Getting into Hot Water, part 4

I've now had a year with the Geyser HPWH. With the exception of the puddle on the floor in July 2011, it has performed consistently. It's performance has not been thrilling, though. In the summer, it was making hot water at about 0.13 - 0.15 kWh/gallon, with incoming water in the mid-60Fs and basement air temperature around 70F. In the winter, with basement temperatures in the low to mid 50Fs, and incoming water at 50F or a bit below, this consumption ratio increased to 0.25 kWh/gallon. I switched to using only the upper electric element in mid-January 2012 and the consumption ratio was 0.31 kWh/gallon, so the HPWH was saving about 20%, actually more, since the HPWH was heating the entire tank, and the electric element only was heating the upper 30% of the tank. This was verifiable by the way with the infrared camera - a sharp temperature gradient below the element location.

If I didn't give myself the flexibility with the 85 gallon tank to do the HPWH or add solar thermal hot water I would have installed a 50 gallon Marathon instead. We have very good data from the Eliakim's Way homes that show about 0.21-0.23 kWh/gallon. So over a year I'm not sure my HPWH and the larger tank saved me anything over a smaller electric water heater. One very significant factor is our very low hot water usage of about 13 gpd. This means that the HPWH spends a significant portion of its operating time working against the standby losses, which means that it's cycling in the 110F - 120F water temperature range, where it is least efficient. And of course that energy is not being used to heat hot water to replace hot water we've used.

I have data on another HPWH, the Accelera made by Stiebel Eltron.

It was installed late this past winter in a Deep Energy Retrofit South Mountain did on a small house in Chilmark. The basement had about R-20 walls and R-3 floor. There is a ducted miniplit heat pump air handler and insulated ducts in the basement as well as the Stiebel Eltron. The S-E has an 80 gallon tank which has the refrigerant heating coil wrapped around the outside of the tank beneath the insulation. It has a 1.7 kW back-up electric element with a separate thermostat. This unit was set to make 130F water. We installed a water meter on the cold water inlet and measured the electrical usage with the Powerhouse Dynamics eMonitor. Over the first six months, the household averaged 45 gpd of DHW usage.

As in other MV homes, the incoming water temperature varies, starting at 50F in early March and rising into the low 60Fs in August. Basement temperature began in the upper 50Fs and rose to the upper 60Fs. The HPWH made 7,980 gallons of hot water and used 477 kWh of electricity, a consumption ratio of 0.060 kWh/gallon. Over three times more efficient than the 50 gallon Marathon tanks at Eliakim's Way, which used 0.20 kWh/gallon over the same months in six houses that averaged 43 gpd. This performance is in a whole other ballpark than that of the Geyser. Also, the unit seems to have low standby losses. On days with no usage it was using about 1/2 kWh. My biggest question is, how long will this expensive device (list price about $2,600) last?

It's worth noting that a HPWH takes heat from the house, at least during the heating season, so how you heat the house matters. Here are some cases to consider:

1 - The HPWH is in a basement with a gas furnace and leaky uninsulated ducts that keep the basement at 70F. The HPWH is operating efficiently because it is taking heat from nice warm air, and that heat is only indirectly getting to the living space. Probably a good application.
2 - The HPWH is in the thermal envelope of a direct gain passive solar house with a wood stove back-up. Again, the heat pump is operating in a favorable temperature regime, and the source of the heat is either the sun or firewood. And often during the winter the space may be overheated and the cooling is not objectionable.
3 - The HPWH is in the thermal envelope of an electrically heated house. Each unit of energy removed from the air is replaced by electric resistance heat. Not a good choice.
4 - The HPWH is in the thermal envelope of a house heated with minisplit heat pumps that operate at a COP of 2.5. The HPWH COP of 2 is effectively reduced to 1.4 because of the energy required by the heat pump to offset the cooling effect of the HPWH. If the house is in heating mode for six months of the year, and the rest of the time the cooling effect of the heat pump is negligible or welcome, then this changes to 1.7.
And finally, the more the climate shifts towards being cooling-dominated, the better the HPWH looks. A HPWH in your house in Florida supplies free cooling and dehumidification as it heats water.

The other thing we've learned with the S-E is the effect it has on the basement humidity. We know a HPWH will both cool and remove moisture from the air, but we didn't know if it would make that air higher or lower relative humidity. It could possibly cool the air and not remove enough moisture to keep the RH from rising as the air was cooled. Here's a 3-1/2 hour run of the HPWH, and the conditions of the air at the start and the end:



What we see is that the basement both cools and drops in relative humidity. As my friend and SMC colleague John Guadagno says, good stuff, good stuff! The reason it's good is that the moisture content of materials is based on the relative humidity of the surrounding air, and lower moisture content means lower opportunity for mold. I agree with JG!

To sum up how I'm thinking on how to make domestic hot water, given my preference to think in terms of electrically powered buildings to mate with renewable power generation:

- Low DHW users, say up to 20 gpd, use electric resistance in either a superinsulated tank or maybe distributed instantaneous electric heaters (caveat emptor - lots and lots of amps!)

- Medium DHW users, say 20 - 50 gpd, consider a heat pump water heater. Pick the highest efficiency and one with a large tank, which keeps the electric back-up off.

- Large users of DHW, consider solar thermal DHW. Look at the Wagner system, which is a clever packaged drainback system, as one possibility.

Comments

[Tad Montgomery]

Marc -- a couple thoughts on this great post of yours.

1) With regard to your Geyser analysis, you write: "In the winter, with basement temperatures in the low to mid 50Fs, and incoming water at 50F or a bit below, this consumption ratio increased to 0.25 kWh/gallon. I switched to using only the upper electric element in mid-January 2012 and the consumption ratio was 0.31 kWh/gallon, so the HPWH was saving about 20%, actually more, since the HPWH was heating the entire tank, and the electric element only was heating the upper 30% of the tank."

I'm wondering about that 'actually more' statement, since the 0.31 kWh/gal. figure is for heating less than one-third of the tank and the 0.25 kWh/gal. fig. is for the whole tank being heated. Since your usage is so low (13 gpd in an 85 gal. tank: 83/3 = only 28 gal. of HW at any time) I bet you never noticed a lack of HW, and that ~2/3 of the standby losses of the tank should be factored in as a benefit of the Geyser's performance, making it actually a good bit better. How well insulated is that tank? Can you quantify the standby losses? In this worst-case scenario (dead of winter) the Geyser might actually have been working 40-60% better than the heating element.

2) In your relative humidity calculation you wrote that the measurements took place after the S.E. had been running for 3.5 hours. I don't know what the run cycles of these things are, but I'd guess that's a long on cycle which would be followed by a long off cycle. If this is the case, the room's temp. would then gradually rise back to ambient when the system cycles off, but with rapidly declining RH, thus leading to much better drying effect than what was measured. This, of course, would be different for different conditions, and dependent on things such as the size and leakiness of the room, thermal mass present, whether it's inside or outside of the thermal envelope, etc. Unfortunately, your graph doesn't show this, but has the RH climbing at the end of the cycle, leading me to scratch my head and wonder where additional air moisture might have been coming from.

[Marc Rosenbaum]

Hi Tad, thanks for your comments.
1 - You're correct - I meant to imply that the heat pump was doing better than the consumption ratio would imply, since it heated the entire tank.
2 - The unit was off for about 14 hours after this long on cycle (because it finished at 8:45 pm and likely there wasn't much usage until some time the next day). Both temp and RH rose slightly, although not up to where they were when the unit began the long run cycle. There is some evidence that materials are desorbing more when the temp rises so that the RH increases rather than decreasing, which it would do if there were no absorption/desorption occurring. But there can be all kinds of other stuff going on in basements, such as laundry, so I'm hesitant to draw major conclusions!

[David Fay]

Marc,

Is that blue thing in the tray beside the Stiebel Eltron a condensate pump? If so, did you monitor its electrical usage with the eMonitor? Is it significant? Is it factored into the kWh/gal?

David

[Marc Rosenbaum]

Yes its a condensate pump and no I haven't measured their energy usage. Good idea! Likely minimal.

[Jan Juran]

Hi Marc: great job re data collection.
Re your Geyser and 85 gal. Marathon, you estimate 1.5 Kwh/day standby heat losses and 2 Kwh/day actual water heating load, or 43% standby losses and 57% load. Standby losses might consume a much higher portion of the Geyser's electricity use, since the heat pump is less efficient in the 110F to 120F hot water range.
Perhaps you might consider superinsulating the Marathon 85 gallon tank and piping? Say, add R-20 to the tank exterior via rigid foamboard or Thermax outside (or those commercially available water heater covers which have an exterior vinyl jacket); fiberglass insulation inside or even cellulose with appropriate containment; of course cut-outs for access to the heating element, the Geyser piping, etc.
This should be more cost effective vs. a new $2600 Stiebel Eltron, for example.
BTW most water heaters lose BTUs to the cold basement floor, placing appropriate insulation (rigid foamboard or other) beneath them can be worthwhile.

[Marc Rosenbaum]

Good comments, Jan, thank you. I think the Marathon doesn't need the foam on the slab so much as the tank insulation is complete around the rounded bottom. I think my standby losses are from the piping above the tank, because all three connections - cold in, hot out, and pressure relief - come in at the top, and I plumbed them all with copper, which I wouldn't do again. So even insulated there's a lot of heat exchanger up there! I think that the Stiebel, with a low cold water inlet, and a side connection for hot out, does better with less effort.

[Jan Juran]

Hi Marc: yes, I see from your pictures there is a very impressive array of copper piping up top, including a nine branch uninsulated length of copper hot water manifold. Convective upward heat losses may be significant. Wonder if it might be cost effective to attempt to superinsulate all of those copper hot water pipes up top? Depending on the Geyser's COP in the 110F-120F range, a 25%-50% electricity savings may be at least theoretically possible?

[Marc Rosenbaum]

Since the manifold is after the dip of the hot water line down to the mixing valve, it is not warm. The losses are off of the lines before they dip down.

[Phil Hawkes]

Hi Marc,

Are you aware of any products that use outside air? I'm working on a house on slab, with no garage, in a cold climate.

Thanks
Phil

[Marc Rosenbaum]

Phil, I don't know of HPWHs that use outdoor air (essentially temperatures below 45F or so). The Japanese make such devices as far as I know, but they aren't imported.

[Ben Graham]

Marc,
I love the simplicity and clarity of your summary!
Wondering what the difference in efficiency would be between a Daikon Altherma and a HP + HPHW. I was surprised to see the low COP of the HPHW. Which leads me to believe the Altherma would do better at both heat + hot water with a COP around 3(At least down to 8deg.) I am in -20F land so it might not be that much better at that point. I know the Mitsubishi hovers around the 3 COP land at those temps, but with the HPHW at or below a COP of 2, it seems weighted down, not to mention the fact the the HP works harder with the HPHW in the envelope.
Have you compared these strategies anywhere?

[Marc Rosenbaum]

Hi Ben
We have a couple of Althermas in summer houses here on MV. The installations are a bit more complicated than I'd like. The simpler of the two seems to have made DHW at a COP of close to 3 for the summer (when it would do the best because it's warm outdoors.) In the heating season we'll see how they do, but there won't be DHW loads because they are summer homes, and they aren't heated to 70F in the winter either.
The Altherma in my opinion (which is not based on much experience, but includes another user's detailed observations) does not have its controls optimized for making DHW. And I haven't seen evidence one way or the other that shows it operating to make heat at a COP of 3 down to 8F. Daikin's eng'g manual shows an integrated COP of 2.02 at 28F when making 122F output for the 030 unit, which is the best one. That COP goes to 3.33 if the output temperature is 86F, but that's not DHW temperature.