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Will a Heat Pump Water Heater Freeze Your Basement?

Will A Heat Pump Water Heater Freeze Your Basement?

Are you considering replacing your current water heater with a heat pump water heater but worried that it’ll freeze your basement in winter?  Well, enough homes have this new technology that we now have some data on this topic.  A new paper by Slipstream is just out with some results that may interest you.  Titled Installed Performance of Heat Pump Water Heaters in a Cold Climate, the paper covers many facets of heat pump water use.  I’m going to discuss only one here:  the effect on air temperature.

The homes in the study

First, note that I’m going to show data below from two different parts of the study.  One was a field study with 9 homes.  The other part was a survey that included the 9 field study homes plus another 72 homes, for 81 total.

The 9 homes in the field study were in rural Michigan.  The heat pump water heaters were all in partially conditioned basements.  Seven were in the main part of the basement; two were in a separate room.  All of the water heaters were 50 gallon models from Bradford White, Rheem, and AO Smith.  The Slipstream researchers monitored water temperature (hot and cold), water flow rates, electricity consumption, inlet air temperature, and indoor and outdoor temperatures.

The objectives of the study were:

  1. HPWH Performance:  Characterize measured energy factor and effective capacity at different operating modes.
  2. Economics:  Calculate cost and energy savings compared to an electric resistance water heater.
  3. Space heating impacts:  Quantify changes in basement temperature and estimate HPWH’s impact on space heating and cooling operation.
  4. Customer satisfaction:  Survey on experience and satisfaction with installation and performance.

I encourage you to download and read the whole study if you’re interested in this topic (link below).  There’s a lot in it!

Effect on basement temperature in the field study

First, the field study monitored the basement temperature at four heights at a distance of 5 to 10 feet from the heat pump water heater.  The graph below shows the cycles of the heat pump water heater at the top.  In this case, there were three cycles during that 28 hour period.

The lower part shows the air temperature in the basement at the four temperature sensors.  One interesting feature of the temperature data is what they show about the cycling of the water heater and the furnace.  The floor sensor shows the water heater cycles most clearly—the big dips coincident with the water heater cycles.  The ceiling sensor shows the furnace cycles most clearly—the choppiness of the curve, especially between about 1 am and 6 pm.

Changes in air temperature near heat pump water heater
Changes in air temperature near heat pump water heater (Fig. 19 from the Slipstream paper)

In terms of the effect on basement temperature, the data above show a drop of about 4 °F near the floor as the heat pump water heat operates.  Then the temperature recovers pretty quickly.  For the 9 homes overall, the average temperature drop during a cycle was 2.3 °F.  After a four-hour recovery period, the average basement temperature was 0.1 °F lower than it was before the cycle.  The study didn’t measure the long-term effect on basement temperature over the course of a whole winter, but that would be interesting to see.

Note that these basements aren’t really cold.  The range of temperatures at the beginning of a heat pump water heater cycle was 62 to 65 degrees Fahrenheit.  My unheated, uninsulated (for now!) basement in Atlanta has been about 60 °F for the past six weeks.  The basements in the field study, recall, are partially conditioned, but they aren’t independently heated.  In other words, the thermostat is upstairs.

Satisfaction with effect on temperature and humidity

The other part of the study was a survey, and one of the questions participants were asked was, “”Do you like the changes to temperature and humidity?”  The results, separately for summer and winter, are shown in the chart below.  In both seasons, the majority had no opinion.  Seventeen people liked the changes in winter.  Only 9 people didn’t like the cooler basements.

I have no idea why anyone would like their basement to be cooler in winter.  Then again, I don’t understand why someone would choose to live in a cold climate that’s not near a mountain with 2,000 vertical feet of ski runs.

Heat pump water heater survey results on the question: "Do you like the changes to temperature and humidity?" (Fig. 25 from the Slipstream paper)
Heat pump water heater survey results on the question: “Do you like the changes to temperature and humidity?” (Fig. 25 from the Slipstream paper)

It’s easier to understand the summer results for this question.  If a heat pump water heater keeps a basement cooler and drier in summer, that’s a great thing.  One of the survey respondents wrote:  “One added benefit: I do not have to run my dehumidifier in the basement anymore.”

Factors to consider

If you’re on the fence about putting a  heat pump water heater in your basement (or elsewhere inside your house), here are some things that can affect how much impact it will have.

  • Amount of hot water use.  The more hot water you use, the more the heat pump water heater will run and cool off the house.
  • Water heater mode.  These things come with a heat pump and electric resistance heating (strip heat).  You can operate them in one of three modes:  all heat pump, all strip heat, or a hybrid mode that supplements the heat pump with strip heat as needed.  If you use heat pump only, it will do more cooling.
  • Size of the basement.  The bigger the basement, the more air there is to dilute the cooling effect.
  • Basement use.  If you have an office in the basement, you may need a way to heat the space independently of the upstairs.  If your main use of the basement is for exercise, you may welcome the cooling.

The Slipstream heat pump water heater study shows that fears of freezing your basement are overblown.  And if you find that your basement does get too cold, you can always turn it to strip heat mode.

Download the report

Slipstream report:  Installed Performance of Heat Pump Water Heaters in a Cold Climate (pdf)

 

Register for the Hot Water Forum!  If you want to get yourself in hot water, this is the place.  Well, I guess it’s not a place anymore since the conference organizers decided to take it virtual, but the agenda is fantastic!  Ben Knopp and I are speaking about actual heat pump water heater performance in homes. There are also talks about gas heat pump water heaters, right-sizing of hot water pipes, 120 volt heat pump water heaters, and much more.  The conference is hosted by the American Council for an Energy Efficient Economy (ACEEE) , which does all kinds of great work.

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.

 

Related Articles

Living With a Heat Pump Water Heater

Water Heating Is a System, Not Just a Water Heater

Why Your Hot Water Takes So Long

 

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This Post Has 33 Comments

  1. Interesting. My situation makes me less enthusiastic for making the switch. I live in the frozen north (Massachusetts) and my basement is part of the insulated envelope of the whole house. I designed the house and supervised the construction, with engineering help from Vanguard. The basement is heavily insulated from the outside and not insulated at all from upstairs. The basement is unfinished but I spend most of my waking time there, running my business. There is a basement heating and A/C zone, but it never runs, because the basement stays in the 60s year-round, regardless of the weather. In the winter this is fine, but in the summer, when I am more lightly dressed, it is uncomfortably cool sometimes. So my reluctance is this: In the winter, the heat that the heat pump is transferring into the water will be expensive heat that I generated to keep the upstairs warm, being drawn down to the basement when the water heater lowers the temp in the basement and thereby increases the delta T between basement and first floor. In the summer, it seems likely to make the basement uncomfortably chilly, unless I dress like it was winter, which will be inconvenient. If I were willing to let it draw the basement down to 60F in the summer, then I suppose it would help keep the upstairs cool, but the upstairs is usually comfortable without A/C except for about 30 scattered days during the summer. So there’s not much value in that, though perhaps there is no harm. The main stumbling block for me is feeling like it much be horribly inefficient to heat water by first heating space and then running a heat pump to transfer that expensive heat into the water. On the other hand, my solar panels generate gobs more electricity than I need, so I could look upon that heat as “free” rather than “expensive.” But I now send that extra electricity to the grid where it helps others and I send the result excess bill credits to friends and family on the same utility. So it has value.

    The basement is dry without dehumidification.

    I don’t have a source of free heat, unless I were to duct the DHW tank to the outside in summer. I suppose that’s the answer. Another demotivating factor is we use very, very little gas as it is with our condensing on-demand water heater. So the whole thing feels like an expensive and time-consuming exercise way out on the flat end of the diminishing returns curve of trying to green my life. I’ve got the electric cars, the solar panels, the highly-insulated airtight triple-glazed house, the heat pump clothes dryer, the induction range, the heat pump for space heating (but only down to 35 degrees). What’s left would be a ground-source heat pump for space heating and hot water, I guess.

    1. @James wrote: … in the summer, when I am more lightly dressed, (the basement) is uncomfortably cool sometimes.”

      I don’t have a heat pump water heater but your comment rings true. My basement office has the same problem. All the overhead metal ductwork was causing the basement to over-cool via conducted heat transfer. BTW, my basement is fully below grade and is on a separate zone. Like yours, it has never had a heating or cooling call.

      During the first summer, I had to wear long sleeve flannel top shirts while working. A bigger problem was that the 5+ degree contrast made it seem uncomfortably warm upstairs (yes, I removed the over-shirt), even though the temp was well within our normal comfort range. Lowering the upstairs setpoint a couple of degrees to compensate would have made my office even cooler, not to mention what an energy waste that would have been!

      Fortunately, we hadn’t yet installed the suspended ceilings in the basement so I decided to insulate the ducts while the ceiling was still open to dampen the radiant cooling effect. That, plus the insulation of the ceiling tiles solved the problem.

      The lesson here is when building a super insulated home, extra precautions are sometimes needed for unintended consequences. If I were to install a heat pump water heater, I would duct the evaporator outlet to the mechanical room, which is the heat pump’s return plenum. That would not only isolate the cool output air from the rest of the basement but would serve to pre-cool the return air in summer. In winter, I would switch the outlet duct to an unfinished storage area.

      In any case, if your existing water heater is natural gas, there’s not much incentive to replace it unless you don’t have any other gas appliances and you can eliminate the monthly fixed cost of having gas service. If the water heater is propane, by all means switch to electric. Depending on how much credit you get for annual excess PV production, you might be better off with a conventional electric water heater. I went with a conventional electric water heater because in my case, the additional PV needed for the water heater cost less than the delta-cost of a HPWH.

    2. You are sort of right. I live in Central Massachusetts and having cool air blowing in my basement in February is kind of silly. During summer however it is quite nice to have dehumidification and its an improvement since in the past moist dripping pipes and
      a sweating water softener system have frustrated me. I’ve turned the unit to electric only which works fine but I’ve seen some of the highest electric bills as a side effect. One of the biggest reasons I went with the hybrid unit for now to to buy time. I have a hydronic boiler from the 1980’s that is getting quite old in the tooth. I don’t want to be forced to replace it at a moments notice by an angry spouse with another oil unit since oil is (Hopefully) the past. I’m hoping to make my future home infrastructure all heat-pump technology based with the addition of cold weather R-32 mini splits. I too have solar (6.5kW). The heat pump water heater has sucked up all my negative balance due with national grid so now I have some moderate bills to pay but at the same time I need a fewer gallons of expensive oil over the heating season.

      1. If you are having to operate the hydronic oil furnace for the sole purpose of generating hot water during summer non-heating months in Massachusetts you may be pleasantly suprised at the oil saved by not doing that, not to mention no longer having a presumably-unwelcome source of heat running all summer.

        This might greatly improve the economics of switching to HPWH.

  2. I appreciate your article – I have recommended to have people simply turn their HP side off in winter. Here, 95% of homes have basements and 100% of them are conditioned. I see no reason to take heat from a conditioned area in winter, but helping cool and lower RH in summer is always a good thing. In our office building, the Mechanical Room has a lot of equipment: pumps (GSHP system), BPS, Transformers, etc. so we have a lot of waste heat. Also makes the Mechanical Room quite comfortable to work in for maintenance staff & contractors. That is the ultimate sales pitch for a HP water heater.

  3. I have a similar comment; if it is winter and the house is sealed, is the heat pump actually saving any energy consumption at all, or just taking heat from the house? if I duct it to the outside, is it still going to have any efficiency in the middle of winter? In the summer it might be lovely to act as a bit of an AC or dehumidifier in the basement.

    1. @Lloyd, ducting the outlet to the outside (hundreds of CFM) will necessarily draw in an equal amount of outside air into the house through a makeup vent or induced infiltration. There’s no free lunch.

      1. It’s not “hundreds” of CFM…not to split hairs, but it’s more like one hundred CFM…let’s keep that in perspective.

        While I get that the cold air discharged from an HPWH seems to be a problem in heating dominated climates, I’m a bit mystified by those who insist upon discharging that “cold” air outdoors. The discharge air temp from an HPWH is typically 15-20*F below ambient. It does not make sense to discharge that air outdoors during cold weather since the makeup / replacement air from the outdoors is likely colder than the discharged air, the net effect adding load to the heating system.

        During mildly cool on up to warm outdoor weather, keeping the HPWH discharge air inside the envelope nearly always makes sense owing to the free dehumidification.

      2. @Curt, HPWH airflow is frustratingly difficult to come by, but I haven’t heard of one that operates at 100 CFM. Any examples?

        Here are several I was able to find: Vaughn Thermal’s Hybrid Water Heater has a 2-speed fan: 450 CFM on high, 250 CFM on low. AO Smith Voltex 475 CFM (per NREL study). Steibel Eltron Accelera 324 CFM. GE Geospring 200 CFM (discontinued, the lowest I’m aware of).

        1. I confess my HPWH CFM estimate arises not from detailed instrumentation or researched specifications but instead a hand waved estimate based on having owned and installed several HPWH over the years coupled with having used a flow hood quite a bit in residential duct systems…I think I kinda know, to within back-of-envelope accuracy, what 100 CFM “feels like”

          I recently moved and took a HPWH with me – when I git roun to reinstalling it I’ll try and remember to nick the flow hood from work and measure it – I’ll have to gin up some sort of high precision beer case cardboard box adapter to mate the HPWH discharge air throat to the flow hood throat.

    2. There’s the CMHC study from 2014 that looked at whole home performance in the twin R-2000 homes in Ottawa’s Canadian Centre for Housing Technology. Surprisingly, not as much interactive effect on heating as you might expect. Perhaps by reducing the local temperature in the room, the delta-T across the foundation is less and therefore there is less heat loss through that part of the building envelope. http://publications.gc.ca/collections/collection_2015/schl-cmhc/nh18-22/NH18-22-114-104-eng.pdf

  4. Living in moderate climate of coastal southern California, HPWH would seem to be obvious. My home is unusual for CA in having a partial basement (Italian and Croatian immigrants must have their wine cellar). I cool about 90% of the year, so more cooling is good. What would be good is a way to vent the warm heat pump hvac exhaust into the HPWH INtake.
    Is anyone considering combining BOTH HVAC and HPWH in a single unit? Oh, condensate makes excellent water for the garden.

  5. When using air from the conditioned space for the DHW heat pump, remember this air may have been heated using a heat pump, in which case it’s not expensive Energy your using. Using warm air from the house will provide more energy into the DHW. In the summer to prevent cooling the basement, divert warm air from outside to mix with DHW air

    1. It may also be a concern of available heating capacity rather than just cost. Our office HP DHW unit produces around 1.5 Ton of cooling. We are taking waste heat from a mechanical room full of heat producing items. In a home, it may mean the heating load of the building has now grown some which could exceed the overall heating unit’s capacity. The Manual J may not have included additional heating needed due to a HP DHW. It may make economic and available heating capacity sense to not cannibalize heat from the home to make hot water.

      1. While it may not make sense for everyone, there are a lot of places in the US where a modern HPWH can be put into a garage space, for example, or even, in warmer climates, take outdoor air in and send output air outside. The current Rheem units say that they have a COP of 4.0, operate at temperatures from 37F to 145F, and can be ducted in and out. They do require a minimum of 1000 cu ft for satisfactory operation if in an enclosed space with no air inlets or outlets. We have our 4 year old unit in a garage, and it works fine year around, not using any conditioned air.

  6. Readers who are considering buying a heat pump water heater should be aware that there are additional variations to the models available that might impact your choice. Not all have back-up electric heating elements, as implied by the article, and are strictly heat pump water heaters only. In that category, some are 240V units and some are 120V units, which is possible because of the much lower electricity demand when there are no electric resistance heating elements present. As a consequence, the “normal” heat pump water heater, often called a “hybrid” water heater, with electric resistance heaters that can be used either alone or in combination with the heat pump to achieve a desired output control, requires 240V and a 30A dedicated circuit, whereas models without the resistance heating can be operated on lower-current circuits at 120V and 15A. In situations where it is less likely to need the resistance heating supplement or alternative modes, a cost savings can be had by buying the heat pump only, 120V version, which may not require any new wiring, or even if it does, it would be less costly than the heavier wiring and double, higher-value circuit breaker required for the combination (and more typical) unit. If you are directly replacing an existing electric water heater with a heat pump version, the circuit is likely to already be 240V and 30A.

  7. Interestingly, a heat pump clothes dryer does not cool your house at all. In fact, it heats it a bit.

  8. Interesting article. We installed a HWHP for the added hot water storage capacity it gives us. We are in NH. We have primary heat with an oil fired hot water boiler with an indirect tank. We’ve plumbed our heater so that if we choose we can shut the boiler off completely in the summer and rely just on the hwhp. (albeit with less storage capacity). We usually only need the extra hot water if we have guests. The electric elements on our heater are turned off. Like others it has eliminated the need for a dehumidifyer in the summer. We do notice that the basement is a bit cooler but that is where we have our workout equipment so it works out well. We do need to rework the valving a bit to give ourselves the ability to make either the indirect or the hwhp the primary. Our plumber misunderstood our desires and while we can run the hwhp independently with the boiler off we are unable to to use the hwhp as a pre-heater to feed the indirect when we would want to do that.

  9. Added audio noise of a HP hot water system, though moderate, is another unmentioned factor to consider. In a particularly quiet environment it is noticeable. Considering the upfront cost (vs a well insulated electric tank) the payback period is lengthy, approaching questionable. And heat pumps do fail and require maintenance. A straight electric tank is almost bullet-proof. The greatest gain in the equation is replacing an old poorly insulated tank with a normal electric tank surrounded by thick foam insulation.

    1. Ed: I covered noise and payback in my previous article on heat pump water heaters. The noise depends on which model you have. Mine is very quiet. And the payback isn’t always lengthy. The simple payback on mine 3.5 years.

      1. And ours paid us back in 3 years (self-installed, but bought at retail and delivered).

  10. It’s too bad no one offers a heat pump water heater with lifetime tank warranty like the non-metallic- tank Marathon line of resistance-heated models offered by Rheem. Time flies. I only want to do this once.

  11. what if a small solar air collector or vertical heatwall could add just enough heat to offset the heat pump’s needs in a basement . Could also apply for air to air heat pumps ..

  12. I have often wondered exactly where the heat absorbed by a HPWH comes from. Obviously some comes from whatever source heats the home. But I would guess that some is being absorbed through the floor and walls of the basement when the room temperature drops below 50 or so. The only winter solution I can think of, given the lack of a winter waste heat source in homes, is to enclose the water heater in a room of its own (12×12 ft) and leave the outside walls and floor uninsulated, but insulate the interior walls and ceiling. This would become the house’s “cool room” where your apple, carrot, potatoes & wine could be stored, and anything that benefits from cool storage in summer, such as winter woolens. If it gets too cool for the heat pump, you would have to open a door. I do like Bob Danielson’s idea for a solar air heater to add heat to the room, but that runs the cost up further.
    That leaves only one other option I can think of: The SanCO2 ECO2 split heat pump water heater [ https://www.eco2waterheater.com ] . It has the packaged heat pump outside and a 43 or 83 gallon stainless steel water tank inside (15 yr warranty). They are connected by a pair of 1/2″ insulated water lines with several strategies to prevent freezing, including auto-drain valves if power fails. This HPWH uses CO2 as a refrigerant, so if it leaks (it happens) instead of a GWP of 2085 it has a GWP of 1. If we are going to convert everything to heat pumps, we cannot continue using R-410a, R-134, or even R-32 with their high GWPs. Since the ECO2 HPWH has a self-contained refrigeration system in the outdoor unit, your plumber can install it; no on-site refrigeration work is needed. if you have a super-insulated house and follow their strict guidelines you can even supply hot water to a heating terminal and heat your house, but since this heat pump is not reversible, it cannot be used for cooling too.

    1. @Gene, isolating a HPWH in an uninsulated room that small with no heat source simply won’t work. The room would quickly cool below outside ambient so you’d need to leave the door open most of the time! And even with the door open, room temperature would mostly remain below outside ambient, especially at night.

      Keep in mind that output and efficiency drop off significantly with temperature, and heat pump mode will shut down when the inlet temperature reaches the heat pump’s min operating temperature. Bottom line, in a cold climate, this strategy needlessly increases reliance on straight electric mode, robbing yourself of some of the efficiency you paid for.

      These considerations obviously don’t apply in summer. In fact, the garage may be the better option in hot climates, especially hot-dry where dehumidification is undesirable.

      In general, your concern over stealing heat from the home’s heating system is unfounded. That’s exactly what’s supposed to happen! The only time that doesn’t make sense is if the heating system’s cost per BTU delivered is higher than straight electric (can sometimes happen with propane or fuel oil, depending on local energy rates). Be sure to check out Siegenthaler’s article on cascading heat pumps linked above in Foley’s comment. When the house is heated by a heat pump, the HPWH takes advantage of the HVAC’s higher COP. The net COP, while somewhat lower than the HPWH’s rated efficiency, will still be higher than a straight electric water heater.

      A CO2 system like the Sanden is obviously a better solution in terms of GWP, but the high cost isn’t economically justified except for applications with heavy hot water usage such as a commercial kitchen in a location without access to natural gas.

  13. If you are heating your basement/crawl to a specified conditioned temperature with resistance heaters, seems like there’s no benefit to a non-split heat pump. Any savings you get would probably be offset by the additional cost to heat the basement/crawl back up.

    1. Cindi: That’s not completely true. Yes, it’s true that your COP in winter is limited by the COP of your heating system. But in cooling season, the HPWH is helping to cool the house.

    1. Cindi: It doesn’t matter if you don’t need the cooling in summer. The HPWH will operate at a much higher efficiency in the summer when it’s not using heat that you’re getting from electric resistance.

  14. In a four season environment, HPWH could result in some input by the occupant. Run the HPWP in summer to add to the cooling of the home – perhaps switch to resistance DWH mode in winter so as not to add to the heating load of the home. In a predominantly cooling environment, the HPWH should run in HP mode all the time. In a predominantly heating mode, perhaps using a GSHP system with a de-superheater water heating attachment will be a better option for ultimate efficiency.

    1. I’ve heard that; makes sense. Each climate is so different as well. In my climate there is typically no cooling required even in summer. Of course things could change with global warming, we have already seen temperature shifts. But it’s still 50 degrees at night in summer and it’s a climate where the air is never warm, just the sun is warm. I have friends with sort-of-well insulated homes (but nowhere near as insulated as the one I’m building), and their house is so cold I’d be running heat in the summer. And in my current old house I’m usually bundled up like winter until mid-afternoon when the sun comes in the windows for a couple hours (or I go outside into the sun.) We actually chose windows that would intentionally let more sun heating through even in summer.
      But I am heating/cooling with mini-splits.

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