I may have painted with too broad a brush condemning HPWH north of I-40. Perhaps I-80 describes a better line.

Much depends on the specifics of the installation. My own seems to run at a COP of 2.0 based on kwh used for water heating before and after its installation. I have 9 months of daily data to support that claim. The room it is in has yet to drop below 65 degrees, but I'm south of I-10. Dehumidification only occurs when ambient RH exceeds about 50%, typical of portable dehumidifiers. I collect the HPWH's condensate in a tub, so as to be aware of how much dehu is occurring, and I have a thermostat with humidity indication fairly near (though not in same room as the unit).

I don't think the situation would arise where a resi humidifier is having to add back water condensed by an HPWH.

When the unit dehumidifies it is not adding to the space's sensible heat load, so the portion of heat derived from dehu is in some sense 'free' in terms of house heat load. Some of the heat it transfers derives from the standby loss of the tank, so that heat is essentially recycled back into the tank along with the power used by the HPWH.

Key to a successful install is providing the unit enough air to work with. An HPWH in a closet without good airflow isn't going to work well and could lead to a mold problem by dropping temps within closet wall assemblies to below ambient dewpoint.

In general I think HPWHs are good technology and am pleased they are now being sold by some of the big name brands. I think analysis will find that they provide better ROI and less complexity / maintenance than solar panels in many situations.

Posted By engineer on 01 Apr 2010 10:49 AM

Much depends on the specifics of the installation. My own seems to run at a COP of 2.0 based on kwh used for water heating before and after its installation. I have 9 months of daily data to support that claim.

A 2 COP sounds more accurate than some of the 3+ claims I've heard.

I don't think the situation would arise where a resi humidifier is having to add back water condensed by an HPWH.

Spoke of a dehumi, have no concerns about adding humidity.

When the unit dehumidifies it is not adding to the space's sensible heat load, so the portion of heat derived from dehu is in some sense 'free' in terms of house heat load.

Again not a northern concern.

Some of the heat it transfers derives from the standby loss of the tank, so that heat is essentially recycled back into the tank along with the power used by the HPWH.

Fair point.

Key to a successful install is providing the unit enough air to work with. An HPWH in a closet without good airflow isn't going to work well and could lead to a mold problem by dropping temps within closet wall assemblies to below ambient dewpoint.
 
You also mentioned air in the room with your water heater only dropped to 65* that is a starting point in mid MI (which will certainly impact COP).

In general I think HPWHs are good technology and am pleased they are now being sold by some of the big name brands. I think analysis will find that they provide better ROI and less complexity / maintenance than solar panels in many situations.

I agree, but do not think universal application to be demonstrably supported at this time.

I don't think we're far apart on this. I appreciate your hard data.
A similar test in a unit installed in a MI basement would be interesting.
J

One reason people don't agree on COP is the tank standby losses. 3X better than electric doesn't mean COP=3 for the entire system - an electric tank water heater doesn't achieve COP=1.

A project I'm calculating currently shows water heating with a 4.16 COP for 43% of requirement due to DSH and electric elements 57% of the time. Their actual cost of making this hot water is $55 for geo contribution and $302 for elements or total of $357/year. If Engineers 2.0 COP were accurate in Michigan (which remains to be seen with a lower than 65* bottom temp to extract heat from), we would see twice the geo consumption- $110, and half the element consumption - $151 or $261/year. At less than $100/year savings the payback is not very compelling.
If it adds heating load to a dwelling (and more operating hours to expensive heating equipment) net benefit could be zero.
Incidently the numbers above are for hot water for a family of 5 and a kw electric cost of .075. If we change this to a family of 2 empty nesters which will happen during the payback period of above scenario the net savings plunges to $15/yr as DSH begins to fulfill 68% of DHW needs.
Not everyone's electric rates are as low as ours, so...milage may vary.

Jon, your observations about standby loss are correct but true of either system so mostly a wash. this is among the reasons I ask for field data such as engineer provides, not peak lab efficiency, which necessarily measure loss in the equation.

Point remains these systems may not be universally beneficial. It will take more compelling info to persuade me.
Joe

Might as well finish the calculation - what's your delta cost (after tax credit) and payback in years?  7-10?

> standby loss are correct but true of either system so mostly a wash...and half the element consumption
If you account for them equally - you did the COP=2 so cost = 2x mistake. Either use a COP closer to 3 or add to the cost of resistance electric water heat.

COP will certainly drop with ambient room temperature. If room temp drops far enough, perhaps to about 50, the HPWH will have to defrost itself, a sure COP-killer.

In my case it may be more accurate to say that my EF has doubled rather than COP, since I base my doubling on a halving of water heating kwh during times of no DSH contribution (such as this week). Published EF for my storage electric water heater is 0.92, but mine probably runs a bit better owing to lower setpoint (110), warmer ambient, and generous pipe insulation, so the difference is minimal.

The waters muddy a bit during high DSH times. Standby losses become a greater share of my overall hot water cost, particularly in summer when DSH is basically free, although I dutifully charge 72 Wh per geo unit hour as a cost of hot water since the DSH circ pump is part of the system, and I haven't discerned a drop in compressor current from stopping or starting DSH while in cooling mode, though theory suggests there should be a reduction in compressor power in cooling mode while DSH is engaged. The HPWH does less well during shorter runtimes associated with a warm buffer tank and standby losses only, and its lines and heat exchanger increase standby losses. I get the doubled EF when the HPWH runs for an hour or so at a time.

My HPWH claims a COP of 3-4 and I haven't seen that. EF is a better measure of HPWH cost reduction and most units claim to run in the low 2 range. Again, I like the GE design best for minimizing complexity, standby losses and higher claimed EF.

Joe and I are very close on this. We all owe our clients accurate ROI analysis, and my system is indefensible as to ROI. I did it out of curiosity and a compelling interest in absolutely minimizing power use.

Some folks aren't just concerned about ROI for any number of reasons ranging from perceived 'Greenness' to bragging rights. I think HPWH compete quite well against pricey and ugly solar thermal collectors which serve also to give a hurricane a handle to pull off a roof. We were quoted $7k on a high end solar thermal system for domestic HW only, and I managed to suppress my laughter until no longer in the presence of the seller.

"Might as well finish the calculation - what's your delta cost (after tax credit) and payback in years? 7-10?"
Ummm-no more like 15 years (or longer than life cycle of most water heater liners) to never. Over the counter cost difference is about $1,300. Retail difference will be greater.
It is fair to note that I compared to an electric water heater that had DSH available, pay back against an electric or gas water heater without DSH would improve dramatically though we still don't know what the impact would be to a heating bill in MI.
Curt, you should be able to calculate the BTU's required to heat the volume of water you use (that would tell us the additional burden placed on heating equipment if space around water heater is conditioned).

"> standby loss are correct but true of either system so mostly a wash...and half the element consumption
If you account for them equally - you did the COP=2 so cost = 2x mistake. Either use a COP closer to 3 or add to the cost of resistance electric water heat." 
Fair enough. I calculated operating cost (which includes standby loss) for my customer (software I use has proven reliability). Engineer provided us with actual operating efficiency of his equipment.
There are flaws to my calculations, but they cut both ways. Don't forget, standby loss increases and operating COP diminish with cooler temps. since our basements start around 65* around here, we certainly wouldn't maintain 65* with a HPWH operating downstairs, unless we heat the space (which is what I am suspicious will have to happen).

Curiously I figured my customer's hot water temp at 125* so his op cost would go down at 110* and payback would stretch longer.

Again, I see definate application for this equipment, I just don't know that a resi. basement in MI is one of them.
Joe

I would like to get a clear idea of the costs. As I understand it:

GE Geospring $1600 - 30% credit = $1120
good 50 gal electric = $400

difference = $720, payback = 7.5 years (using unusual example above), fairly attractive and better than many green projects

I would never consider 110F on a water heater - far too much bacteria risk. 125F, maybe for myself, not for a customer.

"Water heater temperatures should be maintained at 135ºF or higher up to the point of use to keep Legionella bacteria out of the potable water. Water heaters set at 120ºF may save some energy, but they DO NOT protect people."

difference = $720, payback = 7.5 years

Hmm... $8/month?

Not nearly worth the added complexity (IMO, of course).

Simple is better than better.

"Curt, you should be able to calculate the BTU's required to heat the volume of water you use (that would tell us the additional burden placed on heating equipment if space around water heater is conditioned)."

OK, here goes:

Using my pre-HPWH, non-DSH days of 6 kwh per day that works out to ~20 kbtu / day. The HPWH at a COP of two would therefore be pulling 10 kbtu / day out of the ambient air, not a significant load in my circumstances.

National average hot water energy use is roughly double mine, so the hit on ambient air should roughly double. It is also peaky, depressing ambient air temps a bit during periods of heavy recovery.

I agree that if much of the year the house air heating system has to make up the heat absorbed by the HPWH then the thing isn't worth having. A Michigan basement wouldn't be the first place I'd install an HPWH. Up north I'd consider a W-W HPWH if a geo loop is available, keeping in mind that its use would increase strip heat use during cold weather.

If a WW unit delivered at COP 3, the loop in a home using national average energy for hot water would have to provide roughly an extra 1000 btuh.

A WF EW020 could reasonably be expected to run at COP 3 or better, though at what installed cost I do not know, and if a DSH is already in the picture, payback gets pushed out.

For national average I'm using EPA's 64.3 gal / day figure and a rise of 80F, both of which dovetail fairly well with the annual kwh use noted on the yellow sticker affixed to stotage electric water heaters.

As usual, the circumstances of each situation obtain over broad generalizations.

"GE Geospring $1600 - 30% credit = $1120
good 50 gal electric = $400"

Ummm I only pay 1/2 that for an electric heater such as the one in my op cost study. A "good" $400 tank would be the same tank with a longer liner warranty.
Regardless were your water heaters going to install themselves? Retail on a gas or electric tank installed $600-$700 on a HPWH $2,500 to $3,000. Change over from propane for instance will require assisstance of an electrician and two permits.
You made the mistake of figuring the price of the equipment = price of job.
Yes you could do it yourself and ignore the law (not pull permits), but the context of the discussion was advice to my customers.

"I would never consider 110F on a water heater - far too much bacteria risk. 125F, maybe for myself, not for a customer."
How many customers do you have?...kidding. Customers are educated on temperature adjustments and are in control of settings. I simply pick a number for the purpose of calculation.

"If a WW unit delivered at COP 3, the loop in a home using national average energy for hot water would have to provide roughly an extra 1000 btuh."

That would add 2% to the load of my example house. Not huge by any standard, but it is a cost that impacts COP and drops savings in scenario above to ~$80/yr.

" (using unusual example above)"
Not sure what qualifies my example as unusual except that real operating cost calculators exist for other water heaters, and I can only use Curt's data on HPWH as I am unaware of any other. Perhaps you were refering to the use of a DSH in the equation? This is a geo forum.


Your actual price breakdown would be $2,500 less 30% = $1,750 less electric water heater installed cost $600 =$1,150 (I used low end price for both). $1,150/80 =14.375 yrs. If you are changing out a gas water heater you would use high end price for both; $3,000 less 30% =$2,100 less price of other heater $700 = $1400/$80 = 17.5 years.

"Hmm... $8/month?

Not nearly worth the added complexity (IMO, of course)."

We are neglecting another point in this discussion. Repairs and lifespan.
How long are the heat exchangers going to last in MI well water? One refrigeration circuit repair could kick payback well beyond life expectancy of tank.
An electric water heater element is a repair many homeowners do themselves. In fact I have replaced less than a dozen electric elements in my 22 years in this biz.

Give me a manufacturing building with lots of heat from machinary and I'd recommend these things all day long. Still can't see it in a resi application in MI.
joe

Posted By joe.ami on 03 Apr 2010 09:46 AM
I only pay 1/2 that for an electric heater...

Joe, can homeowners go out any buy a $200 water heater (if so, where and what size), or is this the cost to installers?  I was under the impression that tanks with longer warranties have a larger anode (I know anode value has been debated here and elsewhere) to help protect the tank from rusting.  Thanks

$200 gets you a bargain basement storage electric water heater, short warranty and lower EF, at Lowes or HD. As a beginning contractor I'm not yet aware of cheaper sources wholesale only.

If you Google HPWH as I did this morning while procrastinating from my real job you'll find several PPT slide shows and PDFs with pros and cons of HPWHs. Utilities are interested in the demand side reduction potential of these things, but have been burned in the past by technologies (including early HPWHs) that failed to pan out.

Several presentations support what Joe and I quite agree upon: that suitability varies hugely by climate. I didn't anywhere see consideration that HPWH operation in a cold climate could drive up house heating load and impose extra cost. The laws of thermodynamics are strictly enforced even while not acknowledged...

Though I prefer the GE for its simplicity and higher EF, Rheem has an "Ecosense" presentation online making a credible argument for their design's higher first hour rating and arguably greater likelihood that the end-user will leave it in the heat pump mode. I prefer an old school brute force approach to First Hour Ratings - get a bigger tank, but I can't argue with the marketing decision to pack the most punch into a 50 gallon nominal package, since the bulk of sales are in that size range.

Though wary of making blanket statements I'm nearly certain that if ROI is an important or sole consideration then a HPWH will not make sense in any application wherein a geo DSH is operating or planned. Exceptions might exist in cases of very high kwh rate combined with suitable climate and very high hot water demand, but that trio of circumstances is rarely found.

Mixing retail and wholesale prices leads to inaccurate conclusions, as does comparing cheap tanks to better ones. No mistake - you say labor on a HPWH install is ~$1000 and an electric tank install is ~$200 because of what, a drain line (especially with a tax credit on the HPWH labor)?  Much more accurate to call labor a wash and exclude it.

Posted By joe.ami on 03 Apr 2010 09:46 AM
We are neglecting another point in this discussion. Repairs and lifespan.

The discussion is also neglecting the time value of money.

It's hard to justify $700-$1k up front expense to save $8/mo
if you're carrying any balance on a credit card -- where the
guaranteed savings could easily be double that.

Posted By jonr on 03 Apr 2010 11:14 AM
Mixing retail and wholesale prices leads to inaccurate conclusions,
I agree, much better to include it than pretend it doesn't exist. Though we left out sales tax which likely would add a year to ROI.
as does comparing cheap tanks to better ones.
Your selection of the term "better ones" was not mine. I simply gave you the price of tank in my op cost example.
No mistake - you say labor on a HPWH install is ~$1000 and an electric tank install is ~$200 because of what, a drain line (especially with a tax credit on the HPWH labor)?
No I said retail price of a tank installed which includes sundries such as tax, labor, permit, warranty.....the usual over head.
But if you care to be accurate I said installed cost of electric storage tank was $400 to $500 and HPWH ~$900 I didn't say labor cost that much.
Much more accurate to call labor a wash and exclude it.
No way, when I do like water heater to like water heater install is less than 1 hour. Changing style much more effort and opp for trouble.

J

To my knowledge HPWHs cost $1300-1700 BEFORE installation.

"Much more accurate to call labor a wash and exclude it.......
No way, when I do like water heater to like water heater install is less than 1 hour. Changing style much more effort and opp for trouble."
To clarify I know people here get "cost of doing business", but I thought I should expound.
1 man can do an apples to apples water heater change out in an hour or two. ASHPWH in a basement install alone requires second laborer for delivery. Not only due to weight but also due to potential cost of mishap (greater property damage or greater loss of dropping a tank that costs 8 times as much). Additional piping and materials plus up to ~ 8 times the sales tax, up to a man day of labor (3-4 hours X 2), greater warranty liability (more time consuming repairs) and that drain easily add $400-$500 to installation.
Does anyone really presume a company that intends to stay in business would install one of these for the same labor price as a big box budget tank?
Can a DIY skew these numbers in their favor? Of course. But isn't that true of everthing? If that's the case can you characterize this as a better pay back for a green project?
Of course not. My geos' payback in under five against oil or propane and under ten against nat gas.
j