Heat Pump Hot Water Heater

So, you're trying to decide how to heat the domestic hot water in the zero energy house you are designing for Denver.

Solar hot water heaters are an excellent choice. However, the logistics of a solar thermal installation are daunting compared to a typical appliance installation.

A logistically easier solution is to use a heat pump water heater (HPWH) in conjunction with PV solar.

Rheem has just released a new nice-looking unit.

The jury is still out as to which configuration will be best. Recent test results and common sense are beginning to favor HPWH with PV. The reasons for this are that solar DHW is insufficient and inefficient in the winter, and underutilized in the summer. A grid-tied PV system has the advantage of constant efficiency and 100% utilization year round. In addition, large PV cost reductions are coming, but none are expected in solar thermal. State-of-the-art solar thermal collectors still use supply-restricted copper, aluminum, and glass. The labor content of solar thermal installations is also quite high.

Some puzzles remain with the implementation of HPWH systems. Per the manufacturer's recommendations: "Because the heat pump’s exhaust air is cooler than the surrounding atmosphere, these water heaters generally do not belong in or near busy living areas of the home." Since they pull the heat out of the surrounding air, a small, airtight mechanical room isn't the best place for it either.

Discussion of HPWH locations:

1. The garage. A new, well insulated garage might work. An uninsulated garage slab gives off enough low temperature free geothermal heat* to prevent freezing of the pipes to the heater. There are at least two problems with the garage, however: 1. In summer, the HPWH gives off cold air that would be more desirable inside the living space than in the garage. Ducting air from the garage to the house is a CO hazard, and not recommended. 2. If the (insulated) garage door is inadvertently left open in the winter for an extended period, the water lines to the HPWH are at risk of freezing.

2. Mechanical room with forced ventilation. The control scheme and ducting layout for this ventilation can get complicated and costly.

3. Basement. This is where you usually find the water heater in new construction, and will work just fine if you don't mind a cold basement in the winter. If you finish the basement and add heat to it, that heating system will be feeding the HPWH the heat it needs, and this will spoil the overall efficiency of your system.

"The House as a System" philosophy of home design will get a real workout on this one. Please enter a comment if you've heard of any other solutions.

*A perimeter insulated slab in an insulated garage gives off 5-15 Btu/hr/ft2, based on preliminary research. This is a great match for a 7000 btuh HPWH.

The Greenest Building is Usually the One Already Built, but Not Always

I'm really not trying to anger preservationists or tree-huggers here, I'm just analyzing a trite slogan from an engineering perspective. I'm a pragmatic preservationist.

The question is whether or not you should rehab and do a green retrofit on an existing building or replace it with a well-designed low energy new building. The correct answer can only be found after making some reasonable assumptions, cost estimates, and estimates of resale value.

You don't "waste btu's" when you tear down an old building. Those btu's were spent when the building was originally built, and there's nothing you can do to get 'em back.

You can only choose not to spend new btu's on a new building.

And btu's are just another form of dollars.

So it's always an economic problem, eg., what's the present value of retrofitting the old building vs. building an entirely new one. This analysis must be done for every building using each case's unique problems and assumptions.

In residential at least, it's fairly easy to build a new "zero energy" replacement house, but fairly hard to retrofit an old house to zero energy. The cost of the new house can be estimated pretty accurately, but trying to estimate the retrofit cost of the old house is risky. Every old building is different.

The embodied energy of the new building is just part of the down payment on a really good investment. The value of that energy is in the cost of construction. The landfill space required for the old building is also given a dollar value in the cost of demolition. The embodied energy of the old building is a "sunk cost" and doesn't factor in the analysis.

The main thing that doesn't have a dollar value in this analysis is the generation of CO2. Lawmakers have begun trying to put a dollar value on that. Eventually this "carbon tax" will be pretty accurate, and will favor the old building.

Most estimates put the embodied energy of a new building at 5-15% of the lifetime energy usage of the building. That means the operating energy usage of the building is about ten times more important than the original energy content.

Therefore, it may be a catchy phrase, but it's very unscientific to generalize: "The Greenest Building is the One Already Built". Be sure to say "usually".

Mini-Splits or PTACS are the Key to Net Zero Energy Homes

High SEER minisplit heat pumps have finally caught the attention of high efficiency homebuilders. Postgreen, a redeveloper in Philadelphia, touts "geothermal efficiency at a fraction of the cost": http://www.postgreenhomes.com/models/
When you add PV solar to a house and a high efficiency and lo-temp-capable heat pump, then you really don't need to pipe natural gas to the home. (Cooking meals with gas may be preferred by chefs, but it is neither healthy nor energy efficient. Gas ranges put out a lot of CO). That saves the $12/month gas hookup fee.
Besides, you can't send natural gas back to the utility the way PV solar sends electricity back . In superinsulated homes, this scenario also makes solar thermal obsolete for heating and domestic hot water (DHW)
Why? Because even at a solar conversion efficiency of only 10%, in the summer, you are putting energy back on the grid and dollars in your pocket Every. Single. Day. Thus, the yearly system efficiency and ROI is better than solar thermal.
It's becoming accepted worldwide that the only way to acheive true net zero energy is with an all-electric house and PV. Minisplits are part of the puzzle.
A few more of benefits of these heat pumps:
1. Less floor space used inside the house for mechanicals.
2. Compression-cycle air conditioning included "for free".
3. Better zoning is possible than with any sort of central forced air system.
4. In smaller homes, the installed cost is a fraction of a centrally ducted system.