Sure, but my understanding is that modern air-to-air heat pumps with a COP of at least 2.5 at 17F are commercially available. Hence my statement about "much/most of the heating season in much/most of the US."
Cheers, Wayne
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Natural Gas vs Heat pumps for heating
- Thread starter wwhitney
- Start date
Cheers, Wayne
A modern heat pump water heater is 400% more efficient than straight electricity. It doesn't use electricity to make heat. It uses it to MOVE heat. It isn't even close.
Not only that, the UEF of 4x for Rheem/Ruud water heaters is for super cold climates. If you're able to duct attic heat on the intake you can actually achieve up to 6x average year around UEF.
RKCRLR
Active Member
I'll chime in here with a little bit of knowledge (which means it is dangerous).
I have a hybrid propane furnace/heat pump for my home heating. Theoretically, I could use the heat pump to about 20F to heat my home before before the heat loss exceeds the capacity of my heat pump. But that would require running the heat pump continuously and no temperature set-backs. And there is the issue of defrost cycle which uses energy that isn't captured in the COPV (do hybrid water heaters have a defrost cycle?). So, from a practical aspect, my electric lockout temperature (the point I let the system switch from electric to propane if needed) is set to 40F and the heat pump lockout is set to 25F. If it was a standard heat pump it would probably be switching to supplemental resistance heating about then.
But the main reason all electric wouldn't work for me is the unreliability of the power grid. My power goes down often. Even more often now that PG&E has instituted a new policy that triggers a shutdown sooner. And once triggered, they have to inspect all the lines before power can be restored. So a minor outage lasts for hours. And if it goes down in the evening then it is out overnight until they can get the helicopters out to inspect the lines in the morning.
When my propane water heater went out I considered replacing it with an electric hybrid system. But that would require me running my generator during extended power outages in the winter. So I chose a propane on-demand water heater instead.
I have a hybrid propane furnace/heat pump for my home heating. Theoretically, I could use the heat pump to about 20F to heat my home before before the heat loss exceeds the capacity of my heat pump. But that would require running the heat pump continuously and no temperature set-backs. And there is the issue of defrost cycle which uses energy that isn't captured in the COPV (do hybrid water heaters have a defrost cycle?). So, from a practical aspect, my electric lockout temperature (the point I let the system switch from electric to propane if needed) is set to 40F and the heat pump lockout is set to 25F. If it was a standard heat pump it would probably be switching to supplemental resistance heating about then.
But the main reason all electric wouldn't work for me is the unreliability of the power grid. My power goes down often. Even more often now that PG&E has instituted a new policy that triggers a shutdown sooner. And once triggered, they have to inspect all the lines before power can be restored. So a minor outage lasts for hours. And if it goes down in the evening then it is out overnight until they can get the helicopters out to inspect the lines in the morning.
When my propane water heater went out I considered replacing it with an electric hybrid system. But that would require me running my generator during extended power outages in the winter. So I chose a propane on-demand water heater instead.
You raise some good points, but I just wanted to comment on the last part above. Your narrative reads as if continuous running and no temperature set backs are a negative, but I don't believe that's the case. Running continuously is fine for efficiency, and certainly better for the equipment than short cycling.
And temperature setbacks are an energy savings measure to be used when the HVAC system is grossly oversized, meaning that it can get the temperature back to target quickly after the setback period. A right-sized HVAC system that doesn't have that capability can be used without setbacks and will still be more efficient overall than the oversized system. Even though the lack of setbacks means the system will have to deliver/move a bit more heat than the oversized system with setbacks.
Cheers, Wayne
I overhauled my home HVAC a year and a half ago (2x outdoor condensing units, 2x indoor gas furnaces, 2x indoor evaporators). If i went with variable speed heat pumps, my system would have almost doubled in cost. Going with 96% efficient gas furnaces would have been like a 30% premium.
So unless they come back with the "energy upgrade California" incentive for people to get the best of the best, I think it's cost prohibitive at this time to get all this latest tech ya'll keep talking about.
So unless they come back with the "energy upgrade California" incentive for people to get the best of the best, I think it's cost prohibitive at this time to get all this latest tech ya'll keep talking about.
aesculus
Still Trying to Figure This All Out
I am going to have to look into this a bit. Getting a new high efficiency heat pump replacement soon (well if they ever deliver the outdoor unit that has been back logged for 2 months). In the past I would offset my heating/cooling temps because of TOU costs. With a dramatic cost difference of 2x, I thought that made sense. But with a more efficient unit I might just ride through peak periods with no off set.
I'll still have to run one zone at a different setting because that is a comfort thing where we like to sleep in a much colder environment, but that is a small percentage of the overall space and is designed for that.
A heat pump would not be a good choice in the winter where it drops below freezing but modern heat pumps are pretty efficient now down to freezing.
Old style NG plants are 40% efficient. Combined cycle NG plants are around 60% (The National Academies presents: What You Need to Know About Energy).
Even if we we assume an old NG plant at 40% (UEF of 0.4) and an old heat pump water heater with a UEF of 2.5 we're still looking at a heat pump water heater with an NG plant to heated water of UEF 1.5 (0.4 x 2.5) or 59% (1.5 / 0.94) more efficient than heating with NG alone at home assuming that you're comparing against the most efficient NG water heater made with a UEF of 0.94.
Combined cycle NG plants combined with heat pumps in the UEF 4 range and NG burned in a plant driving a heat pump water heater at home is 2.55 times more efficient using 39% the energy to heat water that straight NG takes.
In California, if we look at total system generation for 2020 so that we're not gaming peak renewable production and such i.e. there was an article recently that stated we hit 95% of grid power from renewables but that was for one day, not for the entire year:
39.8% of the production came from non zero carbon based sources like coal and NG.
Assuming a crappy efficiency of 40% for those carbon based sources, we're still at 76% grid efficiency when it comes carbon emissions i.e a UEF of 0.76.
That makes the heat pump water heat with a UEF of 4.0 (0.76*4.0/0.94) 3.23 times more efficient than heating the water directly with NG at home or just 31% of the carbon produced over a 0.94 UEF NG water heater.
And the UEF efficiency ratings of heat pumps assume an average mix of day and night operation around the clock not trying to game it. For instance the UEF goes up if you only use a heat pump to heat during the day and not at night when its cold. We never run our heater at night while we're sleeping.
It's not just heat pumps, but hot water heating needs, for both homes and pools if you have them. We built a new house a couple years ago and used Mitsubishi heat pumps without gas heating stages. In our mild climate, the efficiency is quite good, and costs less than gas even with PG&E prices.
But hot water is a different story I have a very high efficiency condensing water heater (an HTP unit) that operates at multiple temp stages. Think of a well insulated tank with a tankless style heater heating it. Heat pumps trying to get up to 120 degrees or higher (I run the HTP at 140 and then use a mixing valve to take it down to 120 for distribution so the hot water lasts longer). The pool heater is a 400,000 BTU unit. Replacing these with electric heat pump based units a much bigger hit because of the increased delta-T and a much higher capex. for the same amount of BTU output.
My HTP is a 160,000 BTU unit. If I do the simple math that the unit is a 4500 watt heater (as some of the bigger heatpump ratings indicate), that's only about 15000 BTU, assuming 100% efficiency. This is not at all equivalent to in terms of my HTP that is literally 10X heat capacity which is a huge driver of quick recovery time.. For a large family that means you will run out of hot water compared to high efficiency gas unit that will take a long time to heat an 80 gallon tank. And don't forgot that the tank may fail due to corrosion well before the heatpump does. So there is a big lifetime capex delta as well.
Good luck trying to find an all electric pool heater that comes close to the BTU ratings.
Getting rid of nat gas feeds to homes means cold showers and cold pools? And don't forgot because of the BTU differences recovery times are mich higher and that means these units will have to run a lot longer - in some cases almost continuously, and not just offpeak. This whole approach is wrongheaded, and CA will be a nat gas driven system for many many years to come if the current state strategy holds. So why are we trying to force all these changes again?
But hot water is a different story I have a very high efficiency condensing water heater (an HTP unit) that operates at multiple temp stages. Think of a well insulated tank with a tankless style heater heating it. Heat pumps trying to get up to 120 degrees or higher (I run the HTP at 140 and then use a mixing valve to take it down to 120 for distribution so the hot water lasts longer). The pool heater is a 400,000 BTU unit. Replacing these with electric heat pump based units a much bigger hit because of the increased delta-T and a much higher capex. for the same amount of BTU output.
My HTP is a 160,000 BTU unit. If I do the simple math that the unit is a 4500 watt heater (as some of the bigger heatpump ratings indicate), that's only about 15000 BTU, assuming 100% efficiency. This is not at all equivalent to in terms of my HTP that is literally 10X heat capacity which is a huge driver of quick recovery time.. For a large family that means you will run out of hot water compared to high efficiency gas unit that will take a long time to heat an 80 gallon tank. And don't forgot that the tank may fail due to corrosion well before the heatpump does. So there is a big lifetime capex delta as well.
Good luck trying to find an all electric pool heater that comes close to the BTU ratings.
Getting rid of nat gas feeds to homes means cold showers and cold pools? And don't forgot because of the BTU differences recovery times are mich higher and that means these units will have to run a lot longer - in some cases almost continuously, and not just offpeak. This whole approach is wrongheaded, and CA will be a nat gas driven system for many many years to come if the current state strategy holds. So why are we trying to force all these changes again?
No doubt that the "first hour" rating of heat pump water heaters are even lower than straight electric. For heating speed it's hard to beat gas fired directly. Ironically this has turned into a huge problem in our house. Our daughter takes showers that last until the hot water is gone. Because our current water heater is propane (twice the cost of NG / btu), she's able to take very long showers(don't get me started on how we should be able to control our teenager). When we're home and we detect that she's taking a shower, the rest of us jump in to take our much shorter showers so that we limit the amount of hot water she can use but that only works when we're at home and some of us haven't yet showered.
This is also why a tankless is the last thing we'd ever get since the hot water never ends.
I'm in favor of outlawing all NG/Propane appliances even if it means it being inconvenient for some. For those that have 50 gallon NG water heaters that feel that the recovery time might not be fast enough, they should consider an 80 gallon hybrid water heater to compensate.
roblab
Active Member
Several people I know, myself included, put solar panels on the roof AND THE GROUND, so actually a lot more could put in more panels if they used ground mounted panels. They look nice along the fence and make more shade for the groundhogs. I mean dogs. My relative in WA put his 5' off the ground and claims he can brush the snow off easier there. It seems interesting that people worry about using some NG, especially in the winter. Do what you can. Eventually add batteries and use less NG. The gas company doesn't require a minimum use here, at least, but I'm sure we use that much if they did.
I wanted a ground install. I have 1.3 acres and plenty of space. Tesla doesn't do ground installs at all and I wanted their $1.5 / watt deal (after ITC).
RKCRLR
Active Member
Admittedly, I'm a little prejudiced towards "oversizing" a system. My first new home in Sacramento had AC sized per the rules. My first summer in the house sometimes the outside temperatures were 110F or higher. MY AC was running continuously and it was 90F inside. I called the warranty department and was told that the AC was sized for a 20F temperature drop from the outside temperature and that the system was working properly. I swore never again.
But, as I remember, my hybrid system size was driven by the heat pump operation. When I first moved in my wife and I were only home 12 hours a day (if that). It made no sense to maintain the inside temperature during moderate temperature days that the system could easily recover from a set-back temperature. It's only during the extreme days that the system needs to run constantly to maintain the temperature. But my smart thermostat (which is rather advanced) doesn't have the option to monitor temperatures and predicted weather forecasts to determine if set-back temperatures need to be bypassed.
Also, because of the required defrost cycles, the heat pump can't run continuously in heating mode (at least mine can't). It basically runs in AC mode during the defrost cycle. So it has to be able to recover from the defrost cycles. It has an option of weather or not to run the furnace during the defrost cycle which I have set to on. Its very uncomfortable during the defrost cycle otherwise and I can't imagine how long it would take to recover if it wasn't used.
I believe that information is out of date. Heat pumps can certainly work when the 99% design temperature is below freezing, certainly down to 17F, maybe lower.
Cheers, Wayne
That was obviously misdesigned or misinstalled. It's certainly possible to design and install a system that will maintain a 35F delta between inside and outside when running continuously. The flip side is then accepting that if it hits 115F outside, it may rise to 80F inside (will rise if it last long enough). So you need to pick your design temperature properly and build in a forecast for future climate warming. If you only want that to happen once a year, pick the 99.7% forecast temperature as your design temperature.
Cheers, Wayne
RKCRLR
Active Member
There's no way I could way I could agree to banning NG/Propane appliances until we have a reliable grid. And the reliable grid would have to come first.
My condensing tankless water heater has a power button and I'm not afraid to use it if my daughter stays in the shower too long. It's a Navien 199kBTU unit with a 0.95 UEF. The smart recirc system ensures that my shower is hot in 5 seconds, saving water that would be wasted while waiting for the hot water to make it to the fixture.
RKCRLR
Active Member
The 20F temperature drop was the new construction guideline for Sacramento county at the time. It was to be measured at 5' above the floor. The builder sent me a copy because I thought it was BS.
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