Getting into Hot Water, part 2

In the last post I looked at the basics of our DHW load, and looked briefly at adding a solar hot water system to satisfy most of that load. What I decided to do first was to try a heat pump water heater (HPWH), partly because it was a much simpler and less costly installation, and partly because I was just curious to see how well one would work. Since I already had the 85 gallon Marathon electric water heater, I didn't need additional storage, so I looked at the add-on products available and chose the Geyser, which is made in Maine by Nyle Systems. It costs in the range of $1,000, plus installation.

The Geyser is a small cube about 1-1/2 feet on a side, which can sit on the floor or on a platform adjacent to the water heater tank. The way it (and other HPWHs) operates is that it removes heat from the air in the space in which it is located, and transfers that heat to the water. The Geyser has a compressor to operate the refrigeration cycle to take the heat out of the air (like an air conditioner or dehumidifier), a fan to move air across its refrigerant coil, and a pump to circulate water to and from the adjacent storage tank. It connects to the tank at or near the bottom, so it heats the coldest water, maximizing the efficiency of the heat pump. Here's a photo of the interior of the Geyser showing the key components:

As a HPWH extracts heat from the air, it also cools it, which means that it acts as a dehumidifier as well, with one key difference. Conventional dehumidifiers cool the air, condense moisture out, and then reheat the dehumidified air, so they remove moisture and add heat to the air. Since warming the air lowers its relative humidity, the twin effects of moisture removal and heat addition are exactly what we want - lower relative humidity means lower moisture absorption by materials in the space being dehumidified, and therefore less mold. A HPWH removes moisture from the air, but cools it rather than heats it, because the heat goes into the DHW, so it isn't as effective a dehumidifier. I put a Hobo four channel datalogger in place to look at the effects of the HPWH on the basement environment.

The normal Geyser installation uses a clever tube-in-tube fitting that is inserted in the drain fitting on a conventional electric water heater (or other water heater tank). This allows the Geyser to both extract water to be heated, and return heated water through the same port on the water heater. In the case of the Marathon, the drain port location precludes this. I chose to do something a bit unconventional - I removed the lower electric heating element (residential electric water heaters have an upper element and a lower one, and only one operates at a time) and used an adapter to connect the heated water returning from the Geyser to this location. The water to be heated comes from the drain connection.

This approach loses the functionality of having the lower element as back-up to the HPWH - only the top element is usable. The Geyser uses the existing line voltage thermostat on the water heater to tell it when to operate. In a typical electric water heater, the controls first ensure that the top thermostat, that controls the upper element, is satisfied, then the lower thermostat and the lower element, which properly prioritizes the water at the top of the tank, which is drawn off first. In my set-up, once the upper thermostat is satisfied, the power is switched to the lower thermostat, which can call for the Geyser to operate if needed. I've set the upper thermostat to about 90F, and the lower thermostat to 120F, which forces the heat pump to do all of the water heating. I installed the Geyser on July 5th, 2011, and we haven't used any power for electric resistance heating since then, all water heating has been done by the Geyser. Note that HPWHs are slow heaters relative to typical water heaters. The Geyser's output is under 2 kW, whereas an electric water heater is typically 4.5 kW, and gas or oil heaters are much more. This is why the tank size is large for a HPWH. We've had a house full of guests a number of times this summer and fall, and no problem with enough DHW.

Since this is a new technology for me, I wanted to understand how well it was working. I installed a DLJ75 water meter on the cold water feed line to the water heater, before it splits to the thermostatic mixing valve, so I would be able to measure how much DHW we were using. This is a simple, time-tested mechanical water meter. We're installing a couple of others right now that have a pulse output that can be counted and totaled, so they won't need to be read by a human daily to get a sense of the usage patterns. The Geyser is a 120V machine that simply plugs into an outlet, so I'm tracking its electrical usage with a Kill-A-Watt meter. I also have been using the two exterior channels of my Hobo logger to measure cold water inlet and DHW outlet temperatures. With flow and temperature differential I can calculate the energy demand of the DHW system, and with the kWh I can calculate the system efficiency at satisfying that energy.

Like dehumidifiers, HPWHs remove moisture from the air and therefore generate liquid water as that water vapor condenses on the coil. The Geyser has a drain pan to catch this condensate and a tap that can be connected to a plumbing trap or a condensate pump. Mine is simply draining to a 5 gallon pail. Interestingly enough, in the middle of the summer, when the basement temperature and relative humidity was highest, there was a puddle on the concrete below the unit, which went away as we left the peak cooling season behind. I tried to determine the issue and the folks at Nyle were very helpful, but in the end I think that somehow there is a leak in the drain pan, probably at the corners, that only is operative when the rate of condensate is high. Alternatively, there may be condensation on a component that is not above the drain pan. I may need to wait to next summer to resolve this one.

Here's what the system looks like:

I have been pleased that the machine is fairly quiet, and so far, completely reliable.

If I had decided at the beginning that I wanted to install a HPWH, I probably would have selected an integrated unit, that has the heat pump built-in on top of a storage tank. These are made by Stiebel Eltron, GE, AO Smith, and Hubbell, amongst others. There is an excellent report on the field performance of HPWHs authored by Steven Winter Associates.

In the next post, I will comment on other aspects of the DHW installation, then in a subsequent post share some data on the performance of the HPWH, and its effect on the temperature and moisture level of the basement air.

Comments

[c.]

Before I get too excited about a new piece of technology did you only find these for electric water heaters or do they make ones for gas heaters too? The de-humidifier aspect is very appealing. Especially in our very old home we run one to two dehumidifiers all summer long. Working on other things to reduce that need but still it's huge and bad.

[Marc Rosenbaum]

I think one of these could be adapted to a gas water heater. If you are on natural gas, it's likely that the cost of making DHW is less than with a HPWH, though.

[Heating and Plumbing]

Excellent job done, I am completely impressed by your work. It's quite a big heater but you managed it easily.

[Mark]

Marc, the link to the report does not work.

[Marc Rosenbaum]

Thanks for the feedback on the link, please try again now.

[Jordan 11]

I think one of these could be adapted to a gas water heater.

[j chesnut]

Thanks again for going into such detail. I particularly appreciate the information about how you are metering and collecting data.

Any chance that the HPWH is cooling the air enough to bring the adjacent part of the concrete slab under the dewpoint causing the puddle under the 5g pail?

[Marc Rosenbaum]

Good thought, j chesnut, but I can see the water dripping onto the stand first...

[Canada Goose]

in the middle of the summer, when the basement temperature and relative humidity was highest, there was a puddle on the concrete below the unit, which went away as we left the peak cooling season behind. I tried to determine the issue and the folks at Nyle were very helpful, but in the end I think that somehow there is a leak in the drain pan, probably at the corners, that only is operative when the rate of condensate is high. Alternatively, there may be condensation on a component that is not above the drain pan. I may need to wait to next summer to resolve this one.

[Bob Lemaire]

Marc, I was curious why you chose to use the lower element fitting for part of the heat pump loop. Marathon recommends using the drain plug and cold water inlet when connecting to a heat pump. I realize that the lower element is useless to you anyway (and the upper for that matter) but is there some efficiency consideration?

[Marc Rosenbaum]

The Marathon is configured so it's not possible to use the tube-within-a-tube fitting that Nyle supplies. I could have brought the heated water into the cold water dip tube as well. Since that water can be quite cool in the earlier part of the heating cycle (say you've just drawn off 10 gallons and the bottom of the tank is 55F - the heated water coming from the heat pump might be 60F) I didn't want to bring it down through the hot water at the top of the tank. Does this make a lot of difference in terms of efficiency? Probably not. But I don't want the water temp at the tap to be lowered by this effect, so I played it safe.

[Erik Haugsjaa]

Marc, we also have a Marathon (105 gallon) and a Geyser setup going on 1 year in our conditioned basement. I find it a little loud (home office in the open basement) so I have it on a beefy $10 timer to run on off-hours. The only time we run into problems is when we have 3+ kid baths happening back to back... (we are 8 people... 4 kids at the moment). If I know this is coming up, I turn on the resistance elements ahead of time which are set a little hotter and recover faster.

We had a similar puddle once.

I need to do your hack to help keep things better stratified. Nice.

My calcs are that we have 100% solar fraction (net) solar hot water via slightly larger PV array and the geyser. For same price as a traditional solar hot water system (at 60-70% fraction).

One other thought I've had:
The 120V Geyser (often running around 700W) might come in handy during a power outage since any generator or solar backup wouldn't easily handle the 220V resistance elements -- 4500W.

[Hal Levin]

This is a great conversation for someone like me considering adding a HPWH. My interest is in using a HPWH for heating water for a couple of radiators to heat the house. I already have solar DHW and my house is all-electric with an electric energy cost of minus $27 last year, thanks to my PVs, a time of use meter, careful timing of my electricity use, and a generous peak period rate about 3x the off-peak rate from PG&E. My solar DHW system provides all my DHW needs for the warmer (sunnier) part of the year on California's Central Coast. It provides a small contribution to DHW needs during the cooler and cloudier part of the year.

I am very concerned about the noise factor since the HPWH COP is highest with higher air temps, so I want to install it within an enclosed space inside the structure. (No attic or basement in this house.)

So.... have either of you (Marc or Erik) actually measured the noise level and, if so, can you give me your numbers? My solar DHW system provides all my DHW needs for the warmer (sunnier) part of the year on California's Central Coast.

On a different note, for those self-helpers with attics, an integrated passive solar hot water heater system might be the answer. If you install the usual two-tank system below the roof in the attic and cover it with a high performance skylight (high solar heat gain coefficient, low emissivity) directly in the plane of the roof, you can avoid drain down, pumps, and a certain amount of piping. However, in cold climates, the night time losses may be large so an operable shutter or other insulator should be installed for use at night.

[Marc Rosenbaum]

Eric, how do you have the Geyser piped - through the cold water dip tube? And what are your solar calcs? Do you have gallons and kWh measured into the tank, including the resistance elements? Note that my set-up gives up the lower element.

[Bob Lemaire]

Hal, Seems to me that using a HPWH inside the living space to heat radiators is a something like leaving the refrigerator door open to cool the house. You might be better off with something like a Daikin Altherma outside.

[Jan Juran]

Hi Marc: If you have enough vertical drop from the shower drain to the main exit drain pipe to the septic or sewer system, you may be able to install a gravity film exchange drainpipe device. One GFX film heat exchanger can handle all the showers in the house if you can "gang" the shower drainpipes together into one drainpipe. One 5 foot long 3 or 4 inch diameter GFX should be able to save and recycle just over 50% of the heat BTUs used for showers. No moving parts, passive technology, lasts essentially forever. (I want to install one in my house however the mostly single floor design makes it difficult to get enough vertical drop without putting in a float switched pump which can eventually fail and need to be diagnosed then replaced).
If you have implemented a low energy dishwasher, cold water washer etc. then the GFX drainwater heat recovery device could recapture most of the hot water BTUs in your house, bonus=double the first hour capacity of the HPWH in case of many guests, etc.
Check out www.gfxtechnology.com also there are some competitors sold at big box retailers such as the Power Pipe.
Best regards

[Marc Rosenbaum]

I've used a bunch of drainwater heat recovery systems and I do like the PowerPipe design best. We don't have the drop in the basement because the graywater line exits near the top of the foundation wall. We could put one with some effort in a first floor closet and it would serve the upstairs shower, which we use more often. If I were doing new construction I'd integrate it into the system, as a retrofit in this case the effort seems more than its worth.

[Jan Juran]

Hi Marc: my house has the same retrofit problem, insufficient vertical drop in the basement. Perhaps if one of the manufacturers (i.e. Power Pipe, GFX) might make a "retrofit" version, with a float switch activated pump at the bottom which returns the drain water up to the same elevation (via a PVC pipe) as the device's drain water intake? Drainwater heat recovery products can save a terrific amount of wasted heat BTUs, very cost effective over time.

[Marc Rosenbaum]

That certainly could be done. I have a strong reluctance to interfere in any process where gravity is providing the needed service for free though!

[Jan Juran]

Hi Marc: very true, that is a very important consideration. The failure mode here would still drain correctly and for free, passively via gravity, if the pump failed (power failure, pump eventually burns out, etc.) The drainwater heat recovery system simply temporarily would become in effect a second larger S trap; the exhaust point of the loop would be an inch or several inches lower than the input point of the DHR system "loop"--a benign failure mode.

[Richelle Loughney]

I'm planning to replace our water heater tank for our apartment, and we may choose that one since we are so many here. Thanks for the installation tips.

[Jan Juran]

Hi Marc: BTW I recently swapped out all the showerheads in my house with the Niagara and the AM Conservation 1.25 gpm non-aerated models. I cannot discern any difference re quality of shower compared with the old 2.5 gpm standard showerheads. Simple way to approx halve the water usage, halve the heat BTUs used, double the HW heater capacity for showers, and extend the est. life of both the septic system and HW heater. Efi.org carries the AM Conservation 1.25 model for $5.

[Marc Rosenbaum]

Which of the two showerheads did you prefer?

[Jan Juran]

Both the Niagara and the AM Conservation 1.25 gpm models work well. I like the Niagara a little bit better, also I prefer the look of the chrome Niagara vs. the white AM Conservation.
I also swapped out the bathroom sink faucet aerators for 0.5gpm and 1.0 gpm models. The flow of the 1.0 is more than adequate, while the 0.5 flow is a bit less than adequate. Have been looking for something in between, but cannot find a manufacturer making a 0.7 (or thereabouts) gpm faucet aerator.

[Marc Rosenbaum]

Take the 1.0 aerator and make an insert from a piece of copper flashing that restricts the opening a bit.