Is a heat pump water heater a good choice?
We are designing for new construction of a family home (five people) in Omaha, NE. Our average daily water use in our current home is around 200 gallons. We estimate that this could increase somewhat as we will be adding a shower and the kids will be taking more showers. I’ve looked at heat pump water heaters like the Rheem HP-50 (http://www.rheem.com/product.aspx?id=BF7C55FB-67BA-41B5-8EEA-B7022B9DC671) or the GE Geospring (http://www.geappliances.com/heat-pump-hot-water-heater/). We also may use a WaterFurnace Synergy 3D ground source heat pump (http://www.waterfurnace.com/products.aspx?prd=Synergy3D) for our heating and cooling. I believe this unit has ability for DHW pre-heating.
Given the situation described above, does anyone have feedback on these heat pumps or suggestions for alternative solutions? Our goal is long-term water heating efficiency. We plan to use water conservation fixtures throughout the house, use Energy Star appliances, insulate our hot water pipes, and maintain the equipment in a conditioned basement mechanical room.
Thanks in advance for your feedback.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Michael,
If your heat-pump water heater uses an air-source heat pump, it will steal heat from the room in which it is located. This will help cool your house during the summer, but it will raise your space-heating bill during the winter.
There is no simple device that will lower your water heating bill. The best way to lower your water heating bill is:
-- Use less hot water;
-- Insulate your hot-water pipes;
-- Install your water heater in a location that is closest to the room with the greatest level of usage;
-- Keep the diameter of your hot water pipes as small as possible;
-- If your family prefers showers to baths, install a drain water heat-recovery device.
I agree with Martin's list with one caveat. "Keep the diameter of your hot water pipes as small as possible" works only with what is called "home-run" plumbing systems, or parallel supply systems, in which each fixture gets an independent pipe (or PEX tube) for hot and cold water.
Otherwise, trunk (or common) supply pipes need to be sized sufficiently to keep the water velocity below 8 fps for cold and 5 fps for hot - lower velocities are better. High water velocity causes turbulence, noise, erosion of metal piping (particularly at direction changes) and water hammer (which can damage both pipes and appliances).
Using PEX rather than copper for supply lines is an excellent way to reduce turbulence, since the walls are much smoother, there are no elbows or other fittings except at ends, and there are no internal burrs from cutting.
Generally, PEX is the better alternative for homes on private wells and copper for homes on municipal water. This is because chlorine can cause health problems in plastic pipe (it's a health problem in itself, as well), and copper will leach into drinking water with water softeners or acidic water (copper is also a recognized health threat above certain thresholds).
A few more items, though, that I would add to his list is:
- teach your children to take short showers or share bathwater and to NOT run water while soaping up or brushing teeth at the lavatory or washing dishes in the kitchen sink (these work best by example)
- minimize use of outdoor water (Americans consume 40%-60% of their water outdoors, for sprinkling, washing cars, and pools)
- and consider your non-residential consumption of water (US per capita water consumption is 70 gal/day for domestic use, 200 gal/day for commercial and public use, and 1200-1900 gal/day for our extremely resource-intensive agriculture - organic farming uses much less)
- fix fixture drips as soon as you notice them (leaks and drips can account for 13% of daily use)
heat pump water heaters make the most sense in hot humd areas like FL where most water heaters are located in unconditioned, attached garages. Install solar water heating. It makes great sense in new construction. After taking available incentives and rolling the cost into your mortgage, they can be break even from year one or even have positive cash flow for a big household like yours.
Thanks for the responses. Fortunately, we are already practicing most of both Martin and Robert's suggestions. We plan to continue doing these things in our new home, also.
I do plan to use PEX. However, local code here restricts it, but I've been told I can get a waiver if I present a good reason for using it. We will be on well water, so I think that helps my case.
We would like to do solar hot water and actually began there, but water is so inexpensive in our area ($15 monthly flat fee, regardless of usage) and we have no state incentives, that the payback on solar water takes longer. What would you recommend for stretches of time where we aren't getting enough sunlight?
If the sun's not shining, then you're not sweating and there's no need to take showers ;-)
I'm only half joking. Another item I decided not to add to the list of conservation measures, because it goes against the grain of American obsessive-compulsive culture, is to reduce the number of showers per person per week.
Americans tend to bathe far more often than our European counterparts (though early Americans thought bathing was sinful and unnecessary, and Ben Franklin had the first indoor bathtub hundreds of years after first settlement), and I suspect it has helped undermine our health since skin has natural oils for protection which soap removes.
I remain quite healthy and clean by bathing once a week without soap in my outdoor, wood-fired hot tub (where I'm heading as soon as I shut down this computer).
There is already a thread on this topic: https://www.greenbuildingadvisor.com/community/forum/mechanicals/14216/seeking-air-water-heat-pump
Has anyone seen short home runs done in 3/8" PEX? In a small home, this might eliminate the need for a DHW recirculation pump, which has become almost a necessity in semi-custom homes.
In regard to a GSHP, if you plan to superinsulate your home, that huge investment will wear out long before it pays for itself.
As Robert says, a HPWH will steal needed heat from your home in Omaha, so here's a discussion about where you should put it in your house, and whether PV is better than solar thermal: http://greenbuildingindenver.blogspot.com/2009/08/heat-pump-hot-water-heater.html
I did some calculations showing that a well-insulated 2 car garage with an uninsulated slab might be the ideal place to put your HPWH:
http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/45292/afv/topic/Default.aspx
Kevin,
Depending on the home's water pressure, short runs of 3/8-in. PEX work fine for a lavatory, but not for filling a bathtub or a kitchen sink, where you want to be able to fill a pasta pot quickly.
Michael, given the high volume of DHW you're looking at daily, is there a reason you aren't considering a tankless system? I realize that you're going to be on a well supply, but if your water needs a softener system and it's located prior to the tankless feed, then that shouldn't be an obstacle. If it doesn't have hard mineral deposits which may foul the heat exchanger, then obviously no problem there either. With the condensing unit folks like Rinnai make these days, they can make sense for higher-volume applications.
Also, as for insulating your water pipes, have you looked at Aquatherm piping as a possibility? We've had good success with it on our projects in both well and municipal situations. It also tends to be fairly easy to get plan review acceptance for during the permitting stages. The only trick may be whether or not you have a trained installer in your area.
Andy,
I guess I don't consider our usage "high" relative to the average household in our area (less than half the national per person average). That being said, I've had people tell me that the tankless units can't always handle the capacity a family might need (i.e. simultaneous hot showers or washer/dishwasher cycles). Our plumbing is mostly centrally located, but our Kitchen is the outlier. I'm concerned about needing to install two units.
I'll take a look at Aquatherm. Thanks for the tip.
Michael, perhaps I misunderstood your initial post. I read it as 200 gal per day for 5 people. So I take that to mean 40 gal / day / person of DHW. That's quite high even factoring in dish washing, clothes washing, etc. However, if you simply meant 200 gal / day total water consumption (and not just HOT water) then that's a whole different ball game entirely.
In any event, regarding your concern about the unit being able to keep up with demand:
- Their delivery to outlying areas happens as quickly as any tank style storage system so that wouldn't be an issue.
- The newer units trigger at much lower flow thresholds, so even with WaterSense faucets, you still get heat demand (a problem with most older units and some current "cheap" units).
- If you get a high capacity unit from Rinnai with a stated production of 9+ gpm and then downgrade that for a Delta T of say 70 deg, (figuring a set temp of 110 with incoming at 40) you'll still get real time production of 4+ gpm. That should be enough to run 2 low-flow WaterSense shower heads or one shower and one appliance with relative ease.
- During the warmer months when your Delta T is closer to 40 deg, you'll get full capacity and it can handle anything your house could possibly throw at it.
The only other possible snag would be fuel supply (since you mention well water). While there are "whole house" electric models available, they are pathetic when it comes to efficiency. You would either need natural gas or propane to install one of the models I'm referring to.
Finally, some reports I've seen estimate that if you average 1-2 typical residential tank cycles per day @ 50 gal tank size then the math for tankless may not make sense. But when you reach 3 or 4 (or more) cycles (which you would if you're using 200+ gal / day) then they make good sense in short order, both financially and logistically.
Andy,
My apologies. The 200 gal/per day number was TOTAL water consumption. Regarding fuel supply, I have natural gas and electric on the property, so I could use either. We only really use hot water for showers and the dishwasher, maybe a little for the faucets and the washing machine. Given those levels of DHW usage, do you think tankless is economical for my situation?
Okay, that number makes a lot more sense (200 gal / day TOTAL). So ... figure for a house of 5 people on average:
- 5 showers / day @ 8 min ea (I average 5 min but my daughters average 10 ... and only then because I press them about it) and using a WaterSense 2 gpm head = 40 min run time @ 2 gpm = 80 gal / day for showering. Roughly half of that will be hot water based on the time of year and incoming supply temp from the well. So call it 40 gal /day for hot for showering
- one load of dishes per day @ 5 gal / load for a full size Energy Star model and again, divide by 2 = 2.5 gal / day / hot for dishes
- Figure another 5 gal / day for misc. face washing, rinsing, etc.
- Since it’s new construction, I’ll assume that you’re going to have a new HE front loading clothes washer and that you’ll use one of the new cold-water detergents for all of your loads. So maybe figure 1 gal / day just to temper the water per the machine’s design
- So @ 40 + 2.5 + 5 + 1 = you could still be looking at only 1 tank cycle per day for a 50 gal tank.
Based on that you certainly wouldn’t fall into the “high use” range. Quite the opposite based on most of my clients habits, unfortunately for them. So now the question becomes life cycle costs vs. energy savings vs. budget and how long you plan to stay in the house.
If you’re going to be in the house “until you die” you may want to think about tankless anyway. Not because of cost savings, but because of logistics:
- there is no stand-by loss
- they can be expected to have a 20-30 year life cycle (double or triple a tank unit = less mfg energy and less scrap to be recycled or landfilled)
- they don’t draw any heat from your existing space
- you can significantly reduce the mechanical closet size because they are wall mounted
- you only pay to heat what you actually use
- as your family needs increase in the short term you’ll be covered and then when you become “empty nesters” and are spending time away from the home years from now you’re not paying to heat water you’re not using
- since there’s no tank, if there’s ever a leak, the extent of possible damages are greatly reduced.
But ... on the other hand ... even though there are plenty of positive reasons to consider tankless ... sometimes it just isn’t in the budget or your personal preference. So then you’re back to tank-style models. On these I tend to part ways with some “conventional wisdom.”
Specifically, I recommend considering electric resistance with a timer. This freaks a lot of folks out, but there’s method to the madness:
- any style tank can be expected to have a similar life span regardless of heating method before the tank itself could actually fail and leak
- with electric tanks there are zero stand-by losses due to flue gas venting (although this is admittedly greatly reduced with newer condensing gas models, there is still a degree of loss, and another penetration in the building envelope)
- the electric delivery process does have significant losses between the generating plant and your home, but if you convert to / add PV at a later date you can completely negate that on-site (not an option once you commit to gas) And in the interim, there are many “clean power” programs available where you can “buy” wind or solar from the grid (actually a process of credits, but at least your money is going to a better producer)
- By placing the unit on a simple water-heater specific timer (from Intermatic and easily available on line from Grainger supply) you can control it’s run time down to as little as 6 hours / day (ours is set to run from 5 AM - 8 AM and again from 6 PM - 9 PM for a family of four). This is not an option with a gas tank-style.
- The same timer also allows you to easily switch the unit off entirely when you’re going away for a day or more. (I put the timer switch at the top of the basement stairs so this is a painless process) Again, not an option with a gas tank-style regardless of efficiency level.
- The cost to implement this with a simple Energy Star electric model and the timer is much cheaper than the newer HPHW tanks and way cheaper than a tankless install.
- The maintenance and possibilities for failure are significantly reduced by less complex components
I’ll be the first to admit that if you put electric resistance against HE condensing gas or HPHW on a 24 hour run cycle, electric loses every time. But ... when you put the electric @ 6 hrs / day vs. gas @ 24 hrs / day, the electric wins (especially if/when you throw in vacations, etc.). And if you’re able to throw PV into the mix, then it really becomes an unfair fight.
So, bottom line, if budget is flexible but space isn’t and/or you're going to be in the house for decades (and gas is available in sufficient volume and pressure) then I like tankless. But if budget is a concern, or you already have PV, or you’re not planning to “die in the house” then electric on a timer is a nice alternative.
Michael,
If you're considering hydronic or radiant heat, then I would recommend a good quality condensing boiler (like the Triangle Tube Prestige) with an indirect tank or a combination boiler/DHW unit (like the Triangle Tube Excellence).
An indirect tank also has no flue losses, negligible storage losses and provides virtually unlimited hot water. A combo unit has a small reservoir capacity, so it can provide hot water quicker than a tankless.
A stainless steel indirect tank will outlast any tank-type water heater, and this eliminates the need for a secondary heating unit for domestic hot water. It also has the advantage of cycling the boiler in the summer, when there is no demand for space heating, thus keeping the combustion unit clean.
A wall-hung combo unit takes little more space than a tankless water heater and it provides both heat and hot water.
Robert,
Off topic, but can the wall-hung combo unit also work as a retrofit for an existing radiator system? I.e. replace an outdated boiler and tank water heater in one shot? Also, does it matter whether the rads are steam or recirculating? We have very few boilers left down here in the mid-Atlantic, but we just looked at a project with an outdated system last week, They like the rads so we're looking for solutions.
(Michael, sorry for jumping off your topic for a second...)
Andy,
I don't know what you would use to replace a steam boiler, and the only combo unit I'm familiar with is the Triangle Tube Excellence, which comes only in a 30,000-110,000 BTU input (with natural gas) and AFUE of 95%.
It would certainly be compatible with any hydronic distribution system if the heating load was within those limits. However, if there is room for an indirect tank, it would be less expensive and provide more domestic hot water volume than with the combo (which now costs more than their identical boiler and a 30 gallon tank, not including the additional piping and circulator).