Drainwater Heat Recovery Can Lower Your HERS Score

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Drainwater Heat Recovery Can Lower Your HERS Score

Although these simple devices have always saved energy, they weren’t recognized in HERS Index calculations until recently

Posted on Nov 4 2016 by Martin Holladay
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Drainwater heat recovery (DHR) devices have been around for more than twenty years. By now, over 60,000 of the units have been installed in North America. When one of these devices is installed in a typical single-family home, it can reduce the amount of energy used for domestic hot water by 15% to 22%.

Two years ago, changes were made to the rules governing HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Index calculations to give credit for drainwater heat recovery devices when calculating a HERS rating. For builders who are interested in advertising an impressive HERS Index, this is important news — because installing a drainwater heat recovery device can be a relatively inexpensive way to get a two-point improvement in a HERS number.

Researchers measure energy savings

The first such drainwater heat recovery device to hit the market was called the GFX. In the December 1996 issue of Energy Design Update, editor Ned Nisson wrote, “The GFX is a ‘coil-and-tube’ counterflow heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. that is installed vertically in the home’s plumbing waste line and connected to the cold water main. Warm wastewater from showers and sinks runs down through the central copper pipe while incoming cold supply water runs up through the tightly would coil of copper tubing.”

Over the years, a parade of researchers looked into the potential savings attributable to the use of a drainwater heat recovery device:

  • In September 1997, Energy Design Update reported the results of a residential study conducted by Northeast Utilities in Hartford, Connecticut that documented hot water energy savings of 20%.
  • In October 2000, Energy Design Update reported the results of a study conducted at a multi-unit residential building in Duluth, Minnesota by Oak Ridge National Laboratory that documented hot water energy savings of 15% to 22%.
  • In October 2006, I wrote an article for Energy Design Update that reported the results of a field study of 18 Manitoba homes conducted by Manitoba Hydro that documented hot water energy savings of 16%.

While the GFX is still being manufactured by WaterFilm Energy of Medford, New York, all of the other manufacturers of drainwater heat recovery devices are Canadian. These manufacturers include:

Prices for these devices range from about $450 to $800.

[Image credit: Lyndon Than]

Important facts to keep in mind

Here’s what we know about drainwater heat recovery devices:

  • The devices work with tankless water heaters as well as tank-style water heaters.
  • To work properly, most types of drainwater heat recovery devices need to be installed vertically. (The exception is a device called the EcoDrain, which is designed to be installed horizontally.)
  • If a house has a basement, it’s usually fairly easy to install a drainwater heat recovery device. However, drainwater heat recovery devices don’t work well in single-story homes with a slab-on-grade foundation. For a slab-on-grade home, this type of device will only work if the house has two stories and the shower is upstairs.
  • Savings percentages are higher in cold climates than in warm climates, because the temperature of incoming cold water is lower in cold climates than warm climates. A higher delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. between the cold supply water and the warm drain water improves the efficiency of a drainwater heat recovery device. Moreover, homeowners in Miami (where incoming cold water is never really very cold) have lower hot water energy bills than homeowners in Montreal, so opportunities for savings are lower in Miami.
  • Energy savings are only possible if water is flowing down the drain at the same time as hot water is being drawn, so savings are much higher in homes where occupants prefer showers than in homes where occupants prefer baths.
  • Other factors being equal, the longer the drainwater heat recovery device, the higher the energy savings. A typical unit is 5 feet long; shorter units are also available.
  • If you have a tank-style water heater, a drainwater heat recovery device will extend the length of time that you can run your shower before running out of hot water. In other words, it increases the apparent capacity of a water heater, in effect making a 30-gallon water heater perform like a 40-gallon or 50-gallon water heater.

[Image credit: RenewAbility]

Make sure that you have an “equal flow” installation

Although manufacturers of drainwater heat recovery devices provide clear installation instructions, some installers still get the details wrong. If you care about energy efficiency, the most important detail is to follow the directions for a so-called “equal flow” installation.

Here’s the key detail: the preheated water leaving the drainwater heat recovery device must be connected to a tee. One side of the tee is connected to the water heater’s cold inlet, while the other side of the tee is connected to the cold supply line to the shower or showers. An installation that simply sends the preheated water to the water heater's cold supply will have reduced energy savings compared to an “equal flow” installation.

Lower your HERS Index

Drainwater heat recovery devices have always saved energy. But until recently, the devices had no affect on a home’s HERS Index score.

Two years ago, changes in the rules for calculating the HERS Index allowed energy raters to give a credit for drainwater heat recovery devices.

At the recent EEBA conference in Frisco, Texas, Rod Buchalter, the vice president of sales for RenewAbility Energy, a manufacturer of drainwater heat recovery devices, told conference attendees that including a drainwater heat recovery device in a home could lower (that is, improve) an HERS Index by about 2 points. “For example, for a house in Baltimore that was HERS 85, the drainwater credit changes it to a HERS 83, almost an 82,” said Buchalter. “In southern climates, energy savings will not be as much as up north. In Duluth or Alaska, you might get a 4 point reduction. In Phoenix and Miami, you might not get any change to the HERS Index. But in Orlando, Florida, a two-story home can get a 2 point reduction in the HERS Index by installing drainwater heat recovery.”

In short, Buchalter concluded, “Builders include drainwater heat recovery because it’s a low-cost way to lower the HERS Index or to gain LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. points. Effectively, it also increases the water heater capacity. It’s an easy installation, and there is no maintenance required.”

Martin Holladay’s previous blog: “States are Amending, then Adopting, the 2015 IECC.”

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Image Credits:

  1. Image #1: Lyndon Than / PassiveHouseOne
  2. Image #2: David Pill
  3. Image #3: Larry Weingarten

1.
Nov 4, 2016 9:06 AM ET

Other facts to consider.
by D Dorsett

It's usually the case that the additional marginal cost of a bigger (= both fatter & longer) drainwater heat recovery unit "pays off" quicker than going a smaller (= skinnier &/or shorter) unit. The installation labor is about the same independent of size, so installing the largest one that fits the available space is usually going to be "worth it", even if it is a few hundred USD more for the unit than a skinny stubby one.

Natural Resources Canada developed an efficiency testing protocol to enable apples-to-apples comparisons between different models & manufacturers. They maintain a database of third party tested return efficiency (at their test parameters), and an updated spreadsheet can be downloaded from a link on this site:

http://oee.nrcan.gc.ca/pml-lmp/index.cfm?language_langue=en&action=app%2...

The NRCan test protocol is at a full flow shower rate (~2.5gpm), but lower flow rates increase the energy return efficiency, whereas higher flow rates lower the return efficiency.

Get pre-approval from the building inspectors- there have been instances where local inspectors condemned them on a (flawed IMHO) basis that potable piping in contact with drain piping (even though it's double-walled construction, and it's manufactured in a way that they can't abrade or interfere with one another.) Some models also use thicker tubing than used for typical potable piping, and lack the stripe indicating that it's suitable for potable piping per ASTM specs. The quality of that tubing is actually higher than typical potable piping, but it's thicker walled than the materials spec that falls under that which can be marked. This to has led to problems with inspectors, and condemned heat recovery units.

This is an inspector or letter-of-code problem, not a safety problem . Those same units have been condemned in some locations for lack of a marking stripe are pre-approved in Massachusetts, which has fairly stringent rules for what may/may-not be connected to potable plumbing, and the state maintains a (searchable, online: http://license.reg.state.ma.us/pubLic/pl_products/pb_pre_form.asp ) database of fixtures that are allowed to be hooked up. Anything not listed requires a variance in Massachusetts, and not all drainwater heat recovery units are on the list (though they could be- the manufacturer has to apply.)

Some states offer rebate subsidies for drainwater heat recovery based on efficiency, though the details vary.


2.
Nov 4, 2016 9:31 AM ET

Response to Dana Dorsett
by Martin Holladay

Dana,
Thanks for your useful comments. I was unaware that some building inspectors were raising questions about these units; thanks for the warning.

Battling building inspector ignorance and obstructionism sometimes feels like a full-time job.


3.
Nov 4, 2016 11:03 AM ET

Times change, but it can be slow.
by Dana Dorsett

This thread is going 6 years old, but chronicles a particular case where this occured:

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/14/afv/topic/aft/78...

The same product condemned by that inspector is a pre-approved plumbing fixture in Massachusetts:

http://license.reg.state.ma.us/pubLic/pl_products/pb_search.asp?type=P&m...


4.
Nov 4, 2016 11:15 AM ET

Note that the cost
by Jon R

Note that the cost effectiveness is highly influenced by the usage and the cost of heat. With the 1 GPM I use, the ROI wasn't at all attractive.


5.
Nov 4, 2016 5:39 PM ET

If it saves you the cost of upsizing...
by Dana Dorsett

If it saves you the cost of upsizing from 50 gallon heat pump water heater to an 80 gallon or 100 gallon HPWH it pays for itself up front.

If the goal is to actually hit your Net Zero Energy goals it can be cheaper than the additional insulation / window /solar it would take to get there.

It's not always about the net present value of first + financing costs relative to future energy cost savings. The title of this piece is, after all

"Drainwater Heat Recovery Can Lower Your HERS Score"

If the goal is to limbo under some HERS target, it can cheaper than some other methods of getting there.


6.
Nov 4, 2016 11:24 PM ET

Mandatory in Manitoba
by Scott Benson Climate Zone 7A

I was just reading a revised building code document for the province of Manitoba and it stated DWHR devices are mandatory for all new home construction. It states that they are only to be installed where physically possible (not for mainfloor shower with no basement). But doesn't state if all showers require them or other appliances or fixtures require them.

Would it make sense to install for a washing machine or dishwasher?

http://www.mboa.mb.ca/admin/uploads/files/NBC%20Section%209_36%20at%20MBOA%20(Oct%2022%202015)(3).pdf


7.
Nov 5, 2016 5:32 AM ET

Response to Scott Benson
by Martin Holladay

Scott,
If a drainwater heat recovery device only serves a washing machine or dishwasher, it would save so little energy that the payback period would be greatly extended. Most analysts would conclude that this type of installation is a waste of perfectly good copper.

Thanks for letting us know about the Manitoba requirement. As far as I know, that's a first.

I can't help noting one unfortunate feature of the PowerPoint presentation you linked to: it includes an illustration that depicts a flawed installation. In the example shown in the illustration, the pre-heated water is sent to the cold inlet of the water heater, instead of to a tee as required for an "equal flow" installation.

.

Drainwater heat recovery image - Manitoba PowerPoint.jpg


8.
Nov 5, 2016 10:35 AM ET

Doubly flawed
by Charlie Sullivan

Not only is that illustration missing the T--it's also connected backwards. Heat exchangers work much better in a "counterflow" configuration, with the supply water coming in at the bottom (near the exit of the drain water) and coming out at the top.


9.
Nov 5, 2016 11:19 AM ET

Response to Charlie Sullivan
by Martin Holladay

Good catch! You're right.


10.
Nov 5, 2016 4:22 PM ET

If it isn't already obvious
by Eric Habegger

If you have marginal requirements for longer hot water shower times than you are currently getting AND you don't currently have a low flow shower head then save some money and time and go for the low flow shower head and see if it works for you. I know, everyone here already has them but I figure its better to be comprehensive than not allow for outliers. There are some pretty good ones now that don't feel like low flow showerheads.


11.
Nov 6, 2016 11:41 AM ET

Different waste sources
by Malcolm Taylor

I understand their effectiveness with simultaneous water use and drain situations, but can someone tell me how they work where that doesn't occur? With baths, dishwashers and washing machines, where there is a delay between water use and drainage, how does their efficiency suffer? The pre-heated water in the coil would presumably be available for some period of time, but would be cooled if any other cold water drains into the plumbing stack.

Are these effectively shower heat recovery devices? And if so would there be any benefit to twinning the drains so that they only served showers?


12.
Nov 6, 2016 12:31 PM ET

Edited Nov 6, 2016 12:46 PM ET.

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
For all intents and purposes, these devices recover the heat that goes down the drain from a shower.

Of course, there will be some incidental heat recovery from other uses of hot water -- for example, when someone washes dishes in a sink, or on those rare (coincidental) occurrences when some family member is drawing hot water at a sink when water is being drained from a bathtub -- but those occurrences aren't enough to justify the cost of the drainwater heat recovery device.


13.
Nov 6, 2016 4:22 PM ET

Martin
by Malcolm Taylor

Thanks for the reply. I have been thinking about these units through the lens of the last two houses I designed.

The first was two storey slab on grade, with one shower upstairs and another on the main floor, so the unit could only serve one.The owners have been surprised by their electrical bills, but that appears to be largely due to the demands of washing the clothes of their two infant children. Perhaps this balance will shift over time towards more showers.

The second is on a crawlspace, and the occupants favour baths over showers. Their total annual electrical consumption represents somewhere in the region of $1000.

From a design perspective the questions I face are whether they work well enough for a generic family that it is worth including them even if they don't make sense for the present occupants, and whether they work well enough that it is worth making design changes to try and incorporate them, where otherwise they couldn't be installed.


14.
Nov 7, 2016 1:18 PM ET

It's probably more than you ever wanted to know but...(@Malcolm)
by Dana Dorsett

The NRCan modeling and test protocols were developed from research summarized here in some detail:

http://www.regie-energie.qc.ca/audiences/3637-07_2/DDR3637_2/RepDDR/B-12...

There is very little thermal mass in gravity film heat exchangers themselves, so unless flow is simultaneous the amount of heat recovered falls below what is easily measurable. The only residental use-case for this technology really boils down to showers- even hand washing & toilet flushing heat recovered is so tiny as to be difficult to measure, though it can be theoretically calculated.

Focusing that use-case entirely on NPV of future energy cost savings is to miss a piece of the boat. There is some savings on the size/capacity requirements of the hot water heating mechanicals as well. It's true that some families will be shower-bathers who literally never shower, at which point it's value is limited to scrap copper, but those instances are probably more than one standard deviation out. Whether it makes good policy sense to require them by building codes requires a bit more research on hot water use patterns across a large number of households to know the distribution of shower users vs. tub-only bathers in the region. (Hopefully that research was actually done in Manitoba.)

From a total carbon footprint of the heat exchanger, a few years ago I made a napkin-math estimate that with a standard gas tank type water heater and 20 minutes of 2 gpm showering use per day the "payoff" on atmospheric carbon emissions was less than one year. Most of the carbon emissions are from the smelting of copper from ore, with much lesser amounts attributable to transportation & fabrication energy. With more use data and a survey of water heating sources one could convert that into lifecycle carbon tons avoided to tons emitted ratio, and the relative cost to come up with a $/ton number, which would also be part of the policy discussion.


15.
Nov 7, 2016 4:23 PM ET

Dana
by Malcolm Taylor

"It's probably more than you ever wanted to know but.."

Summarizing that sort of stuff is kind of what I've come to rely on you for on such topics :)


16.
Nov 8, 2016 10:05 AM ET

Chimney pipe heat recovery
by ADK Creator

Can this concept be applied to a chimney pipe coming off a wood stove or masonry heater? Are there examples of this and products designed for this application?

Thanks


17.
Nov 8, 2016 10:35 AM ET

Response to ADK Creator
by Martin Holladay

AKD,
Any device that lowers the temperature of a wood-stove flue increases the rate at which creosote accumulates and clogs the flue. This raises the frequency of required cleanings and increases the risk of chimney fires. That's why lowering the temperature of a wood-stove flue is a bad idea.


18.
Nov 8, 2016 10:38 AM ET

Martin
by ADK Creator

Understood. Thanks


19.
Nov 9, 2016 1:14 PM ET

Edited Nov 9, 2016 1:18 PM ET.

Been working well for me for over 3 years
by John Charlesworth

Installed a 42x4" PowerPipe in our basement 3 years ago here in Victoria BC. (Zone 4 Coastal)

In winter, here was the heat recovered while using the shower (it stabilized within 30 seconds of starting the shower):
40F: temp of water from street
100F: temp of water from shower head
90F: temp of drain water by the time it reaches the plumbing stack
68F: temp of warm water leaving DWHR unit
This gave us a 28F temp gain "for free" while running the shower (28/60=43% energy use reduction).

This year I added a second PowerPipe (66x4") in series with the original one. Drain water from the 2nd floor shower runs through both units while water from the ground floor shower runs only through the original one.

The temp gains from the ground floor shower remained the same. However the heat gains from the 2nd floor shower are now close to the theoretical limit. My measurements show:

40F: temp of water from street
100F: temp of water from shower head
90F: temp of drain water by the time it reaches the plumbing stack
82F: temp of warm water leaving DWHR unit
This gave us a 42F temp gain "for free" while running the upstairs shower (42/60=70% energy use reduction).

Payback for this system will be under 4 years. Performance is better in colder climates (where incoming water temperature is lower). There is no significant heat recovery from dishwasher, toilets, or baths.

Having the two heat exchangers in series has worked well even when only the lower one is being fed with drain water because its outlet temperature (68F) is room temperature and therefore is energy neutral as it runs through the upper unit.

Highly recommended for those in cooler climates.

.../john


20.
Nov 9, 2016 2:58 PM ET

Bigger really IS better, in this instance @ John Charlesworth
by Dana Dorsett

Those numbers correlate reasonably will with NRCan test data. The 4x 42" PowerPipe tested ate 43.7% steady state under the standard test conditoins:

http://oee.nrcan.gc.ca/pml-lmp/index.cfm?action=app.formHandler&operatio...

Have you bucket-tested the actual shower flows?

The much higher return efficiency of longer units almost always pays back sooner than shorter ones. I only had space for a 4" x 48" running at ~50% average return efficiency (in service since late 2008) at my bucket tested ~2 gpm, but it's been worth it, despite the decline in the retail cost of natural gas that has occurred since it was installed.


21.
Nov 10, 2016 11:03 AM ET

Actual efficiency even higher
by John Charlesworth

I haven't measured the gpm flow yet--I should do that.

The biggest loss is the cooling that happens as the shower water falls through the air from shower head to bathtub drain--almost 10F.

Looking at the heat recovery of the heat exchangers themselves based on the actual temperature of the drain water entering them, it's even higher as a percentage.

For the R4-42 unit on its own, actual measured efficiency is 28/50=56% and for the combination of the two in series, it's 42/50=84%.

Both of these are better than the NRCan ratings so I must be at the sweet spot regarding flow rates. I do have it plumbed so that the pre-heated water feeds both the water heater tank *and* the "cold" side of the shower controls.


22.
Nov 10, 2016 11:47 AM ET

NRCan measures the net efficiency, not the simple efficiency.
by Dana Dorsett

The test protocol measures the temperature at the showerhead, not the temp at the drain for determining the efficiency, since that's the fraction of actual net energy returned of the raw energy input.

That's a more relevant number than the raw efficiency of the heat exchanger itself in a closed system, without the losses to the room that happen between the shower head and the drain.

One could argue that in a heating dominated climate a large fraction of the energy lost between the showerhead & drain isn't really lost, since it offsets the heating load, but that gets a lot squishier, and isn't what they are really trying to measure.


23.
Nov 11, 2016 1:04 PM ET

What about commercial use?
by Buzz Burger

I can't help but think that in certain commercial applications there could be a tremendous savings/quick ROI. At my local health club, for example there is usually continuous multiple shower use for about 15 hours a day, Is the appropriate hardware available for that situation or can the same hardware work? Often, but not always there is no basement below which could be a problem.

I've also wondered if the hardware exists to somehow connect the heating of water with refrigeration. Probably totally impractical for a single family home, but what about a large restaurant or other food service facility that requires a lot of refrigeration and a lot of hot water use? Is there a practical way to suck the heat out of a freezer with a heat pump and transfer it to water?


24.
Nov 11, 2016 5:27 PM ET

Response to Buzz Burger
by Martin Holladay

Buzz,
Manufacturers of drainwater heat recovery devices are very familiar with commercial installations at establishments like hotels, laundromats, etc. Owners of commercial establishments should talk to these manufacturers about their needs -- most manufacturers can help design a good installation.

If you use the right terms in the GBA search box -- maybe "hybrid refrigerator water heater," without the quotes -- you'll read lots of threads on the topic. The topic comes up on GBA like clockwork, twice a year. Makes no sense for a residential installation, as you guessed.


25.
Jan 30, 2017 9:46 AM ET

Can a drainwater heat recovery unit serve two showers?
by Timothy Godshall

I have two showers located close together and am wondering if one of these units can serve both of them? If so, I suppose the equal flow installation would have to include a pipe serving both showers. Also, how far away from the shower can the drainwater heat recover unit be located and still be effective? Due to the wall layout in the basement under my bathrooms, the vertical drop won't occur till about 15-20 feet away from the shower drain.


26.
Jan 30, 2017 9:52 AM ET

Response to Timothy Godshall
by Martin Holladay

Timothy,
Q. "I have two showers located close together and am wondering if one of these units can serve both of them?"

A. Yes -- certainly.

Q. "If so, I suppose the equal flow installation would have to include a pipe serving both showers."

A. That's correct.

Q. "How far away from the shower can the drainwater heat recovery unit be located and still be effective? Due to the wall layout in the basement under my bathrooms, the vertical drop won't occur till about 15-20 feet away from the shower drain."

A. It's still worth installing the drainwater heat recovery device. If you want to boost the performance of the system slightly, you could wrap the drainpipe that extends from the showers to the drainwater unit with insulation.


27.
Jan 30, 2017 10:36 AM ET

No need to insulate the distributed output of the heat exchanger
by D Dorsett

The easiest way to have the heat exchanger's output serve multiple showers is to have it supply the entire cold water distribution for the house. The output of the heat exchanger is typically between 68-75F (depending on incoming water temperature, flow, and the size of the heat exchanger), It would take an unusually large heat exchanger for it's output to be much more than that.

So if the distribution plumbing is inside of conditioned space (even if it's a unheated 60F basement), the total amount of heat loss from even 50' of distribution plumbing is pretty low, not enough to rationalize insulating the tepid heat exchanger output plumbing to the shower(s) and water heater.

Losses from the drain distribution are slightly higher due to higher temperature of the drain, but again the losses are pretty small, especially with plastic drain pipe. The losses will be slightly higher on vertical sections of drain due to the gravity-film distribution on the interior surface, but on the horizontal sections the water is flowing in a tight stream along the bottom of the pipe. Only with very long drain plumbing would insulating it be worthwhile.


28.
Feb 1, 2017 7:17 AM ET

Response to D Dorsett
by Timothy Godshall

Thanks for your response. I have a question about your recommendation: "The easiest way to have the heat exchanger's output serve multiple showers is to have it supply the entire cold water distribution for the house." Wouldn't this dilute the efficiency of the system since the hot water gains are being dissipated in the whole house's cold water lines rather than going directly to the shower that is drawing the water? (I acknowledge that you did say "easiest" not "most efficient"!)


29.
Feb 1, 2017 8:04 AM ET

Not much (response to Timothy Godshall)
by D Dorsett

"Wouldn't this dilute the efficiency of the system since the hot water gains are being dissipated in the whole house's cold water lines rather than going directly to the shower that is drawing the water?"

The tepid water only flows toward the open taps, and it's at roughly room temperature, with very little heat being lost to the rooms.

If a sink or toilet is filling while the shower is running there is some loss, since the heat going into the sink or toilet isn't being returned to the hot water heater, shower mixer, or the drain. If free-flowing (cold only) into the sink at high flow the dilution factor will be real enough. But in practical terms those events aren't usually long in duration or very frequent in most homes.


30.
Jun 18, 2017 3:16 PM ET

Why not just send the heated water straight to the shower?
by Eric Woodhouse

So, dumb question, perhaps: Assuming the DHR device is attached to a single shower drain, why is the "equal flow" installation better than just sending all the preheated water to the cold side of the shower mixing valve? I was in a class on heat pumps for water heating a couple weeks ago and it was mentioned that the COP of the heat pump drops as the cold supply temperature increases, so that returning preheated water to a HP tank isn't a good idea... but as I think about it, it seems like using the preheated water immediately would be most efficient, regardless of the heat source. Am I missing something here?


31.
Jun 18, 2017 4:29 PM ET

Response to Eric Woodhouse
by Charlie Sullivan

The efficiency of heat recovery goes up when the volume of incoming water going through it increases, and goes down when it's lower. So you simply get the best heat recovery when all the incoming water flowing at that time goes through the heat exchanger. It's true that the heat pump COP goes down when the water temperature goes up, but it's really the mix temperature in the tank, not the incoming water temperature, that matters. If you are significantly dropping the tank temperature, the people showering aren't going to be happy, so there isn't really a good way to make use of that phenomenon.


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