it yourself. Also ours was installed with no access to the intake side if the inside condenser coil. A little bit of dust and lint gets buy the house filter.
After 30 years it was so full of lint you could not see the fins. The system was only flowing about 30% of what it could do. I cut neat weatherproof access panel in the duct as it go through the roof. I measured my hand held vacuum to know what size to make them.
On some of the old systems the compressors last forever. So if you fix some of the cheap problems the main expensive parts will keep going. Also check your fan and blower for balance. Ours used to shake and rumble the house. I bent the fan blades so they all follow the same track. The blower would not bend so I added small pieces of medium lead solder to it. Now the system purrs. We also have a Bard PH1130 at the other end of the house. It had problems with worn and burnt contactor points. I replaced them. Also did the coil vacuum access thing to that one. That one is only 20 years old. The average service person seems to recomend anything after 10 years old you should replace it. They just to also happen be installers. Hmmmmm.....

 

[Hankjones]

I did think about replacing the heat pump, but during July and Aug, my electric bill was never above $130. During this winter (i was burning on and off) my electric bill didn't go over $170. I think i'd replace it when it dies, but apparently you can't stop a trane...

I guess on the other hand, like what cottonwoodsteve says, maybe i should really learn to maintain this thing.

Here's a graph of what I found distributing and made me call the HVAC guy:

I was burning all day, but noticed that the heat pump was doing something.... The blue line measures outside the ducts right after blower. Like clockwork, you can see a slight temp increase every 90min or so. The temperature would go up, but the blower wouldn't be running. At 5pm, you can see a huge spike. That's when i saw the temp increase so i manually turned on the blower. Now in hindsight..... i think i know what's going on... Kinda of a brain fart. I may not have turned off the heat pump entirely. I think what I may have done was set the temp to about 60F and because i was burning wood, it would never kick in. The heat pump probably went into defrost since it was cold and wet outside even though it was never heating the house. Maybe next year i will turn it off completely and save a few more bucks!

 

[begreen]

That's odd. Our's never comes on for just defrost. It can sit idle for days if we are burning hard and it's cold outside.

 

[fdegree]

That seems odd to me too. I'm not sure I follow exactly where you are taking the temperature for the blue line. But, when the heat pump goes into defrost, it is actually running the refrigerant (freon) in the same direction it would normally go when in the cooling mode. Which would mean the temperature you are monitoring should drop a little when the defrost cycle happens. Unless the electric heat is coming on at the same time...which is scary if your fan is not running.

I doubt a heat pump that old would have an automatic defrost that energizes itself based on outside air conditions...maybe I'm wrong. I would think, since you have your t'stat set below room temperature, nothing on the heat pump should be coming on...not even the defrost cycle.

I think something is definitely wrong, and maybe a little dangerous IF the electric heat is coming on without the fan.

 

[cottonwoodsteve]

Yes, something is not right.
We do the same thing with the thermostat low and the wood stove going. The heat pump does not do anything, including defrost, if the wood stove keeps the house above the low temp set on the thermostat. Also like fdegree said the temp should go lower.
I think it is a thermostat or thermostat wiring problem. Maybe go to the manufactures site and see it they have a F.A.Q. area or trouble shooting area. The heat pump has wires labled W, Y, R etc. See if these are conected to the proper terminals in the thermostat. Also make sure they are not shorted or open (broken).
Make sure the wire labled G goes to the thermostat input G etc. Do not assume the green wire is wired to the G terminal in the heatpump.
Another problem is a relay might have stuck contacts. They get pitted and rough from constant arcing on and off. A little peak of a rough spot makes so much heat because of a poor connection it will make a tiny weld to the other contact. The open spring is very weak so it can not overcome the slight welding. So you might have a relay ON even though the thermostat is not doing anything.
Take a plastic handle of a screw driver and give each relay many firm raps from all sides. We had a relay stick and the inside fan just kept running forever. I got up on the roof, opened up the electrical side and ***** it with a screw driver and solved the problem. Then I ordered a new one.
I have attached pictures.
Small relay on left stuck, but it is sealed so you can't see what the contacs are doing. But I knew just how to hit it :>)
The large relay is for the auxiliary heat. The contact bar actually broke off from heat and fatigue. The lower contacts were not used so I just moved the wires down to the "new " set.