HEATING WATER IN A ZERO NET-ENERGY HOUSE

Air Source Heat Pumps versus Electric Resistance Water Heaters — Lesson Learned.

You’ll find that many zero net-energy (“ZNE”) homes use a ’high efficient’ air-source heat pump water heater (“HPWH”).  Many net-zero authors recommend these systems as part of the over-all strategy to lower the home’s energy demand.  This recommendation sounds logical, especially when the EnergyGuide label on such water tanks advertise several hundred dollars of annual savings compared to other less efficient systems such as electric resistance water heaters (”ERWH”).  In addition, many jurisdictions are trying to encourage the use of HPWH systems either via incentives or regulations.

BUT, WHOA NELLIE, HOLD YOUR HORSES!!!  

These air-source systems aren’t as good as advertised and may not be your best option to heat water, especially in a colder climate.  I’ll explain this shortly, but first, if you opt to use a HPWH, you need to make that choice early in your design process, otherwise you may find you can’t properly situate your HPWH.

Air source heat pump water heaters:

• Should not be located in a room/closet with less than 1000 cubic feet of volume.

• Have a fan, which produces noise.

• Have slower hot water recovery times, meaning if you run out of hot water, it takes a long time for your water to reheat .  (Some models have a backup, supplemental, electric resistance heater to deal with this issue, but they lose efficiency when operating in backup mode.) 

• Require the installation of a drain for disposal of condensate (water) produced by the heat pump.

• Are taller (and heavier) than conventional water heaters and need more clearance.

• Lose net operating efficiencies in many real life settings, especially when installed in homes in colder climate zones.

• Are more costly to purchase and install.

In short, in a lab setting, air-source heat pumps make sense to heat water, but in actual residential settings, especially in colder climates, the benefits and practicality of using these systems can be less than advertised, especially if your water tank cannot be located in an unconditioned space.  

In colder climates, the hot water tank is normally situated inside the heated (or partly heated) envelop of the house. This is done to prevent freezing of the water pipes.  This in-door location is usually sub-optimal when using an air-source heat pump as the heat pump draws heat from the surrounding indoor air to heat the water.  As a result, your water gets hotter, but your indoor air gets cooler. That’s fine in the summer if you are trying to cool your house, but in the winter you’ve got a problem.  Your home heating system is heating the air in your house, but your hot water system is cooling your in-door air.  The two heating systems are fighting each other.  

One suggested method to minimize this problem is to locate your HPWH water tank in a partly unconditioned space, such as an unheated basement that remains above freezing.  However, this space is not totally thermally isolated from the house, meaning that at least some of the heat being used to heat the water is being drawn from the interior of the house.  This efficiency loss, and other in-situ operational losses, are not accounted for in most of the advertised efficiency metrics for HWHPs. 

In a cold climate, the problem with properly situating a HPWH cannot be solved be installing the HPWH outdoors, as then there is an issue with freezing, and most HPWH are not designed to work in these low temperatures.  In addition, once a HWHP operates in cold ambient air, it loses operating efficiencies as it is harder to draw heat from colder air.  Also, many HWHPs are actually hybrid systems that incorporate a supplemental electric resistance heater to speed up the hot water recovery times.  These hybrid heaters will switch to electric resistance mode once the air ambient temperature falls below a pre-set threshold.  (For example, one model has a pre-set threshold of 50 degrees Fahrenheit).  In that scenario, your ‘highly efficient’ HWHP is merely operating in an inefficient electric resistance mode.

In short, while HWHPs are efficient, they may not be as efficient in actual practice as compared to the advertised lab-based efficiency metrics.  In addition, you likely will have a problem situating your HPWH water tank.  Ideally, you need a space which doesn’t get too cold, is larger than 1000 cubic feet, has a way to drain or capture condensate, and possibly has sound insulation.  Plus, depending on the HPWH model you choose, you may run out of hot water quicker than you’d like. – That said, most zero net-energy houses are using HPWHs as they are usually much more efficient than ERWHs when properly situated (although maybe not always as efficient as advertised). — However, I did not use a HPWH for my house; here’s why….

For THE HAYFIELD HOUSE, I did not think about how I was going to heat water until the house’s framing was completed.  At that point, there was no logical place to put an HPWH.  So instead, I used a less efficient electric resistance hot water tank.  BUT, all this really meant was that a few more solar panels were needed to off-set the incremental electrical demand from heating water when using an electric resistance heater.  So let’s look at how many extra solar panels were needed.

The HERS certificate estimates that 11.3 MMBtu of energy is being used annually to heat hot water at THE HAYFIELD HOUSE using an ERHW.  This converts to about 3311 kWh of electricity.  Each solar panel on the HAYFIELD HOUSE produces nearly 400 kWh on an annual basis, meaning that about 8 solar panels are needed to heat the water.  But that number is the entire number of panels needed to heat all the water, not the incremental number of panels needed when using less efficient electric resistance versus a more efficient air-source heat pump.  I then used an energy model to estimate the incremental increase in annual watts used from switching from an HPWH to an ERHW.  The result was that I needed about 3 extra solar panels to produce the additional watts. Alternately, one can use the theoretical efficiency gain based on COP (coefficient of performance) wherein some authors claim that HPWH are ‘three times more efficient than ERHW’.  If so, that would imply about 5.5 extra solar panels are needed.  For sake of argument, let’s assume 4 extra panels are needed. — The obvious next question is “What was the cost to install four extra solar panels on THE HAYFIELD HOUSE?”

 
The total installed cost of the 16.64KW solar system for the HAYFIELD HOUSE was about $61,000.  Since this is a 52 solar panel system, this equates to $1173 per solar panel.  For sake of simplicity, I will use this figure (although that estimate overstates the true marginal cost as this figure includes not only the cost of the panels, but also the cost of the inverters).  Therefore, to install 4 additional solar panels would cost 4 X $1173 = $4,692.   BUT WAIT… That figure is pre-tax, pre-incentives.  On an after-tax, after incentives basis, the numbers are much less. 

THE HAYFIELD HOUSE qualified for:

• A 30% federal tax credit for solar panels, plus;
• A Massachusetts tax credit, plus;
• Net metering, plus;
• SREC2 credits.

The value of SREC2 credits is determined via periodic auctions, so the net present value of SREC2 credits is uncertain.  However, presently the SREC2 credits for THE HAYFIELD HOUSE equate to about $4000 per year, or about $77 per solar panel per year.  Due to this valuation uncertainty, I won’t do an after-tax, after-incentive calculation, but the end result almost assuredly would reduce the incremental after-tax cost of installing 4 more solar panels to ‘not that much’.

Anyhow, if you plan to use a HPWH, make sure you think through early in your design phase of where you would properly situate the tank.  However, all is not lost if you need to go with an ERWH if your water usage is low, you have space to add more solar panels, and your state has a favorable solar credit program.

So the moral of this story is:

If you aren’t sure which turn to take, it may not matter.