Electric Heat and Bills

[Traderfjp]

Hi,

I have a 4300 sq. home in NC that we are moving into. I have two 10 year old heat pump units. One heats the two main floors and has a propane backup for when it gets real cold. I believe it's rated for 120k btu and has a propane backup for really cold nights. The other heat pump heats two bedroom on the top floor and using electric heat for backup. The total top floor living space is maybe 700 sq ft. total. The house is well insulated and stays warm. I had set the thermostats to 55 for most of January and February. I couldn't believe the bills we were getting. It was costing 10.00 a day to heat the home and that was with everything else basically off. We went to the house at the end of February. I had am HVAC guy come out He put his gauges on both units and said they were working fine but I might want to buy a newer energy efficient units and of course his company sells them.

So we leave at the end of February and I sent the heat pump on the top level to 48 and leave the main level at 55. The new setting took effect on the 22nd. Now my bills are less than half and temps were about the same. The bills went from 350 a month to 150.00. I'm thinking that the top level heat pump was using its backup electric heaters instead of the heat pump because it is not setup properly or the unit is inefficient. I was wondering if there is anything I can do.
Option 1 is to keep the top heat pump at 48 degrees and when we want to use those rooms we pop the heat on, fix something??, or buy a new high efficient split system. I think the top unit is only putting out 30k or 40k btu. Check out the energy usage chart Any insight would be helpful.

 

[Dana]

The theory that the top unit had been running on auxiliary heat strips much or all of the time is plausible. With most heat pumps it's possible to set them up to NEVER run them. Without so much as a vendor name or model number it's hard to say how easy/hard it is to disable or lock out the auxiliary heat strips, or guess how the system might behave at low indoor temperatures. Some heat pumps can disable the heat strips with a setting on the thermostat.

Ten tons/ 120,000 BTU/hr is a fairly high load number for (4300' - 700'=) 3600' of space. A tight, well insulated 2x4 framed 3600' first floor would have a design heat load of 4 tons at NC style 99% outside design temps, and a cooling load of maybe 3 tons. Or is the 120KBTU just the size of the propane burner, not the heat pump? The design heating and cooling loads of a 700' top floor in a tight well insulated house are likely to be about 1 ton, often less.

Wi-Fi thermostats remotely reporting the indoor temp could let you turn the systems WAY down and still have ample freeze protection.

 

[Sponsor]

 

[Traderfjp]

The theory that the top unit had been running on auxiliary heat strips much or all of the time is plausible. With most heat pumps it's possible to set them up to NEVER run them. ton, often less.

Wi-Fi thermostats remotely reporting the indoor temp could let you turn the systems WAY down and still have ample freeze protection.

I'm not at the house so I don't know the model of either unit. I'll have to get that info. in April when I go to the house to work. However, it makes sense that the upstairs unit was the culprit since I turned it down to 48 while I left the other unit at about the same setting. I don't remember seeing a setting to turn off the electric heat backup on the thermostat but maybe it was there. I do remember Auto, hold, and a few other settings but I will check that too. I wish I had more info.

 

[Dana]

If it turns out that the heat pumps are ridiculously oversized for even the design loads (probably are) it may be worth permanently disabling the backup auxiliary heat (including the propane burner) and working out an operating mode strategy using deep setbacks with long recovery ramps.

It doesn't take a high duty cycle for a 10 ton heat pump to serve what could be an average 2 ton average heating load while keeping the indoors at 55F. It will be more efficient to run four 60-72 minute recovery cycles per day than two ~6-8 minute cycles per hour all day & night. Most heat pumps are are just getting their steady-state efficiency six to ten minutes into a cycle, and ideally you'd get at least 15 minutes out of a given cycle.

Very few "dual fuel" systems running on propane will be cheaper to run than auxiliary heating strips + heat pump. A few more will still "pay off" if natural gas. If the heat pump is crazy oversized it'll never need the capacity of the fossil burner, and probably not the heat strips. Even with a slightly undersized heat pump the heat pump is still providing the heavy lifting at some higher efficiency even when the heat strips are in use, whereas a dual-fuel setup has to switch over- the heat pump has to turnoff when the propane furnace is running to ensure that the incoming air entering the furnace isn't too warm.

 

[WorthFlorida]

It’s the thermostat that controls the aux heat when needed. If it’s a programmable/digital type you can change the program to single stage heat, that only allows the heat pump to run. The electric elements are added to the air handler as a option and they’re about the size if a small toaster but can be any kilowatt in size. For homes you may have a 5, 8 or 10 kilowatt unit. To see your rating you need to remove the top air handler cover and at the very top it be labeled on the heater.

The electric heater or AUX heat is turned on by the set temperature and the room temperature. Most thermostats will turn on the aux heater when there is a difference of two degrees or more from the room temperature and the set temperature. Most will turn off the aux heat when you’re in about one degree difference and the heat pump will continue run. Smart thermostats have algorithms to turn on the aux heat by how long it usually takes to bring the temperature up vs the outside air temperature and room temperature. There are options in the programmable thermostats that only allow the aux heater to run and not the heat pump. Digital thermostats will usually display “heat” or “aux heat”. Aux heat usually means both heat sources are on. Some may read or the instructions call it emergency heat, not aux.

If you don’t have one now if is well worth it to upgrade to WIFI smart thermostats. I have this unit but HP have several different models. For a while HP once a month would send an email on how many hours the thermostat ran for cooling and heating. I started to get them then after a few months it stopped. I don’t know why.
http://www.honeywellhome.com/en/products/thermostat/wi-fi-smart-thermostat-rth9580


Ten year old units could be SEER 12, possibly 14. Today SEER 14 is minimum by law. For a few hundred dollars more you can bump it up to SEER 16 and the efficiency and cost savings really bumps up. Both the compressor and a matching air handler need to be changed. If you just change the compressor unit it would be Seer 15 rating.

 

[Traderfjp]

I looked up the digital thermostat and it looks to be very easy to use. We do have two digital thermostats now and while they are not wifi I will check out the settings. I paid someone 200 to come to my house and he never once suggested we could turn off the auxiliary heat and just let the heat pump do its thing. Do you know if the propane backup can also be disabled via the thermostat?.

 

[Traderfjp]

I found some booklets I brought back with me. The tech that came out said that some of the registers were not throwing out enough heat. He suggested a new unit with a bigger blower. I have paper work on a damper control but he never suggested changing any of the settings. I'm reading that sometimes damper get stuck closed or open. My propane backup heater is Carrier Infinity 96 Model 58MVP. It shows it as two speed but the tech never mentioned how to control the speeds when I asked him about the furnace.

Not sure of the size:
Carrier Infinity 16 25HNB6/25HNB9 Heat Pump
Two-stage heat pumps run on low capacity most of the time – switching to high only when a boost in heating or cooling is needed or called for at the thermostat. The low-capacity cycles are longer, so they are quieter and they produce more gentle temperatures with little variance.

The Hybrid Heat performance allows the heat pump to be used with a gas furnace. The heat pump does the heating until the temperatures fall to near or below freezing. Then, the furnace takes over automatically, returning to the heat pump when outside temperatures rise again. Here are all the details.

• 17 SEER and 9.6 HSPF are Energy Star rated.
• 2-stage, scroll compressor.
• Sound blanket and Silencer System II top reduce operating noise to 67dB.
• Filter drier protects the compressor from moisture and sediment in the refrigerant.
• Uses the top-rated Carrier Infinity WiFi controller and can be controlled with a smart device.

The other heat pump is a Carrier 38ydb.

 

[Dana]

Insufficient throw is usually a duct design/implementation issue. Throwing a bigger air handler at it is a pretty dubious approach, IMHO. If there is a lot of poorly installed flex duct it's usually cheaper and easier and higher efficiency to recommission the ducts with tauter straighter flex and longer radiused turns to achieve higher velocity. Flex or hard piped, duct leakage needs to be measured and rectified as-needed.

The smallest 25HB9 is a 3 tonner, the smallest 25HB6 is 2 tons.

Most dual-fuel systems are set up to do switch to the fossil burner at a calculated financial crossover point, not a capacity crossover, since the COP efficiency of the heat pump falls off with lower outdoor temperatures (or a bigger indoor to outdoor temperature, really). See the heating capacity curves & tables in this document. The capacity tables only go down to 65F for an indoor temperature and -3F for an outdoor temperature, but even the 2 & 3 ton 25HB9s still has quite a bit of capacity at +17F out/65F in, and would have even more at 55F indoors.

On page 6 of both manuals it mentions:

4. Outdoor Air Temperature Sensor Designed for use with Carrier Thermostats listed in this publication. This device enables the thermostat to display the outdoor temperature. This device also is required to enable special thermostat features such as auxiliary heat lock out. Usage Guideline: Suggested for all Carrier thermostats listed in this publication.

If the upstairs unit doesn't have the outdoor sensor it could end up using the aux heat strip even when it has plenty of heat pump capacity. (Which it does, even if it's a 2-tonner serving 700 square feet.)

Like the heat pumps, the Carrier Infinity 96 Model 58MVP furnace comes in a range of sizes, from 40,000 BTU/hr to 120,000 BTU/hr.

 

[Traderfjp]

Your post is very informative as others have been. Is there a way to change the crossover point.? Even when it was relatively warm like 40 the propane heater would run for a long time before the heat pump turned on. I also used 100 gallons this year of propane which is about 300.00. I will try to lockout the aux heat option with the thermostat as suggested. I may have done this already if you see the above graph and the amount of electricity usage which changed dramatically in my favor. Not sure. Honestly the amount of air coming out of the registers is not a problem. There are no cold sots in the house and we are toasty warm so a simple fan will hep circulate the heat if I need to use on. I remember the tech saying I had the biggest heat pump Carrier made at that time for the main living area (120k) and the upstairs unit is 30k btu. I will check t see if t here is an outdoor sensor. I guess there should be a spot for it on the main board or something. Would I gain anything if I bought a plit unit with a high weer rating? Would it be a waste of money? If the power went out is there a way to by pass the heat pump and use a generator to run the propane furnace. What do you estimate the cost of all my equipment when it was installed? Thanks

Insufficient throw is usually a duct design/implementation issue. Throwing a bigger air handler at it is a pretty dubious approach, IMHO. If there is a lot of poorly installed flex duct it's usually cheaper and easier and higher efficiency to recommission the ducts with tauter straighter flex and longer radiused turns to achieve higher velocity. Flex or hard piped, duct leakage needs to be measured and rectified as-needed.

The smallest 25HB9 is a 3 tonner, the smallest 25HB6 is 2 tons.

Most dual-fuel systems are set up to do switch to the fossil burner at a calculated financial crossover point, not a capacity crossover, since the COP efficiency of the heat pump falls off with lower outdoor temperatures (or a bigger indoor to outdoor temperature, really). See the heating capacity curves & tables in this document. The capacity tables only go down to 65F for an indoor temperature and -3F for an outdoor temperature, but even the 2 & 3 ton 25HB9s still has quite a bit of capacity at +17F out/65F in, and would have even more at 55F indoors.

On page 6 of both manuals it mentions:

4. Outdoor Air Temperature Sensor Designed for use with Carrier Thermostats listed in this publication. This device enables the thermostat to display the outdoor temperature. This device also is required to enable special thermostat features such as auxiliary heat lock out. Usage Guideline: Suggested for all Carrier thermostats listed in this publication.

If the upstairs unit doesn't have the outdoor sensor it could end up using the aux heat strip even when it has plenty of heat pump capacity. (Which it does, even if it's a 2-tonner serving 700 square feet.)

Like the heat pumps, the Carrier Infinity 96 Model 58MVP furnace comes in a range of sizes, from 40,000 BTU/hr to 120,000 BTU/hr.

 

[Dana]

The biggest in the 25HB9 / 25HB6 series would be 5 tons, or 60,000 BTU/hr cooling rating. A pair of them would be a 120,000 BTU/hr of coolikng

At high stage and 70F indoors, 17F outdoors the 50 ton 25HNB960A + FE4ANB006 air handler can deliver about 35,000 BTU/hr. A pair would be good for 70K. See page 21.

That's not really great heating capacity for a 5 tonner, but should be enough to fully heat 3600' of tight-well insulated 2x4 framed first floor at 17F, and way more than would be needed at 32F. If the windows are single pane glass, no foundation insulation , or the house leaks are like a sieve it might not be able to support a full 70F indoor temp at 17F, without help, but it should be able to handle a 55F temp. Typical 99% outside design temps in NC are north of 17F, except up in the mountains. (The airport at Asheville's 99% temperature bin is +16F.)

The lockout or crossover point is probably settable at the thermostat.

I personally don't have much experience with Carrier heat pumps- most of them crap out on capacity at my local outdoor design temps, and electricity here is pretty expensive. I've seen a few of the 25VNA modulating versions around which retain some capacity down to about 0F before falling off a cliff. The 3 ton 25VNA036A delivers as much heat at 17F as the 4 ton 25HNB948A.

 

[WorthFlorida]

Your Carrier Infinity 96 Model 58MVP is a condensating boiler and can accept an AC cabinet over plenum or just an A coil in the duct work just above the boiler. Not sure what was added before or after the initial installation. But to control both heating systems all depends on how the thermostat is wired. It will have two W terminals, W1 & W2. W1 is the first stage, W2 second stage heat. You'll need to trace what goes where with the W wires (generally the white color wire).

Fan speed control is usualy left up to the furnace or the air handler controller board and able to changed speeds is not an option on a thermostat. Early multi speed fan motors speeds were a set of DIP switches to control the speeds. One set of switches as an example (1-3) would be for heating mode and another set (4-7) for cooling. New units are variable speed and no switches to control the the speed of the fan.

Some AC/Heat pumps that has two stage cooling will also have one or two speed compressors and a variable speed compressor fan motor. All controlled by the controller board for the compressor.

 

[Dana]

"Your Carrier Infinity 96 Model 58MVP is a condensating boiler..."

Huh?

You had me doing a double take there!

The 58MVP looks a whole lot like a 2 stage condensing furnace (and not a boiler)- a pretty nice one at that.

While it's possible to marry a boiler to a heat pump using a hydro-air handler, this isn't one of those.

 

[Traderfjp]

The propane furnace works with the heat pump. Doesn't that mean they are married. Hoping for a breakup with the thermostat.

"Your Carrier Infinity 96 Model 58MVP is a condensating boiler..."

Huh?

You had me doing a double take there!

The 58MVP looks a whole lot like a 2 stage condensing furnace (and not a boiler)- a pretty nice one at that.

While it's possible to marry a boiler to a heat pump using a hydro-air handler, this isn't one of those.

 

[Dana]

The propane furnace works with the heat pump. Doesn't that mean they are married. Hoping for a breakup with the thermostat.

If they're using the same ducts they're "married" in the sense that one can't act without the cooperation of the other. That can usually be managed with the thermostat.