heating hot water with pv panels

[nwdiver]

I don't believe my use of the super off peak rate of $0.35/kWh from my POCO for power is disingenuous as much as it was quite conservative.

Point being that $0.35/kWh is near the highest rate that anyone in the country will pay for a kWh (not to mention ~3x higher than the US average) and this isn't about retail costs but using SURPLUS solar to heat water. Which goes back to my original statement of 'Most people in most places' very few people are paying $0.35/kWh, fewer still among people that have surplus solar generation...

In NM since I produce more than I consume using an additional kWh to heat water 'costs' me ~$0.024/kWh. Meaning the check I get from Xcel is ~$0.024/kWh smaller. At my rental in WA it's $0 since under PSE any annual excess is lost. So as long production exceeds consumption it literally costs nothing to heat water.

Beyond that not everyone has natural gas and it's increasingly falling out of favor. Berkeley recently banned gas for all new homes.

There's a wider context that you're ignoring. Electrification if done in a way that flattens the demand curve can decrease the cost of electricity because fixed costs become a lower percentage of overall cost. Water heaters are one of the cheapest ways to do this since they're essentially a 3kWh thermal battery. PLUS; the amount of solar generation we're losing due to curtailment is only going to increase; Why not use it to heat water?

[Ampster]

I just returned from 3 days in Alaska, with no Internet access, to see this discussion continues to rage on.

Using the numbers that J.P.M. articulated above I was pleasantly surprised to see that my substitution of a HPWH for a 10 year old rusted gas water heater was actually better than I thought. My lowest cost rate for electricity is $0.17 per kWh. Using the example above that brings my cost per therm to $1.75 compared to natural gas at $2.44 per therm. Obviously everyone doesn't enjoy my low rate (EV-A from PG&E).

However when it comes to comparing HPWH to a resistive element water heater., the HPWH uses almost a third less kWhrs in a typical year. (1255 kWhs vs 3493 kWhs).

[NewBostonConst]

I just returned from 3 days in Alaska, with no Internet access, to see this discussion continues to rage on.

Using the numbers that J.P.M. articulated above I was pleasantly surprised to see that my substitution of a HPWH for a 10 year old rusted gas water heater was actually better than I thought. My lowest cost rate for electricity is $0.17 per kWh. Using the example above that brings my cost per therm to $1.75 compared to natural gas at $2.44 per therm. Obviously everyone doesn't enjoy my low rate (EV-A from PG&E).

However when it comes to comparing HPWH to a resistive element water heater., the HPWH uses almost a third less kWhrs in a typical year. (1255 kWhs vs 3493 kWhs).

But you got to remember in a Heating Dominated Climate where nearly all hot water heaters are inside the building....In this case the HPWH is taking heat out of the building that has to be replaced by the furnace...so you aren't really gaining much.

[J.P.M.]

But you got to remember in a Heating Dominated Climate where nearly all hot water heaters are inside the building....In this case the HPWH is taking heat out of the building that has to be replaced by the furnace...so you aren't really gaining much.

That's usually but not always the case. In such cases where the HP source for the heat is the conditioned air that's just been heated by nat. gas, a lot (but not all) of the sting is taken out of what amounts to putting a HP between a better/cheaper/faster energy source (nat. gas) and the DHW load.

If the HP source is outside air, the HPWH COP reduction will reflect the colder source temp. when the outside amb. air is colder than the air in the conditioned (and assumed heated) space. In such applications, one cheap and dirty, back of the envelope way to readjust the COP at the conditioned space heat source temp. accounting for a lot of the heat actually coming from fossil fuel combustion is to reduce that HP COP by 1/(furnace efficiency). So, a COP of, say, 4 with a 70 F source temp. and a furn. eff. of, say, 70 % becomes 4 - (1/0.70)) = 4 - 1.43 = 2.57. Depending on the outside air temp. that may actually still be above the HP COP if/when outside air is the heat source, particularly if/where coil defrost is part of the cost considerations.

[bcroe]

However when it comes to comparing HPWH to a resistive element water heater., the HPWH
uses almost a third less kWhrs in a typical year. (1255 kWhs vs 3493 kWhs).

A third less KWHs sounds like a COP of 1.5 which makes me think that is earlier technology. More
like 3 would be more up to date. That would also pull more heat from the building which would be
beneficial in the air conditioning season. Bruce Roe

[J.P.M.]

I just returned from 3 days in Alaska, with no Internet access, to see this discussion continues to rage on.

Using the numbers that J.P.M. articulated above I was pleasantly surprised to see that my substitution of a HPWH for a 10 year old rusted gas water heater was actually better than I thought. My lowest cost rate for electricity is $0.17 per kWh. Using the example above that brings my cost per therm to $1.75 compared to natural gas at $2.44 per therm. Obviously everyone doesn't enjoy my low rate (EV-A from PG&E).

However when it comes to comparing HPWH to a resistive element water heater., the HPWH uses almost a third less kWhrs in a typical year. (1255 kWhs vs 3493 kWhs).

Actually, that cost/therm in terms of electricity prices at $0.17/kWh = (100,000 BTU/therm/3,412 BTU/kWh ) * $0.17/kWh = $4.98/ Therm.

That cost then needs to be divided by the effective COP of any HPWH, however that's figured. That will give the cost of fuel for operation of the HPWH which is not the cost per therm based on per kWh costs of electricity.

Q: Are the 1,255 and 3,493 kWh figures actuals, estimates, or from where ?

As a first approx., for fuel cost considerations only, based on your provided costs, that would mean as long as the net COP of the HPWH stays above ($4.98/($2.44/ combustion. eff. of the nat, gas fired water heater )) = 2.04*(comb. eff.) or so for your application and situation, the fuel cost of operating a HPWH will be less than the fuel cost of operating a nat. gas fired water heater. So, if fuel cost is the only economic figure of merit, the less cost source is f(4 things): HP heat source temp., electricity cost, nat. gas cost and nat. gas combustion eff.

In a warm climate such as yours, I've little doubt that a HPWH may well be more fuel cost effective than a nat. gas fired unit.

Most of what I've written in this thread pertains to HPWH operation and applications in cold(er) climates.

In any climate, I'd still take into consideration what I believe is the likely higher initial investment required for a HPWH as well as what I'd bet will be higher maint. costs and service requirements, and the intangible PITA factor costs that can come from violating the KISS principle by choosing more complicated HPWH over simple gas fired methods to meet a DHW duty.

[Ampster]

But you got to remember in a Heating Dominated Climate where nearly all hot water heaters are inside the building....In this case the HPWH is taking heat out of the building that has to be replaced by the furnace...so you aren't really gaining much.

Nearly all? That is not the situation in all cases where I have deployed HPWHs. Three are in unheated garages and the fourth is behind a vented exterior door. In fact none of the homes I have owned in California have water heaters in the heated space. One modification I am thinking of making in my home is to use air from my attic which is typically warmer than outside ambient air.

The original poster has a resistive element water heater and that is the comparison that is most relevant to this thread. The advantage of HPWHs over resistive element water heaters still remains despite efforts to side track this conversation by making a comparison to Natural Gas.

[nwdiver]

Q: Are the 1,255 and 3,493 kWh figures actuals, estimates, or from where ?


In any climate, I'd still take into consideration what I believe is the likely higher initial investment required for a HPWH as well as what I'd bet will be higher maint. costs and service requirements, and the intangible PITA factor costs that can come from violating the KISS principle by choosing more complicated HPWH over simple gas fired methods to meet a DHW duty.

There are several independent studies which show HPWHs reduce energy use by >70%.


The payback period for a HPWH is typically <5 years FAR less than turn-key solar and a properly designed HPWH (not GE Geospring) doesn't require any more maintenance than a refrigerator. In fact... where I live in SE NM getting a HPWH significantly REDUCES maintenance over a regular water heater since a HPWH doesn't build scale when it's in HP mode.

Also; Most water heaters are located in the garage which means they neither add to furnace demand in the winter nor are they exposed to extreme low temperatures.

[Ampster]

In a warm climate such as yours, I've little doubt that a HPWH may well be more fuel cost effective than a nat. gas fired unit.

Most of what I've written in this thread pertains to HPWH operation and applications in cold(er) climates.

Most of what you have written may not be relevant to the original poster who linked to an Australian product to retrofit an electric water heater. If he is from Australia, and his heat source is electricity then it is likely that a HPWH will be more cost effective than a resistive element to heat water. The discussion began as a discussion about an electric water heater.

In any climate, I'd still take into consideration what I believe is the likely higher initial investment required for a HPWH as well as what I'd bet will be higher maint. costs and service requirements, and the intangible PITA factor costs that can come from violating the KISS principle by choosing more complicated HPWH over simple gas fired methods to meet a DHW duty.

There is no question about the higher initial cost, but with a payback of less than five years the long term cost of operation is better than resistive heat. You can believe what you want about maintenance costs, but your lack of actual experience makes your argument resemble what I have heard from the Gas Company peddlers, which is mostly negative. Focusing on cold climates ignores the benefits provided in warmer climates. I will acknowledge that the Geospring has had some issues.. I fixed mine easily and the issue had nothing to do with the heat pump portion. It is still going strong. My other three units have had no maintenance issues. One is still doing well after 9 years.
There is nothing simple about gas fired water heaters either. I have had to replace control valves and igniters over the years. It is much simpler to design a home with a HPWH because there is no flue or gas line to run. They can be located in conditioned space and draw and vent air from where ever it is most efficient. Furthermore, I have not yet been cited by the KISS police for violating that principle. I did gain more room for panels when I eliminated the water heater vent on my roof.

[Mike90250]

.......There is nothing simple about gas fired water heaters either. ........

Only true nowadays. In the past, the simple pilot light and millivolt thermopile to control the gas valve, was good for the life of the heater (10 + years). I've seen the modern components fail in 2 years. The glow coil ignitor on my home furnace would last 2 seasons max. The electronic brain in my Viking range, 3 replacements in the last 7 years.
Yet when I was growing up, I only recall the water heater being replaced once, and the furnace only needing new air filters. It's a poor trade off to get rid of simple, reliable pilot lights, and replace with electronics that fail. It may save a few therms over their being a lit pilot light, but the waste involved in manufacturing and disposing of glow coils and electronic control modules, is going to be a lot more, but shifted to a different country.

[J.P.M.]

There are several independent studies which show HPWHs reduce energy use by >70%.


The payback period for a HPWH is typically <5 years FAR less than turn-key solar and a properly designed HPWH (not GE Geospring) doesn't require any more maintenance than a refrigerator. In fact... where I live in SE NM getting a HPWH significantly REDUCES maintenance over a regular water heater since a HPWH doesn't build scale when it's in HP mode.

Also; Most water heaters are located in the garage which means they neither add to furnace demand in the winter nor are they exposed to extreme low temperatures.

Thank you for the link. After reading/studying it, I believe it seems to confirm/back up a lot of what I think I know, believe and write about the efficacy of HPWH, particularly the figures/graphs that seem to confirm the idea that HPWH performance where source temps < 40 - 50 F tends to roll off to < 2 or so rather quickly and therefore less likely to be cost effective in colder climates where, whether you and Ampster choose to see or believe it or not, a good portion of the U.S. and the world happens to live.

I'd suggest you and others read it after you educate yourself about heat pump operation. If you do choose to educate yourself and then read you linked report, you may well agree that it confirms what you don't seem to currently agree with.

I'll leave this thread with the probably futile hope that both you and Ampster stick more to what you might know and look at things in a more detached way instead of parochially and often ignorantly relating anecdotal information as fact. Some of what you both sometimes write is interesting but IMO only, it's often wrong (such as the way you treat excess production payments to PV owners), or what you bloviate about simply and often misrepresents realty. I don't believe such behavior is in the best interest of the goal of providing as accurate information as possible, if that's part of what's supposed to happen around here.

You two fit more into the AZWS crowd. Too much of the "I did/read/heard this once, it agrees with what I like and so it must be true" (non) logic with little to back up beyond what seems to me to be mostly ego and ignorant opinions.

Take what you want of the above. Scrap the rest.

[LucMan]

Only true nowadays. In the past, the simple pilot light and millivolt thermopile to control the gas valve, was good for the life of the heater (10 + years). I've seen the modern components fail in 2 years. The glow coil ignitor on my home furnace would last 2 seasons max. The electronic brain in my Viking range, 3 replacements in the last 7 years.
Yet when I was growing up, I only recall the water heater being replaced once, and the furnace only needing new air filters. It's a poor trade off to get rid of simple, reliable pilot lights, and replace with electronics that fail. It may save a few therms over their being a lit pilot light, but the waste involved in manufacturing and disposing of glow coils and electronic control modules, is going to be a lot more, but shifted to a different country.

The total cost to own needs to be considered when purchasing all HVAC and water heating equipment. The energy savings can be quickly equalized by just one repair. The high efficiency equipment initial install is generally much higher than the standard efficiency unit, add in the complexity of the electronics, increased amount of parts and the decreased total lifespan of the system and your savings will be less than you assumed. Expect to get 10-12 yrs if your lucky from your HPHW tank and refrigeration circuit if it is properly maintained and the water quality is within the " FINE PRINT" of the warranty. Warranties cover just the parts not the labor so if you happen to get a "problem" unit there will be negative savings. Some manufacturers really suck when it comes to honoring warranties.
Don't believe the marketing hype from the manufacturers, ask someone in the trade.

[J.P.M.]

Most of what you have written may not be relevant to the original poster who linked to an Australian product to retrofit an electric water heater. If he is from Australia, and his heat source is electricity then it is likely that a HPWH will be more cost effective than a resistive element to heat water. The discussion began as a discussion about an electric water heater.

There is no question about the higher initial cost, but with a payback of less than five years the long term cost of operation is better than resistive heat. You can believe what you want about maintenance costs, but your lack of actual experience makes your argument resemble what I have heard from the Gas Company peddlers, which is mostly negative. Focusing on cold climates ignores the benefits provided in warmer climates. I will acknowledge that the Geospring has had some issues.. I fixed mine easily and the issue had nothing to do with the heat pump portion. It is still going strong. My other three units have had no maintenance issues. One is still doing well after 9 years.
There is nothing simple about gas fired water heaters either. I have had to replace control valves and igniters over the years. It is much simpler to design a home with a HPWH because there is no flue or gas line to run. They can be located in conditioned space and draw and vent air from where ever it is most efficient. Furthermore, I have not yet been cited by the KISS police for violating that principle. I did gain more room for panels when I eliminated the water heater vent on my roof.

Because of what I see as you ignorance of the technical aspects of HP principles and operation, I believe you have no idea of the meaning or significance of what I've written with respect to HPWH operation in this or other threads. Therefore, I believe what you think of it's relevance makes little impact on it's accuracy.

The discussion did indeed begin about a device to add on to electric water heating methods. I'd note that the OP has a single post to this forum and so none since the original the first post to this thread.

The discussion, for this thread anyway, quickly changed to one of HPWHs and their efficacy in cold(er) climates. That direction was initiated by Peakbagger for HPWRs and NewBostonConst. as to their possible poor performance in cold climates.

I added nat. gas to the thread discussions because I believe it represents a commonly (but by no means universally) available, more cost effective and more reliable way to heat water for residential applications.

Continue to fix your HPWR's as you wish. Although that's anecdotal information and so of limited use, I'd think relying on the mostly common sense KISS principle to require less fixing with a simpler method if/when possible is better than more fixing of more complicated systems.

Gas fired domestic water heaters are indeed complicated - compared to some passive solar methods such as batch heating or other methods of heating water. But few situations being aa/nothing, most commonly available gas fired equipment for domestic water heating applications is less complicated than, for example, heating with wood or pellet stoves, etc. It's not all/nothing. My engineering opinion, FWIW, is that available gas fired equipment for heating domestic water is less complicated, more developed and so more reliable, less costly to acquire and, in cold(er) climates, less expensive to operate than most all HPWHs in the same service. Most all of that seems to be borne out, BTW, by nwdiver's link to the Bonneville study of HPWR used for residential water heating.

My acknowledged lack of experience with HPWH refers to the specific application of HP science and technology as it may apply to domestic water heating only. I believe my experience in heat pumps practical applications, particularly as a means of residential space heating is greater than yours.

I'm also pretty sure my technical knowledge of the thermodynamics of HP and refrigerator operation, and how the Thermodynamics, heat transfer and fluid mechanical aspects may relate to the practical aspects of space conditioning using refrigeration and HP equipment is such that I may well have forgotten more about the subject than you'll know for some time - if ever - but I'll leave that to other's to determine for themselves.

From where I sit and type, you seem full of anecdotal and what seems to be casual observational information with little background or explanatory information to back up what you write. I believe you believe what you write. I don't believe what you write adds much that's meaningful to the conversations or the body of knowledge.

But it's all about opinions and they're like noses - everyone has one and all of them smell, at least some of the time.

Take what you want of the above. Scrap the rest.

Mods: I'm, out of this thread.

[nwdiver]

Thank you for the link. After reading/studying it, I believe it seems to confirm/back up a lot of what I think I know, believe and write about the efficacy of HPWH, particularly the figures/graphs that seem to confirm the idea that HPWH performance where source temps < 40 - 50 F tends to roll off to < 2 or so rather quickly and therefore less likely to be cost effective in colder climates where, whether you and Ampster choose to see or believe it or not, a good portion of the U.S. and the world happens to live.

Yeah... that's how you get an average COP of ~3 throughout the year. Even cold areas have a spring, summer and fall. The COP is nearly 5 when the garage is ~90F. Why waste gas heating water in a warm garage AND curtail solar???

Why are you not supportive of electrification? A water heater is one of the best examples of 'value stacking'. Take something you need anyway and make it a 'battery'. Might only be ~3kWh but it's ~3kWh for the price of a wifi connection and some software. Can't do that if you're using gas.

Continue to fix your HPWR's as you wish.

Are you judging all HPWH from one bad anecdotal experience you had?

Aside from the GE Geospring most HPWH will run >10 years with no problems. It's just a refrigerator in reverse.... how much maintenance does your refrigerator need? As I mentioned previously if you're in an area with hard water they significantly reduce maintenance.

[Ampster]

.......
From where I sit and type, you seem full of anecdotal and what seems to be casual observational information with little background or explanatory information to back up what you write. I believe you believe what you write. I don't believe what you write adds much that's meaningful to the conversations or the body of knowledge.
.......

No worries @J.P,M. A few different opinions are what make these kind of forums worthwhile. Everyone has a different learning style and each reader is going to chose to do what makes sense to them. If many are indeed as ignorant as you continually believe them to be, then many are not going to understand your very complete and detailed explanations anyway, The one or two thank yous I do get give me enough feedback that I am going to continue doing what I believe is of some value to a few.