How the heck does this thing work?

[ToddB10k]

We bought a house in July, and the house is 20 years old. Which means a lot of the HVAC system is also 20 years old. And we have single pane windows - and they're large windows. The house is two story, about 2700 square feet, brick all four sides all the way up.

When we moved in I did daily readings on the electric meter for about a week and knew I was in for a nice cooling bill - first bill was 3700 KW/H. Simply due to the age of the system, I know we'll be replacing it soon, but gotta save a few bucks first.

Last night I tried putting the heat on and had a heck of a time - the system has a heat pump and a furnace and I don't know anything about heat pumps, especially when there is also a furnace. Finally gave up and went to bed. Tonight I investigated a little further - turns out when the plumber fixed a gas leak for us he forgot to turn the gas back on to the two furnaces and fire up the pilot lights. Both are manual light pilots - may just call a guy to do that... May just do it myself... Still weighing my cheapness versus my laziness...

But last night was also when I learned a bit about heat pumps. Not a fan of them - AC costs enough in the summer, so I sure don't want to run the thing backwards to get heat during the winter.

In the interim, I have a few questions and hope someone can give me answers.

1. Not quite sure how the thing works on the HEAT mode (as opposed to the SUPL HT mode) - the adjuster has two mercury bulbs in it, and looking at how the thing works and based on reading some things last night, it seems like if it's just a little bit cold in the house the first bulb will trip over and fire up the heat pump. If the heat pump can't do the job and it keeps getting colder then the second bulb trips over and fires up the furnace. Please let me know if I'm even close on my guess here!

2. If I run the thing in the SUPL HT mode, does this bypass the heat pump altogether and just run the gas furnace? This is my preference!!

3. Is there some way to find out the efficiency of the AC units and the gas furnaces?

This is what we have:

Outside Unit 1 - Carrier 38YK048310
Coil 1 - Carrier CE3AXA048T00AAAA
Gas Furnace 1 - Carrier 58PB100150JA
Heat Pump Thing Mounted on Furnace - Carrier Optimizer 2 - 38YH900070
Thermostat - Carrier T874G1055 (HH07AT171)


Outside Unit 2 - Bryant 113RNA030-E
Coil 2 - Carrier CE3AXA030T00AAAA
Gas Furnace 2 - Carrier 58BP050150GA
Thermostat - Carrier T874G1055 (HH07AT171)

Anyway, thanks for any info you might have!!

[pacnw]

We bought a house in July, and the house is 20 years old. Which means a lot of the HVAC system is also 20 years old. And we have single pane windows - and they're large windows. The house is two story, about 2700 square feet, brick all four sides all the way up.

When we moved in I did daily readings on the electric meter for about a week and knew I was in for a nice cooling bill - first bill was 3700 KW/H. Simply due to the age of the system, I know we'll be replacing it soon, but gotta save a few bucks first.

Last night I tried putting the heat on and had a heck of a time - the system has a heat pump and a furnace and I don't know anything about heat pumps, especially when there is also a furnace. Finally gave up and went to bed. Tonight I investigated a little further - turns out when the plumber fixed a gas leak for us he forgot to turn the gas back on to the two furnaces and fire up the pilot lights. Both are manual light pilots - may just call a guy to do that... May just do it myself... Still weighing my cheapness versus my laziness...
As long as it actually is a manual pilot, has instructions that you can follow, you should at least give it a try.

But last night was also when I learned a bit about heat pumps. Not a fan of them - AC costs enough in the summer, so I sure don't want to run the thing backwards to get heat during the winter.

In the interim, I have a few questions and hope someone can give me answers.

1. Not quite sure how the thing works on the HEAT mode (as opposed to the SUPL HT mode) - the adjuster has two mercury bulbs in it, and looking at how the thing works and based on reading some things last night, it seems like if it's just a little bit cold in the house the first bulb will trip over and fire up the heat pump. If the heat pump can't do the job and it keeps getting colder then the second bulb trips over and fires up the furnace. Please let me know if I'm even close on my guess here!This is how it SHOULD work.

2. If I run the thing in the SUPL HT mode, does this bypass the heat pump altogether and just run the gas furnace? This is my preference!!
Again, this is how it SHOULD work.

3. Is there some way to find out the efficiency of the AC units and the gas furnaces?

This is what we have:

Outside Unit 1 - Carrier 38YK048310
Coil 1 - Carrier CE3AXA048T00AAAA
Gas Furnace 1 - Carrier 58PB100150JA
Heat Pump Thing Mounted on Furnace - Carrier Optimizer 2 - 38YH900070
Thermostat - Carrier T874G1055 (HH07AT171)


Outside Unit 2 - Bryant 113RNA030-E
Coil 2 - Carrier CE3AXA030T00AAAA
Gas Furnace 2 - Carrier 58BP050150GA
Thermostat - Carrier T874G1055 (HH07AT171)

Anyway, thanks for any info you might have!!

As for the efficiency, someone else will have to get you that information.

Depending on the utility rates, the heat pump MAY be cheaper. A heat pump does not create heat, or cold for that matter, it merely transfers heat from one location to another. In heat, from outside to inside and in cool, from inside to outside.

Have a knowledgeable technician, you can use the map locator feature on this site, go over the system with you to answer questions, service the units and determine the best course of action.

[bmathews]

If you're using propane. It's usually cheaper to use the heat pump to heat your house. If you're using natural gas. It's cheaper to heat your house with that. It's nice to have both, but more costly to replace. Then you can have your choice of heating with electric or gas.

[ToddB10k]

Thanks for the quick reply!! Would have replied myself sooner but I was watching the video links in your post - not sure which I like better!!

I think you're right about having a tech over for a visit - I have a regular guy that I like. Also got two shiny new CO alarms - I know the heat exchanger in one unit was replaced right before we bought the house and I did NOT want to fire them up without the alarms in place!! But, like I said, I'll get my guy over. He should be able to explain the system to me - pretty knowledgeable guy.

For the replacement, I need to find someone that does the whole Manual J/D and I'll have to look at relocating the returns - two in the kitchen?? What's up with that?? And None in the master bedroom - a good 5 degree difference in there during the summer.

Thanks again for the reply! Hopefully someone will have the efficiency info on these old units!

[ToddB10k]

If you're using propane. It's usually cheaper to use the heat pump to heat your house. If you're using natural gas. It's cheaper to heat your house with that. It's nice to have both, but more costly to replace. Then you can have your choice of heating with electric or gas.

Thanks!! We have natural gas for the furnaces. I'm a big fan of gas for certain things, but at the same time I'm scared to death of it.

Shortly after we moved in the double sinks in the upstairs bathroom were plugged. The drains were connected with a cross (in the county - no code), so I couldn't do anything. Called a plumber and he went into the attic and cut the vent pipe to install a cleanout so he could snake it without climbing on the roof. While up there he said he smelled a bit of gas and suggested a pressure test on the system. He said he didn't think it was too bad. He came back the next day for that and did his thing and found a tiny leak on an elbow by the water heater in the attic. He fixed that and pressured the system up again and it was leaking even faster. He searched and searched and by this time it was 9:30pm so he gave up till the next day - leaving the gas turned off.

Next step was to try to isolate it to a section of pipe, so he cut the one that feeds to the fireplace and the laundry room and capped it and pressured up again. Leaking even faster. Then a bit of luck. He was by the meter outside and both AC units happened to be off - super hot day, so this was pure luck - and he happened to hear a hissing noise. Back inside the house he cut a hole in the drywall in the master bedroom and found the stub pipe coming in had corroded enough that a pin hole had opened up. After he put a new pipe in I used a caulking gun and LOTS of caulk around that opening in the brick!!

Quite a fortuitous sink clog...

Thanks for your post!!

[Cooked]

There is nothing wrong with natural gas. It's perfectly safe if the plumbing and the equipment is up to par. Your water heater(s) are prob gas and you should learn how to light them in in case one blows out on Christmas morning. It also kinda sounds like your plumber might've had his head in the sand but at least he got the prob fixed.

I would've thought a 20 year old house would've had double pane windows and units with a piezo spark ignition but then again the stuff I see in the urban jungle Houston (area) never ceases to amaze me.

[Cooked]

The OP appears to be in Texas. A properly operating heat pump should be cheaper to operate vs. natural gas in that mild climate, especially when compared to a 20 year old gas furnace that is probably running at 80% efficiency, if that. The only thing that would change this is if he has a terrifically low natural gas rate.

Yep he's in Houston but it does get cold here. And remember, Texas is a big state. There are the northern areas like Amarillo that get bitterly cold, like 10 below sometimes.

[ToddB10k]

I would've thought a 20 year old house would've had double pane windows and units with a piezo spark ignition but then again the stuff I see in the urban jungle Houston (area) never ceases to amaze me.

You know how in car engine compartments and behind computer desks someone takes the time to bundle all the wires together to keep them all nice and neat? Well, the guys that did the flex ducts in the attic did the same thing! You ought to see how nice and tight they tied them all together!! And the bends are a thing of beauty - some of them are a full 90 degrees with very nearly a zero bend radius. With that, twenty year old equipment, and single pane windows...

I was reading through some other homeowner posts on this thread and saw some pictures of 50 year old steel ducts on a basement ceiling with beautiful curves and reducers and got all misty-eyed... Why can't they all be like that? And when I get the new system put in, what are the odds I can get steel ducts - you know, something like an actual engineered system - without having to fork over new car money? Ah, dream big, I guess...

Anyone with any efficiency info on my equipment??

Thanks for all the posts!

[cartercrew]

There should be some kind of control package that disables the heat pump and turns on the furnace if the thermostat calls for 2nd stage heat or the outdoor temperature gets below a pre-set balance point. The furnace and heat pump should not run at the same time, system pressure in the heat pump indoor coil will get too high.

If memory serves, that is a 10 - 11 SEER system, with a 2.5 COP at 17* and 3.5 COP at 47*.
At my local rates (Charleston SC), electric is $.108 per kWh, and $1.07 per therm natural gas.

100,000 btu per therm gas through an 80% efficient furnace is 935 btu per $.01, 748 into the house, 187 up the stack.

1 kWh of electric heat produces 3412 btu, 3.5 COP makes the theoretical output is 11,942 btu per kWh. At $.108 per kwh, that is 1151 btu per $.01 at 47* outdoor temp. The cost at 17* outside is 790 btu per $.01

In this region, gas is never cheaper than a properly functioning heat pump for heat delivered to the house. However, unit capacity usually can't meet demand at outdoor temperatures much lower than 30*. That is usually what we set the balance point temperature at.

As you can see, the heat pump is a more cost effective heating source than natural gas. With that said, heat pumps still don't produce a "warm" heat. They will warm a room to 70+ degrees, but the air coming through the vents is not hot. That is why I tell customers to stay out from under the air vents if possible.

[ToddB10k]

The OP appears to be in Texas. A properly operating heat pump should be cheaper to operate vs. natural gas in that mild climate, especially when compared to a 20 year old gas furnace that is probably running at 80% efficiency, if that. The only thing that would change this is if he has a terrifically low natural gas rate.

Yes, as noted, I'm in the Houston area. I don't know much about the heat pumps, other than really basic "how they work" so I can't even guess on the efficiency bits. As far as climate, it is not at all unusual to get down to freezing or slightly below for multiple nights in a row and then have a string of days with high temps in the 40's. So, no, it's not "northern" cold, but it does get cold. I grew up in Nebraska and Iowa, so I do know "northern" cold. But after 10 years here - with 6 months of 90+ per year instead of 6 weeks - when it gets down to the sixties it feels cold to me and I tend to run the furnace so I'm comfortable in shorts. Silly, I know, but it's my preference.

Right now the house is around 67 degrees and I am NOT liking it. But I'll hold off on firing up the heat a bit longer - save my pennies for a new system!!

[cartercrew]

With the system being 10 SEER or better and still functioning (according to your description) I would not focus on system replacement yet. Spend the $$ you already said you have to save for an energy audit that includes a blower door and duct leakage test. Get recomendations from the testing company for where to start tightening up the house so it will use less energy. Calculate how much you want to spend on lowering the heating/cooling load, plan a time-line to get that done, then have the heating/cooling equipment changed to newer, more efficient and SMALLER equipment. Save money with lower demand AND higher efficiency.

[ToddB10k]

There should be some kind of control package that disables the heat pump and turns on the furnace if the thermostat calls for 2nd stage heat or the outdoor temperature gets below a pre-set balance point. The furnace and heat pump should not run at the same time, system pressure in the heat pump indoor coil will get too high.

If memory serves, that is a 10 - 11 SEER system, with a 2.5 COP at 17* and 3.5 COP at 47*.
At my local rates (Charleston SC), electric is $.108 per kWh, and $1.07 per therm natural gas.

100,000 btu per therm gas through an 80% efficient furnace is 935 btu per $.01, 748 into the house, 187 up the stack.

1 kWh of electric heat produces 3412 btu, 3.5 COP makes the theoretical output is 11,942 btu per kWh. At $.108 per kwh, that is 1151 btu per $.01 at 47* outdoor temp. The cost at 17* outside is 790 btu per $.01

In this region, gas is never cheaper than a properly functioning heat pump for heat delivered to the house. However, unit capacity usually can't meet demand at outdoor temperatures much lower than 30*. That is usually what we set the balance point temperature at.

As you can see, the heat pump is a more cost effective heating source than natural gas. With that said, heat pumps still don't produce a "warm" heat. They will warm a room to 70+ degrees, but the air coming through the vents is not hot. That is why I tell customers to stay out from under the air vents if possible.

Wow - great post!! Thanks for all that.

Based on my original post, I believe the thermostat will let me kill the heat pump and go straight to the furnace with the SUPL HT setting, and it will do what is most appropriate - heat pump or gas furnace - on the regular HEAT setting.

I don't fully understand a few parts of your post - like what's a therm - but in the part I made bold, does anything change with a higher efficiency furnace? I mean, do gas and electric "costs" get closer with better efficiency?

Thanks again!

[cartercrew]

100,000 btu per therm gas through an 80% efficient furnace is 935 btu per $.01, 748 into the house, 187 up the stack.

Natural gas is sold in either a Therm or a Cubic Foot. My utility company sells it for $1.07 per therm.

The efficiency rating of a gas furnace basiclly tells how much of the heat that is put into the furnace from the gas actualy makes it into the house. An 80% furnace puts 80% of the heat from burning gas into the air going into the house and 20% into air going up the chimney. Furnace efficiencies are getting close to their theoretical 100% limit, 97% being the highest I have heard. If you put a more efficient furnace in, more of the heat produced goes into the house.



1 kWh of electric heat produces 3412 btu, 3.5 COP makes the theoretical output is 11,942 btu per kWh. At $.108 per kwh, that is 1151 btu per $.01 at 47* outdoor temp. The cost at 17* outside is 790 btu per $.01

Electricity is sold by kWh, or 1000 watts per hour. COP is a calculation to compare how much better, or more efficient, heat pumps are than electric resistance heat. Resistance heat has a COP of 1.0, so your heat pump at 47* outside temperature makes 3.5 times the amount of heat for each watt consumed.


Wow - great post!! Thanks for all that.

Based on my original post, I believe the thermostat will let me kill the heat pump and go straight to the furnace with the SUPL HT setting, and it will do what is most appropriate - heat pump or gas furnace - on the regular HEAT setting.

I don't fully understand a few parts of your post - like what's a therm - but in the part I made bold, does anything change with a higher efficiency furnace? I mean, do gas and electric "costs" get closer with better efficiency?

Thanks again!

Yes, higher efficiency equipment costs less to produce the same amount of heat. But again, if you spend some time getting the house tighter, you will need to produce less heat. Once you have cost-effectively gotten the quantity of heat (and cooling) needed to its lowest level, then it is time to replace the equipment with higher efficiency new equipment. Over-sized high efficiency equipment will not be as efficient to operate.

[ToddB10k]

Carter

Thanks for the info!! As far as I recall (she pays the gas bill - I haven't seen on in years) we use cubic feet here.

I understand what you're saying about sealing up the house. I'm currently in the process of locating a contractor that looks at the whole thing - blower door and Manual J and D and all that.

My plan is to basically start from scratch for the whole system. The house is SO open that two units seem silly, plus the ducts are terrible (tied together, leaking) and the returns are ridiculous. So I'm after a contractor that does true design - and hopefully I can find one that will put in steel ducts!!

[beenthere]

Micahwes.

This is the Ask Our Pro's forum. In order to post advice here, you must have verified qualifications and have been approved by the AOP Committee.

You can find the rules for posting and qualifications here (http://hvac-talk.com/vbb/showthread.php?t=957002).

[pacnw]

For the replacement, I need to find someone that does the whole Manual J/D and I'll have to look at relocating the returns - two in the kitchen?? What's up with that?? And None in the master bedroom - a good 5 degree difference in there during the summer.



usually there are NO returns in the kitchen or bathroom.

you do not want to recirculate the smells or cooking "fumes".