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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


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?


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.


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

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.

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