Jill and I have taken a plunge and bought a small (1100 ft2) house about a mile away in West Tisbury. I have a couple more posts tee'd up for House 5 on window insulation and on the electric mini-boiler I've installed, so stayed tuned (I've been really busy with the Zero Energy Homes Online Course, for one thing). I just blower door tested this new house on Sunday at about 3,100 CFM50, so there is more "opportunity" than House 5 offered :-) - that's about 0.6 CFM50/ft2 of shell, which is 3-1/2 times worse than House 5 was when we started on it. A huge chunk of that leakage is because they made a small attic in this one story home that is vented to the outdoors, and that's where they installed the furnace and ductwork (don't try this at home.)
New house below
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.
I've written an article about House 5 in the latest issue of BuildingEnergy, the magazine of the Northeast Sustainable Energy Association. It's got other excellent articles, too. You can find it here:
You are a NESEA member, aren't you? There's no better community to join if you're passionate about great buildings. Most of my closest friends and colleagues have come from my 30+ year involvement in NESEA, and the most exciting thing these days is the influx of amazing young folks, ready to take over from the tottering geezers like me! I was the second Lifetime Member of NESEA - it was an obvious choice when the category was created - nothing has had as much effect on my professional journey as the relationships I've made within the NESEA community. Join here:
I'm pumped about this. It's taking a huge amount of time - I've taken this week off to get started - but I know it will make me better at communicating this information. I've set a bar that I think exceeds most similar courses - my goal is to give the participants the insights and tools to really design a zero net energy project themselves. Take a look!
I've been AWOL for a while, sorry about that. I'm putting together my first online course, on Zero Net Energy Homes, for the NESEA Building Energy Masters Series. You can read about it here:
Meanwhile, the House 5 PV system just passed 8,000 kWh generated! And we have a surplus credit of over 3,700 kWh, which we can allocate to another meter.
I have plans to post more about the hot water question, especially on heat pump water heaters, and also on the insulating blinds. Stay tuned!
At the end of June we had one full year of operating with the PV and taking energy usage and production data. From July 1st, 2011, through June 30th, 2012 we used 3,755 kWh, which would have cost about $700. This is below the THC target and likely meets the Passivehouse primary energy limit as well. During that period the solar electric system produced 6,779 kWh, meaning that we had a net export of 3,024 kWh – handily achieving zero annual net energy. The surplus could be used to run an electric car over 10,000 miles. It’s important to note that this was an uncommonly warm winter, and I’d expect to use 6-700 kWh more in an average year. It’s also important to note that we are a household of two – add a couple of teens and the energy balance would be different – yet I believe we could still be net zero and meet THC under those circumstances. With the balmy winter, we were actually net zero every month except December and January.
We spent about $26,000 after subsidies to get here. Some of this work was subcontracted and some I did myself. I got some good deals, too. I think another person might have spent $40,000 to have the same work performed. The energy bill of this house when we got it would be in the neighborhood of $3,300 annually, so the simple payback of this effort seems well within the range of reasonable, and we got a more comfortable house with better air quality.
For the first time in my sheltered life, I have a range with a self-cleaning oven. After over a year which included roasting a number of chickens (which we've been raising the past few years) we had an oven covered with enough spattered grease to cause the smoke detector to go off any time we turned the oven on. The manual that came with the range cautioned against using the usual oven cleaners and recommended the use of the self-cleaning feature. This process locks the oven and heats it up to a very high temperature - Wikipedia among others says 900F. I was curious what this would be like - would the range feel really hot to the touch; how much would it smoke; woudl it actually clean the oven; how much energy would it use?
You have to clean any serious accumulations of stuff out first, perhaps so it doesn't combust. Also, you're not supposed to leave the racks in - they warn against the high temps destroying a finish that makes them slide easily. This was a major bummer since we'd been dumb enough to leave the plethora of racks this range comes with in the oven while we only needed one to roast the chicken :-( Duh - they were really grimy and we (OK, not we, Jill) cleaned them by hand. Note to self...
Once we started the process, the oven heats up quickly and smoke comes spewing out the vent. We had windows open and the range hood running. In the first hour of the three hour cycle the visible smoke stopped. I didn't abort the cycle because I thought I should go through it at least one time. The front and sides of the range felt surprisingly un-scary in terms of temperature - there might actually be some reasonable insulation in there
Once it turns off, the door stays locked and it indicates on the display that it's still hot for close to another hour, until it cools down. It did get pretty clean, and there was some ash remaining. And the entire process used 8 kWh of electricity. That night we had friends come to share homemade pizza, so we added another 3 kWh. The day's total of 11 kWh exceeded the usage in some months!
Phil Forest flipped our 4.76 kW Sunpower PV system on during the afternoon of June 9th, 2011. At the end of the day June 8th, 2012, the system had produced 6,694 kWh. On the following day, it made 22.5 kWh, so take half of that for a full year's worth of production - afternoon to afternoon - and the total is 6,705 kWh. I'm really pleased and not a little surprised at this total, it's significantly over what we predicted. The system has some winter shading, too. Nonetheless, the yield was 1.41 kWh/W/year, and during the time when I had a small (1.06 kW) system at my NH home in the late 1990s/early 2000s, that system never made over 1 kWh/W/year. Likely a cloudier climate, and I know this past year has been sunny (Eliakim's Way PV production is up 6% over the first year), but some of this has to also be technology improvements. Sunpower claims their technology is more productive in low light conditions and high temperature conditions than their competitors, and just maybe they're right!
It's now two full years since the eight households moved into their new homes at Eliakim's Way that South Mountain Company designed and built. We've continued to read the sub-meters monthly and now have two full years of data. Here's the comparison between the two years, by house and by end use:
- Heating energy was down due to the mild winter
- PV output was up - 1.42 kWh/W!
- House 4 got within 300 kWh of zero net energy
- Most households increased their use of energy once heating is disregarded
- House 8 used an amazingly low amount of heat
Our meter reader has said Enough! so from now on out we'll be looking once a year at these totals.