More Musings of an Energy Nerd
An increasing percentage of green builders are choosing to install heat-pump water heaters — appliances that use an air-source heat pump to extract heat from the air, transferring that heat to water stored in an insulated tank. Most heat-pump water heaters incorporate both of these components — the heat pump and the insulated tank — into a single unit, with the heat pump compressor mounted above the insulated tank. This type of water heater is typically installed in a basement, mechanical room, or (in warm climates) in an attached garage.
If you look at the heat pump compressor mounted on top of a typical heat-pump water heater, you’ll notice that there are two openings for air flow. The unit has a fan that pulls ambient air through the intake. Inside the unit, this air passes over a copper heat-exchange coil and transfers some of its heat to the refrigerant flowing through the coil. Then the air, now cooled, is expelled from the exhaust side of the appliance.
This process raises the temperature of the refrigerant and, eventually, the water in the tank. The process also cools the air in the room where the heat-pump water heater is located. If a heat-pump water heater is installed in a very small room — a closet or a tiny mechanical room, for example — the room can get so cold that the heat pump can’t operate properly. That’s why the manufacturers of heat-pump water heaters dictate the minimum volume (generally, 1,000 cubic feet) of the room in which this type of appliance can be installed.
The larger the room, the lower the chance that the room’s air temperature will sink to a level that is too low for efficient operation of the water heater.
Ducts provide installers with a little flexibility
Some manufacturers of heat-pump…
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42 Comments
Thanks for the great info Martin.
In the Pacific Northwest we've advised builders that if the unit is located inside the heated space, ducting both intake and exhaust to the outside is a good option for 2 reasons - 1) you avoid pressurize/depressurize issues on the building envelope in a tight house and 2) the outside temperature is above 35 F most of the year up here and I believe that's the cutoff point where most of these units switch to electric resistance heat. Even though the heat pump will run less efficiently at colder air intake temperatures isn't that still better than ER mode?
Oh and the other assumption would be this is a small home without a good spot to dump cold air into...
A Habitat project I'm working with will be installing a Rheem and ducting the exhaust into a conditioned crawl. The sole criteria is really comfort, it seems like a win/win to let the cool air temper in the crawlspace before returning to the house and also keep the intake temps a little higher throughout the cycle. This article is reassuring in that the ducting should have a negligible effect on performance.
I'd like to second the factory filter not filtering very well. The one on my GE at home doesn't filter well and seems overly restrictive at the same time. Removing it is worth a good 5 db of noise when it's running.
I realize this is a bit of topic, but John Semmelhack's comments about upgrading the filter for the HPWH touch on a concern I have about the Habitat home I mentioned above and others like it. That home will have 5 filters in it that need maintenance, 2 mini-split heads, 2 in the ERV, and the HPWH. If many of those are replaceable the cost could add up, and if they're washable the reality is that many homeowners aren't always that good at keeping up with this stuff. Is anyone out there doing long term maintenance contracts to make sure this stuff is getting done? Reminder systems at least. Ideas along these lines for affordable housing? As someone who does a lot of existing home analysis, maintenance is a serious issue. Dealing with the long term operation of residential high performance homes seems like a good topic unto itself. I'm curious what others are doing.
Great point about maintenance Andy. My feeling is that roughly 0% of homeowners can be trusted to clean/change filters. I'm about to build a small home for a client and plan to "strongly suggest" they pay me a small fee to come do periodic ERV / mini-split / HPWH filter cleaning. Also gives me a chance to make sure equipment is running smoothly. Now, how many years can I realistically do that? Not really sure...
I ducted my new heat pump water heater a couple years ago, with baffles, so in the summer I draw from my hot room and exhaust cooler air into the same room (free air conditioning), and in the winter (climate zone 2, always above freezing), I draw from the outside and vent to the outside.
I just installed a Rheem 65 Gallon HPWH in a 500 SF unfinished, unconditioned basement with no other heat source. I'm in Massachusetts so the basement room usually bottoms out around 45 degrees which is where it sits now. During extended operation of the water heater the room will drop to about 43 degrees temporarily. Rheem says this unit can run in heat pump mode down to 37 degrees but I'm noticing the recovery time is very slow, which is expected. My question is this...does it make sense to duct the intake side of the water heater into the adjoining finished 600 SF basement room in order to steal the warmer air thus increasing efficiency and recovery time? I would plan on leaving the unit exhausting into the unfinished basement which would benefit from the drier air, but I could return the exhaust back into the finished room if needed. It seems to me that this would help with efficiency but would it be significant?
Jon,
Your suggested ducting experiment is certainly worth a try.
We are thinking about installing a HPWH in our laundry/mechanical room. The laundry room is 850 cubic feet. The laundry room will also house the HRV system for the house. If we put a supply and return vent from the HRV into that room, will we be able to exchange enough air to negate the cooling effect of the HPWH? The internal ducted multi-position unit for the multi-split will also be in that room. Would another option be to do a significant enough air exchange through that unit to distribute the cooled air throughout the house? I think this latter option would have to be mainly on the supply side to prevent overheating of the room when the HPWH is not running. The multi-position unit will be run in an always-on type set-up in low speed.
Kevin,
Your HRV should be set up to provide the desired cfm of supply air as specified by ASHRAE 62.2 or to meet your family's needs. It seems a shame to devote a portion of your HRV's air flow to the mechanical room, where fresh outdoor air isn't usually required. Of course, an exhaust grille (connected to the HRV system) makes sense -- and if you have an undercut on the door, that exhaust grille will help with air movement into the laundry room.
If this were my house, I'd probably live with the situation and see how cold the room gets. Of course, there is a downside to installing a heat-pump water heater that is smaller than the minimum recommendations set by the HPWH manufacturer -- you may void the warranty if you don't follow instructions. Options include a louvered door on the laundry room (a method approved by some HPWH manufacturers) or ducts connected to the HPWH as advised in the article on this page.
Martin,
Great advice as always. Based on the information in the article and the comments here I decided to go with the Rheem unit in a big part due to noise. It is only 49dB which compares to my current fridge at 44dB (according to the iphone noise meter app). The Rheem calls for a minimum room size of 700 cubic feet (thanks for the link). So I think I should be okay with an HRV exhaust as one might normally do with a laundry room and balanced HVAC connections for normal room conditioning. If I have to duct HPWH I can go out into the hall discharging right above where the main air intake is for the HVAC ducted unit.
Thanks for this Martin. I'm about to install a HPWH on Long Island--likely the Rheem or Ruud. I had been planning on using an intake duct that would draw air from outside in the summer. In the winter, I would disconnect and cap it. The discharge duct would go into the basement year-round for dehumidification.
I know that you strongly discourage a seasonal approach. But I'm one of the zero percent that would remember. I've worked on every inch of this house and pay a lot of attention to my heating/cooling! I'll have to remove it prior to dying, however.
Considering the outside air in the summer could be a bit less than 20 degrees F warmer than the basement air, I'm still tempted to use a duct on the intake side during the summer. Even though our unconditioned basement is huge (perhaps 3-4000 sf) I can't help but think that the differential would provide some benefit. Do I need to be talked down from the cliff?
Actually, the more I think of it, especially considering the cost of running the duct through the exterior wall ($250, unless I do the install myself), it's seeming less and less attractive. I'm being my own therapist here :-)
Any thoughts? Thanks!
User-7665154,
If you pull in outdoor air for your heat-pump water heater, and discharge that air into the basement, you are in effect pressurizing your house. I don't recommend this, for a variety of reasons -- I'm in favor of houses that try to maintain a neutral pressure with respect to the outdoors. Pulling in outdoor air during the summer will also tend to increase your air conditioning costs.
Thanks for the response Martin. Looks like I'll forgo the duct.
You said "Pulling in outdoor air during the summer will also tend to increase your air conditioning costs." I was under the assumption that any warmth in the outside air pulled in would be extracted by the heat pump, sending cool, dry air to the unconditioned basement. What am I missing?
>"What am I missing?"
It may or may not be delivering cooler/drier air than the air that is being pushed out of the house by pressurizing the house, but it's definitely NOT lowering the cooling load by very much even in the best-case scenario.
The humidity of the outdoor air is much higher than that of the indoor air (especially in the "-A" climate zones) in summer, so the temperature & humidity of the output of the heat pump water heater is guaranteed to be higher than if it were drawing from indoor air.
Thank you Dana!
The HPWH reduces the temperature of the incoming air by about 20 degrees. If you're bringing 90 degree air inside, it'll only drop to around 70 degrees. That's not much cooling. And, the dewpoint of that air might not be much less than 70 degrees, making it pretty damp. if you want the HPWH to cool and dry the basement, you should just recirculate the basement air through it. Using outside air will increase the COP of the water heater a bit, but it will likely cause other issues that don't make the exercise worthwhile.
That's really helpful Peter. To your credit, the two energy efficiency experts who came to do estimates didn't know what you guys know!
Kudos to you. Thanks!
I'm thinking to put the HPWH in my small laundry/mech room with a well louvred door.. and also locate an ERV exhaust point there.
This way the ERV is sucking out the cold air from the room and constantly pulling warmer air into the room. Seems like a solid idea?
NMS43,
During the summer, the HPWH will be cooling your indoor air--a desirable feature. Instead of sucking the cold air out of the mechanical room and sending it out of the house, you might want the cool air to linger.
During the winter, pulling cool air into the exhaust duct of your ERV will lower the effectiveness of the ventilation system's ability to warm up the incoming fresh air that is pulled from the outdoors into your ERV's core.
In short, there can be undesirable consequences associated with your plan, and the energy implications are complicated.
Hey Martin! Thanks for chiming in.
For the summer issue you could place a damper on that exhaust point so it's not sucking out the cold air. Being in climate zone 5 I'm more concerned with the unwanted cooling during heating season.
The HPWH has to steal it's heat from somewhere. What would be a better option? In my case there is no attic, crawlspace, or basement to utilize.
My increasing experience with these has led me to a couple conclusions since first reading this article.
1. The HPWH has to steal heat from the house in a cold climate, don't pretend otherwise, just deal with possible comfort issues.
2. In most cases the HPWH only runs a small portion of the day and you're actually a lot less likely to notice it than you think. The room it's in returns to normal quite quickly. This comes from customer feed back, but everyone is different.
3. With the Rheem model you can schedule a tank temp increase in the night time hours, if you have a larger tank, a mixing valve, and run up tank temp, it can do most of it's pumping at night when no one will notice. (depends heavily of usage) It could also do this during the day if no one is home and sun is shining on some PV.
4. I don't think this was mentioned in this article, and I have not had the oportunity to do this, but I believe John Semmelhack has ducted one into the heating/cooling duct work so the temperature is evenly dispersed, thermostat keeps things warm and no one notices a cool spot at all. Duct design must account for (or at least tolerate) the air flow from the HPWH.
Good luck.
Just saw this article while researching a HPHW for a tiny home I am building in northeast Pennsylvania . As I've drawn it, the home is approx. 700 sq ft consisting of three rooms connected by a large covered deck. The bathhouse/mechanical area is about 160 square feet that I plan on only heating the space with a radiant floor (house on piles with 9.25" of dense pack in floor joists, double stud walls, etc..). I have considered a HPHW in an "open direct" radiant setup but then I am cooling the space I am heating. Also considered on demand electric but am worried about efficiency and its ability to keep up with demand (house is planned as a sort of basecamp along the Delaware for family gatherings/camping). Any thoughts/advice on this dilemma would be very helpful. I am attaching a floor plan.
Ted,
Why do you want a radiant floor? You might want to read this article: "All About Radiant Floors."
It's clearly impossible to use a heat-pump water heater to provide space heating for the room in which the heater is located (and it sounds like you have figured that out). A heat-pump water heater lowers the temperature of the room in which it is located.
For a seasonal tiny house, the obvious solution is to use baseboard electric (resistance) heaters.
I just read the article on tankless electric for a vacation home and got some quick answers. I will look into a standard electric water heater.
Although they are more expensive, I think the best solution for HPWH is the Sanden CO2 unit. The condensing unit sits outside separated from the tank and the refrigerant is earth friendly. I studied the unit a lot trying to use it as a heat source for a radiant heat system but the technology doesn't work as well for low temperature differential heating. The Sanden units are most efficient when they are heating cold water to high temperatures and work best in a household that uses a lot of hot water. I lament not going with that unit for my hot water but it really isn't the right choice for a 2 person household either.
Martin,
I’m in the process of building a very tight 2800 sf all electric sip panel house located in climate zone 4a. The house will have a forced air hvac system and zehndar ERV. I want to use a Rheem hybrid HWH. Based on the article above and my climate zone, can I use exterior ducted intake air? If I use interior intake air I’ll de-pressurize the house. What are my other options for intake air? Also the Senden unit is out of my price range.
Thanks,
Ryan
Ryan,
Q. "Can I use exterior ducted intake air? If I use interior intake air I’ll depressurize the house."
A. Your question is confusing. Installed without any ducts, a heat-pump water heater does not depressurize a house. It pulls interior intake air into the unit and exhausts the same volume of air into the room where the water heater is located. No air enters the house, and no air leaves the house, so there is no depressurization. If you are worried about depressurization, skip the ducts.
Martin,
Sorry for the confusion. I want to duct the entire heat pump system from intake to exhaust. Because I live in climate zone 4a, Nassau county, Long Island NY, do you think that it’s wise for my intake to come from exterior air only because the outside air temp will go below freezing sometimes or does that mean the heatpump hot water electrical heating element will turn on more often with this all ducted setup.
That won't work out well. In cool weather, the heat exchanger will frost up and there's no provision for defrosting. In warm weather, you'll lose the option to cool the house. There's no benefit to ducting it to use outside air, except maybe for a few weeks in the fall and a few weeks in the spring.
Very helpful info, thank you.
We are installing a HPWH in the home we are currently building, zone 6A. The basement mechanical/storage room where it will be located is generously sized and well-insulated, but unheated. It doesn't necessitate ducting, but in its location it would be easy to do so and I figured we might as well to realize whatever small gains there may be.
It is directly under the fridge and pantry, and two floors below the top of the vaulted living room ceiling. My plan has been to vent the exhaust air under the fridge, is the pantry a better choice - or are these equal options?
We've also planned to install a small in-line fan at the top of the vaulted living room to send excess warm air down into the mechanical room in the summer or when the woodstove is running. If I'm understanding correctly though, we could skip the fan altogether and just duct it right to the HPWH?
Will a ducted unit still absorb heat from the air around it, or will the duct restrict it to only taking in heat from the ducted location? The mechanical room shares a wall with the wine cellar which will be exhausting its warm air and humidity into the mech room - so I'd like the HP to be able to make use of both sources.
Thank you for any feedback!
Emma,
The exhaust air from a heat-pump water heater will be cold. If you are interested in ducting the exhaust air to the area near your refrigerator or your pantry, only you can decide which place deserves the duct. Do you want the area near your refrigerator to be cooler than it is now? Or do you prefer your pantry to be cooler than it is now?
I'm not a fan of unnecessary ducts. I'm even less of a fan of unnecessary fans. Your plan to run ductwork to the top of your cathedral ceiling, and to install a fan there, is likely to cost more to install and run than any possible energy benefit.
If your mechanical room is big enough, I'd be inclined to leave well enough alone rather than to complicate your system with ducts that have only a seasonal benefit.
Thanks, Martin - I see your point and too appreciate simplicity. It's also easy to get excited about opportunities for recycling waste heat. The idea for the fan began with improving upstairs comfort, and an energy benefit is a nice perk.
The concept of ducting directly to the HP and nixing the fan to the mechanical room feels like an improvement. In the article you mention that a HPWH provides 1/10 ton of cooling - is the inverse true as well? Does it remove 1/10 ton of heat from the air? In terms of a wood stove running in a cathedral ceiling living room that seems like it might be quite nice.
Thanks,
Emma
Emma,
Q. "Is the inverse true as well?"
A. I don't understand your question. The meaning of "1/10 ton of cooling" is "1,200 BTU/h of cooling," because a ton = 12,000 BTU/h.
What's the inverse of that? I'm not sure what you mean.
Q. "Does it remove 1/10 ton of heat from the air?"
A. It removes 1,200 BTU/h from the air, as long as it is running.
Q. "In terms of a wood stove running in a cathedral ceiling living room, that seems like it might be quite nice."
A. Here's my suggestion: If your living room is too hot, don't light a fire in your wood stove.
Constructive stuff, thanks Martin.
Unfortunately our lived experience in our current home with a vaulted living room and wood stove is that it can be perfectly comfortable on the main floor but very hot in the upstairs rooms. Exploring the ducted HP from the vault feels like this century's version of the reversed ceiling fan. I'll keep chewing on your feedback.
Emma,
As an engineer, I am more sympathetic to the impulse to install complex, optimized systems than Martin is, although on my better days I pay attention to his wisdom in that regard. Living in the same general region as you do, I appreciate that even a fairly small amount of cooling and dehumidification can be helpful in the summer, particularly with a pretty good envelope.
Exactly where you get the effect of the cooling and dehumidification depends on where you draw the HPWH intake from, and where you dump its output. To maximize the cooling of a space, you would want to duct both ends, and draw from and dump into the same space; but if you draw from and duct to different places, you would spread the effect around more.
In the winter, drawing from the top of the living room would make the HPWH run quite efficiently, if that air is nice and warm, particularly if the run time of the water heating coincides with firing the wood stove. If you do not duct the output, it's a little hard to know what the net effect on the temperature in the basement would be--there might be modest cooling of that region, but significantly less than there would be without ducting.
In the summer, you might want to maximize dehumidification of the basement, where humidity is more problematic than in other parts of the house, in which case you'd want no ducting. Or, if you use the same duct, you would do less to dehumidify the basement, but you would be circulating the cooler basement air up into the main levels to replace the air that you pulled out with the duct, and so you'd be cooling and dehumidifying the whole house a little bit.
Thanks, Charlie - I appreciate that. My thought is to draw from the top of the living room and exhaust to the pantry. The location allows for straight, simple duct lines, so it's hard to resist.
What I haven't been able to figure out yet is if the HP is ducted, will it also still absorb heat/humidity from the air around it? Or will the ducts restrict it to those locations? I want to make sure that the HP is able to make use of the heat that the wine cellar cooling unit will be dumping into the mechanical room as well. So alternatively I could not duct the HP at all, but have a fan coming down from the top of the living room that we turn on as needed (sorry, Martin).
Sorry I missed that piece of your question: If you duct it, it won't absorb heat and humidity from the region it's in, unless you deliberately provide an opening in the duct to pull some of the air in locally as well as some from a remote location. You could provide a damper to adjust how much comes locally and how much remotely.
It might make sense to install it without the ducting, but plan for where and how to install the ducts if you decide to use them, and run for a while and see how the temperatures settle out before you add ducting.
I'm about to close up the walls of my philadelphia townhouse (3 bed, 2 bath). I have a 40 gal HPWH going into a mechanical closet on the second floor. It's next to one bathroom and under the second. Is there a reason I can't pull intake air from the two bathrooms? The most significant use of hot water is most likely going to be showers - how much heat is lost in steam? Will the HPWH help with condensation, or is it too much moisture for the unit? I have a HRV return in each bathroom, will it undermine my hrv and pull stale throughout the house?
I'm probably going to install louvered and undercut doors and leave the unit un-ducted, but in theory, could this work?
Sasha,
You didn't tell us whether your mechanical closet meets the minimum volume requirements established by the heat pump water heater manufacturer. If it does, you don't need any ducts. I suspect that it doesn't, which is why you're planning to install water heater ducts.
On average, the air in your bathroom won't be particularly humid, because of the HRV exhaust port that you have installed in your bathroom, and because showers don't occur that often in most households. So there is no particular reason to worry about the humidity of the air.
If you duct the supply air side of your HPWH to the bathroom, then there is the potential for two fans to be fighting each other when the HPWH is operating. Two fans will be trying to pull stale air from your bathroom at the same time: the HRV exhaust fan and the HPWH fan.
According to one source, a HPWH fan is rated at 475 cfm. That's a lot more than your HRV fan, which is probably only pulling 40 cfm or less from your bathroom. So when the HPWH is running, the HPWH fan will overpower the HRV fan.
If your goal is to get the stale bathroom air out of your house, your ducting plan will defeat that goal. When the HPWH is running, the stale bathroom air will stay in your house.
Here might be one more reason to add ducts to a HPWH: The plumber installers put the HPWH with the output fan directly facing the basement wall 7.5" away. The air return path is pretty short, so there's not much mixing of air before some of that output air returns directly to the HPWH air intake.
The output fan is near the top of the HPWH. The input to the HPWH is on top of the unit (but on the top side opposite the fan output). There is a bit more than 12" from the fan output grille to the air inlet on top. If the path of the output air goes directly to the basement wall, then bounces up and back to the input grille, its a bit over 2' in length. Not exactly a good way to be mixing air before it gets back into the unit. (I bet that works wonders for the efficiency of the HPWH. ;-) If you put your hand around the back and top of the unit, however, the fan or louvers (can't see back there) seem to push more air toward the left and right, which is good.
It would have been more sensible to rotate the HPWH so the fan faces the wall at a 45 degree angle or more, so the air is forced mostly away from the unit. Thereby it would mix with other air before being sucked back into the unit.
Just putting some cardboard or other barrier between the output and input would create a longer return path for the air and encourage more mixing with other air in the basement. Or a duct to add a greater distance between input and output, to get the air flow to mix with other air in the basement more effectively.
Placing a fan nearby to direct air from the output could help too.
I live in climate zone 6a (coastal Maine) and I'm wanting to install a HPWH. My water is currently heated by a combi-boiler. My boiler / water storage tank is located in a small utility room (~400 cubic feet). The utility room has a 4in passive vent that draws air from the unconditioned attic into the utility room to support combustion in the boiler.
During the heating months, the boiler keeps the utility room quite warm, by far the warmest part of the house. During the non-heating months, it's still warmer than the rest of the house (when it gets really hot in the attic, warm air is drawn through the passive vent into the utility room even though the boiler isn't running).
Is it OK to install a HPWH in this space, even though the utility room is small? It's hard to imagine a scenario when the utility room got really cold.
Eventually, I want to install air source heat pumps as the primary source of heat in my house. In that scenario, would it be problematic for the HPWH if the utility room isn't being heated by the boiler? Would a louvered door to the utility room be helpful then? Would the unit need dedicated ducts?
Thanks.
Ben,
Q. "Is it OK to install a HPWH in this space, even though the utility room is small?"
A. No. You still have to abide by the HPWH manufacturer's requirements concerning the minimum volume of the utility room where the HPWH is installed--unless you decide to install ducts as described in this article.
Q. "Eventually, I want to install air source heat pumps as the primary source of heat in my house."
A. [Later edit: I misread your question, so I am withdrawing my answer. My apologies.]
Thank you, Martin.
I went and looked at some HPWH manufacturer's ventilation guides, and in many cases, the minimum room size is well less than 1000 cubic feet (e.g. https://pdf.lowes.com/productdocuments/4c228d0e-18e5-45cb-9ff8-3d175b4ad80e/60325518.pdf). And the space can be quite small if you install a fully louvered door. This seems like it could be a good alternative to ducts in my case.
I should have been more clear that my plan is to install one or more ductless minisplits to replace the boiler, and my question was whether this would have any implications for having a HPWH in the utility room (since the utility room would no longer be heated by the boiler). Given actual the ventilation requirements, the question is moot.
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