Natural Gas vs Heat pumps for heating

[Sunshine Speed]

When replacing my gas furnace and outdoor electric AC, I originally wanted ductless mini splits, because of the cleanliness (no ducts, no dust/crap collecting in ducts). In the end, I decided to replace my existing 1965 ducting with brand new flex ducting and get a single whole-house 4-ton heat pump.

Benefits of single outdoor heat pump feeding blower in attic with 2 zones and 2 thermostats (upstairs/downstairs):
- For single heat pump, outside of house only has one set of refrigerant/electric line running from heat pump to attic (whereas ductless mini split refrigerant and wiring has several runs from outdoor units on multiple walls to indoor units, and makes outside walls of house look ugly)
- Only one outdoor heat pump to maintain (whereas ductless mini splits require occasional maintenance at every indoor unit)
- Simple vent/register in each room looks better than the ductless mini splits in various rooms.
- Cheaper to install single 4-ton heat pump (vs mini splits) if attic already had previous ducting and every room already has registers/vents.
- Every room in the upstairs or downstairs is always heated or cooled automatically from registers, no need to manage individual rooms and ensure doors to those rooms are closed.
- Even quieter operation compared to mini splits because of continuously variable speed heat pump and blower can run at quiet low speeds in attic.

Benefits of ductless mini splits:
- No need to create ductwork for homes that don't already have it.
- Ducts are disgusting and collect dust and crap over the years, so it's best to not have any.
- If any single unit breaks, other units in various rooms can still cool or heat the home.
- If you want to save money, you can turn off various individual rooms (whereas with my system I can only turn off whole upstairs or whole downstairs).

Hope this helps.

 

[brkaus]

@wwu123 the other thing not mentioned yet about the recirc pump built into the Navien unit is that it is variable speed. When you first start to use hot water, it fires up the heater and then runs the pump at a high flow rate to push the newly heater water through the pipes as fast as it can. When it senses that the return water is hot enough, it slows down or stops, allowing the just the user flow of hot water to go through the system.

It is important to insulate your hot water pipes, including the return, as much as possible or this will lead to significant energy loss. If I had more time when my house was under construction, I would have gone along all the pipes and filled in the gaps between the closed cell foam pipe insulation with spray foam.

So my stupid builder put in a recirculating system and the majority of it is in the slab with little to no insulation

and it’s setup as a long loop, so the farthest fixtures become practically useless if the system isn’t on.

 

[holeydonut]

All I know is I wish there was a TMC but for HVAC. The HVAC Talk forum is turrrribble and impossible for a homeowner/newb to get use out of.

I feel so stupid looking back at my HVAC investment. Like I'd do it so differently now knowing what I know after reading all this stuff on TMC

I didn't hate PG&E enough when I was doing my HVAC build... but now that I know how much evil it is to have reliance on PG&E then I would have avoided NG and somehow found some extra bucks to make that happen.

 

[jjrandorin]

Do ya'll like not have women in the house hold who wash their hair and like blow through your 50 gal gas water heater? We're using about 25 therms per month for hot water.

Maybe I need to wrap my hot water heater with that weird heat reflective insulation/padding crap?

Its just my wife and I now and though we are empty nesters, we only have 1 child. My wife is very (very) understanding of all this stuff, including us setting the dishwasher to run overnight. We dont do that with washing clothes though.

When my daughter was living at home (going to junior college) and working part time, rather than argue about the cost of gas / water (because of long showers etc) I had her pay for those bills instead. Its amazing how, all of a sudden, those previous conversations where I used to ask why the shower was running so long, etc suddenly stopped.

After a couple of months of having to pay part of her (very meager) income she was earning from working at walmart toward gas and water bills, she started asking us if we had to run the sprinklers so long, why were we not getting more drought tolerant plants, etc (lmao).

I wasnt very popular with her at that time, but now that shes moved out (with a significant other, but she is the bread winner in her relationship for the most part) she understands what I was after. She actually told me so, and it almost brought a tear to my eyes, haha.

 

[holeydonut]

Frackkkk

Europe's gas shortage could make the whole world pay more to get warm this winter

Natural gas prices surge as investors fret about the commodity’s availability through the upcoming winter.

www.cnbc.com

 

[Ampster]

today, 35% of electricity in CAISO comes from NG

There was a time, briefly, when much of urban home lighting came from natural gas. Considering the transition to incandescent, florescent and now LED it is not surprising to see this transition. Change is inevitable
.

 

[getakey]

I did not realize the title of this thread changed. My comments now buried were primarily about where the Electricity is going to come from with new homes being forced to be all electric. I understand heat pumps can be very efficient. I have 2 HVAC units: traditional gas furnace with electric AC and second is a heat pump (with emergency gas which also runs in the defrost cycle). It is new as I recently replaced an older gas furnace. I can say that at low temps, it works very hard to keep up.

Anyway, my comments before were that incremental electric use is going to be primarily filled from NG plants especially in the winter for the foreseeable future. Look at the supply curve on a winter day (you can pick the day in the link bellow). After 5pm in the winter, well over 50% comes from NG. About 40% comes from imports. Most of that is probably hydro from Pacific Northwest.

California ISO - Supply, Today's Outlook

View real-time and historical data on generation resources, including renewables, currently on the system.

www.caiso.com

 

[jjrandorin]

I did not realize the title of this thread changed. My comments now buried were primarily about where the Electricity is going to come from with new homes being forced to be all electric. I understand heat pumps can be very efficient. I have 2 HVAC units: traditional gas furnace with electric AC and second is a heat pump (with emergency gas which also runs in the defrost cycle). It is new as I recently replaced an older gas furnace. I can say that at low temps, it works very hard to keep up.

Anyway, my comments before were that incremental electric use is going to be primarily filled from NG plants especially in the winter for the foreseeable future. Look at the supply curve on a winter day (you can pick the day in the link bellow). After 5pm in the winter, well over 50% comes from NG. About 40% comes from imports. Most of that is probably hydro from Pacific Northwest.

California ISO - Supply, Today's Outlook

View real-time and historical data on generation resources, including renewables, currently on the system.

www.caiso.com

that thread still exists btw... I just split off the first couple of pages of posts as the discussion morphed. you can find it here:

Did anyone catch the new PGE EV rate on 8/1? [wrong tariff]

7 cent increase at peak time starting at 4 while part peak and off peak decreased by 4 cents which is when most of the solar production happens. This is a lose lose for everyone unless they are load shifting with storage.

teslamotorsclub.com

 

[wwu123]

While performance improvements in heat pumps show up first in mini-splits, they are also available in conventional central systems. And a "mini-split" outdoor unit can be paired with a high static pressure air handler to replace a central system. Possibly also with one or two additional ductless heads if desired to address shortcomings of the existing ducts, or to condition an addition.

Cheers, Wayne

Say, do you have a pointer to those mini-split outdoor units that can be tied to an air handler - does that mean it could be tied to the furnace blower as well? Our 1960's era nonfunctional A/C and coil are still sitting atop our 10-year new furnace, but we only miss not having A/C during a few heatwaves each summer, and really 1.5 to 2-tons would probably suffice.

Despite my wife having grown up with mini-splits in Tokyo, she abhors the indoor units hanging from the wall. If I could plumb a mini-split outdoor unit into the central ducting, that would solve the WAF. I actually have experimented this summer running a portable 12K BTU (= 1 ton) unit into the central ducting, since there were some vestigial duct ends from an old humidifier (I think), attaching the dual-hoses and using it "backwards" while in heat-pump mode. While it couldn't cool down the house in the afternoon, it kept indoors from getting too warm until I could open the windows each night.

 

[wwhitney]

Most of the mini split makers have that option these days. E.g.

Medium Static Pressure Duct - AIRSTAGE SINGLE-ROOM MINI-SPLIT SYSTEMS - Residential - FUJITSU GENERAL United States & Canada

Medium Static Pressure Duct - AIRSTAGE SINGLE-ROOM MINI-SPLIT SYSTEMS - Residential - FUJITSU GENERAL United States & Canada

www.fujitsu-general.com

Finding a good HVAC designer/installer is hard, but if you do they should know about this option.

Cheers, Wayne

 

[holeydonut]

Who here thinks @wwhitney needs to start a combo solar + ESS + HVAC company? It'll be the only company that builds everything code compliant and works right in the customers' interests.

Sorry Vines you're excluded from this premier short-list since you don't do HVAC.

 

[miimura]

The Navien sounds nice, will have to look into it next time. Can the Navien use the cheat of using the cold water side as a return through? There's no way we can plumb a return all the way from the bathrooms without ripping out all the walls and floors again.

Fortunately or unfortunately, our Noritz is 12-years old and shows no signs of dying anytime soon, even with no maintenance - it's pampered with whole-house filtered and softened water so we don't even do annual cleaning or flush on it. We do have access to wrap all the pipes in the crawlspace, and once they go into the wall cavities, any waste heat would still heat the house the 6. months of the year we're using gas heating anyways.

You really need a properly plumbed return line. Softening the water going to a tankless heater will greatly prolong its life. In fact, I unknowingly let my softener run out of salt during the pandemic and it fouled the flow sensor in the Navien. Fortunately, it was not too expensive to get someone out to diagnose and replace the sensor. I have never flushed it in the 8+ years of service.

 

[wwu123]

Most of the mini split makers have that option these days. E.g.

Medium Static Pressure Duct - AIRSTAGE SINGLE-ROOM MINI-SPLIT SYSTEMS - Residential - FUJITSU GENERAL United States & Canada

Medium Static Pressure Duct - AIRSTAGE SINGLE-ROOM MINI-SPLIT SYSTEMS - Residential - FUJITSU GENERAL United States & Canada

www.fujitsu-general.com

Finding a good HVAC designer/installer is hard, but if you do they should know about this option.

Cheers, Wayne

Ah, thanks, so I guess it's using it's own small powered duct system or air handler that goes into the walls for the indoor unit. Won't tie into existing central ducts or existing central furnace blower, which is what I was hoping for.

Nothing wrong with it, just that somewhat similar to available ductless ceiling or in-wall cassettes. Problem with all is price - that the name brands are ridiculously expensive for the equipment and install costs in the U.S. So would prob cost more than just replacing my nonfunctional A/C condensor and coil in my central ducts. Friend of mine recently got a quote for Mitsubishi mini-splits for the entire house, two outdoor units with 5-6 indoor heads, $35-40K installed. And they probably need A/C even less than the two weeks a year I need it. Now they'll be using their mini-splits for heating as well, but I already have a nice 95% efficient multi-stage furnace that works great for heating.

 

[dareed1]

Ah, thanks, so I guess it's using it's own small powered duct system or air handler that goes into the walls for the indoor unit. Won't tie into existing central ducts or existing central furnace blower, which is what I was hoping for.

Nothing wrong with it, just that somewhat similar to available ductless ceiling or in-wall cassettes. Problem with all is price - that the name brands are ridiculously expensive for the equipment and install costs in the U.S. So would prob cost more than just replacing my nonfunctional A/C condensor and coil in my central ducts. Friend of mine recently got a quote for Mitsubishi mini-splits for the entire house, two outdoor units with 5-6 indoor heads, $35-40K installed. And they probably need A/C even less than the two weeks a year I need it. Now they'll be using their mini-splits for heating as well, but I already have a nice 95% efficient multi-stage furnace that works great for heating.

We installed a ducted Mitsubishi heat pump system into our 2100 sq ft single story house last January. Based on a Manual J calculation, we sized it at 2.5 tons (30,000 BTUh). The system uses a SUZ single zone exterior unit coupled with a SVZ air handler. The air handler fits easily into the spot previously occupied with a 80,000 BTUh 20 year old NG furnace. We also chose to replace all the ducts in the crawl space since they were 50 years old. Total cost was $28K; most of that was labor. Replacing the ducts was $9K, so if our house were newer, the cost would have been <$20K.

The trigger to do this was our desire to add AC for the "two weeks that we need it" and although the furnace was still functioning, it was noisy. Combined with a solar system, it appears that our running energy costs will be almost entirely the natural gas used in the water heater and stove.
Ducted air handler

 

[wwhitney]

Ah, thanks, so I guess it's using it's own small powered duct system or air handler that goes into the walls for the indoor unit. Won't tie into existing central ducts or existing central furnace blower, which is what I was hoping for.

It can certainly be used with existing central ducts, you need the "high static" or "medium static" compact air handler model that has a blower rated for 0.8" or 1.0" static w.c., and the comparatively shallow duct connection geometry on the compact air handler would require a little sheet metal work for the transitions. It would be good to confirm with a manometer that your existing ducts and furnace are operating (with a clean filter) at a static pressure differential in that neighborhood. [Although if the gas furnace airflow required is much higher than the mini-split airflow required, the gas furnace's operating at a higher static pressure differential doesn't necessarily mean that your ducts are too narrow for the compact air handler's lower air flow.] And you may have to pay attention to the filter type, size, and pressure drop. [Always true, just more critical when the blower is "right-sized."]

As to installing it in parallel with an existing gas furnace, it should be possible, although it's not something I have any experience designing. [Although if the heat pump is adequate for the design load, you could just remove the gas furnace and resell it.] I can certainly imagine a parallel installation where the compact air handler and a gas furnace share the same return and supply plenums, with a couple motorized dampers to avoid air flow short-circuiting backwards through the non-operating unit. Or you might be able to put them in series and disconnect one of the two blowers, wiring the other blower to come on when either unit runs.

That said, ducted mini-splits are bit less efficient than ductless ones, so if you want to keep the gas furnace, be sure to compare the price/efficiency of a new ducted mini-split to a new conventional central A/C with a coil that would fit in your existing unit.

Cheers, Wayne

 

[Big Earl]

We installed a ducted Mitsubishi heat pump system into our 2100 sq ft single story house last January. Based on a Manual J calculation, we sized it at 2.5 tons (30,000 BTUh). The system uses a SUZ single zone exterior unit coupled with a SVZ air handler. The air handler fits easily into the spot previously occupied with a 80,000 BTUh 20 year old NG furnace. We also chose to replace all the ducts in the crawl space since they were 50 years old. Total cost was $28K; most of that was labor. Replacing the ducts was $9K, so if our house were newer, the cost would have been <$20K.

The trigger to do this was our desire to add AC for the "two weeks that we need it" and although the furnace was still functioning, it was noisy. Combined with a solar system, it appears that our running energy costs will be almost entirely the natural gas used in the water heater and stove.
Ducted air handler

Since inverter-based HVAC systems (mini-splits and the like) perform best at part load, I'd advise sizing a system slightly higher than what Manual J recommends for cooling. Ideally, size the system for your heating load (if you intend to use it for heat), which in Zone 2 of the country is 35 - 40 BTU/hr per square foot. You might find that your 30,000 BTU/hr system has trouble keeping up on the coldest days. In addition to providing better comfort, sizing for your heating needs will result in higher efficiency when cooling.

Caveat: You wouldn't want to do this with a conventional, fixed speed cooling system since oversizing the cooling will result in poor dehumidification (maybe not a problem in very dry areas). Systems with variable speed fans and compressors (like mini-splits) can provide sufficient dehumidification even under part load.

 

[wwu123]

It can certainly be used with existing central ducts, you need the "high static" or "medium static" compact air handler model that has a blower rated for 0.8" or 1.0" static w.c., and the comparatively shallow duct connection geometry on the compact air handler would require a little sheet metal work for the transitions. It would be good to confirm with a manometer that your existing ducts and furnace are operating (with a clean filter) at a static pressure differential in that neighborhood. [Although if the gas furnace airflow required is much higher than the mini-split airflow required, the gas furnace's operating at a higher static pressure differential doesn't necessarily mean that your ducts are too narrow for the compact air handler's lower air flow.] And you may have to pay attention to the filter type, size, and pressure drop. [Always true, just more critical when the blower is "right-sized."]

As to installing it in parallel with an existing gas furnace, it should be possible, although it's not something I have any experience designing. [Although if the heat pump is adequate for the design load, you could just remove the gas furnace and resell it.] I can certainly imagine a parallel installation where the compact air handler and a gas furnace share the same return and supply plenums, with a couple motorized dampers to avoid air flow short-circuiting backwards through the non-operating unit. Or you might be able to put them in series and disconnect one of the two blowers, wiring the other blower to come on when either unit runs.

That said, ducted mini-splits are bit less efficient than ductless ones, so if you want to keep the gas furnace, be sure to compare the price/efficiency of a new ducted mini-split to a new conventional central A/C with a coil that would fit in your existing unit.

Cheers, Wayne

Thanks for clarifying, as I watched a couple of Youtube videos, and so it always looked like they were fashioning their own supply plenums and ductwork for these ducted mini-splits. Plus all of my ducts are round (flex or rigid) coming off the supply plenum, and these mini-split ducts are always shallow rectangular - I guess it could be hooked together with a bunch of transition sheetmetal. Will look more into it. Ideally like to go dual-fuel - run the furnace for quick recovery in the morning, and then a minisplit heat pump during the day (esp it can get to 60-70 deg highs even on many winter afternoons here in NorCal).

My top-of-the-line Carrier Infinity furnace can measure static pressure with its proprietary controller, with my slightly undersized ductwork, it shows static pressure can be a bit high around 0.8-0.9" when running on top speed - but 95% of the time it's running in low speed with static pressure around 0.5" IIRC. I switched it to an Ecobee smart thermostat after seeing the same unchanging static pressure for 5 years LOL, so I could monitor temps and occupancy in 5-7 different rooms instead.

 

[Big Earl]

Thanks for clarifying, as I watched a couple of Youtube videos, and so it always looked like they were fashioning their own supply plenums and ductwork for these ducted mini-splits. Plus all of my ducts are round (flex or rigid) coming off the supply plenum, and these mini-split ducts are always shallow rectangular - I guess it could be hooked together with a bunch of transition sheetmetal. Will look more into it. Ideally like to go dual-fuel - run the furnace for quick recovery in the morning, and then a minisplit heat pump during the day (esp it can get to 60-70 deg highs even on many winter afternoons here in NorCal).

My top-of-the-line Carrier Infinity furnace can measure static pressure with its proprietary controller, with my slightly undersized ductwork, it shows static pressure can be a bit high around 0.8-0.9" when running on top speed - but 95% of the time it's running in low speed with static pressure around 0.5" IIRC. I switched it to an Ecobee smart thermostat after seeing the same unchanging static pressure for 5 years LOL, so I could monitor temps and occupancy in 5-7 different rooms instead.

Check out this more conventional air handler packaging (random links - they're available from other manufacturers and vendors):

2 to 3 Ton 20 SEER MrCool Universal Central Heat Pump Split System - Upflow/Horizontal

The 2 to 3 Ton 20 SEER MrCool Universal Central Heat Pump Split System is the innovative heating and cooling tool you need to stay warm in winter, cool in summer, and energy efficient every day of the year. Unlike other conventional heat pump split systems, the Universal relies on a DC inverter...

iwae.com

4 to 5 Ton 17 SEER2 MrCool Universal Central Heat Pump Split System - Upflow/Horizontal

The 4 to 5 Ton 17 SEER2 MrCool Universal Central Heat Pump Split System is one of the most high performing, versatile, and simple heating and cooling products available from the industry today. This is a central heat pump that can cool better than most air conditioners, heat better than...

iwae.com

 

[dareed1]

Since inverter-based HVAC systems (mini-splits and the like) perform best at part load, I'd advise sizing a system slightly higher than what Manual J recommends for cooling. Ideally, size the system for your heating load (if you intend to use it for heat), which in Zone 2 of the country is 35 - 40 BTU/hr per square foot. You might find that your 30,000 BTU/hr system has trouble keeping up on the coldest days. In addition to providing better comfort, sizing for your heating needs will result in higher efficiency when cooling.

Caveat: You wouldn't want to do this with a conventional, fixed speed cooling system since oversizing the cooling will result in poor dehumidification (maybe not a problem in very dry areas). Systems with variable speed fans and compressors (like mini-splits) can provide sufficient dehumidification even under part load.

Yes, heat pumps tend to have peak efficiency at part load. It is also true that heat pumps of smaller capacity tend to be more efficient at all outputs than units of larger capacity. But instead of considering tendencies, I used the data for heat output versus kW input. The 30,000 BTUh unit was significantly more efficient at all outputs than a larger unit, and it had enough capacity according to a Manual J calculation of the heating load. We've already been able to evaluate its performance in the lowest temperatures that we'll see, and it does fine.

I'm not sure which climate zone formulation you mean by Zone 2, but where we live the temperature has gone below 32F once in 40 years, and has gone above 100F perhaps 5 times. For us, the heating load is the main criterion.

 

[Big Earl]

Yes, heat pumps tend to have peak efficiency at part load. It is also true that heat pumps of smaller capacity tend to be more efficient at all outputs than units of larger capacity. But instead of considering tendencies, I used the data for heat output versus kW input. The 30,000 BTUh unit was significantly more efficient at all outputs than a larger unit, and it had enough capacity according to a Manual J calculation of the heating load. We've already been able to evaluate its performance in the lowest temperatures that we'll see, and it does fine.

I'm not sure which climate zone formulation you mean by Zone 2, but where we live the temperature has gone below 32F once in 40 years, and has gone above 100F perhaps 5 times. For us, the heating load is the main criterion.

Sorry, I realize now that the map I was using to determine zone was not the IECC map. Glad your system is working out well for you.