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Domestic hot water power demand is no longer an issue?

Traditionally, heating and cooling requirements have been the largest energy draw in homes. The ability to relegate heating and cooling to 2nd or even 3rd place in a home energy draw hierarchy is now possible with better building techniques, such as PH. It has been generally agreed that the new king of the draw is DHW. However, I wonder if that is still the case now that HPHW systems have hit the market. I live in a heating dominated climate. The maximum heating demand allowed per PHI is 4.75 KBtu/SF or 5.2 KBtu/SF per PHIUS+ for my climate zone. If I were to build a 1,500 SF home, that means my max allowable annual heating demand per PHI in KWH would 2,088 ((4.75/3.412)*1500). What should I expect my annual DHW demand to be and how does it compare. I looked at using both an indoor only and split system ASHP for DWH. When I modeled the indoor in WUFI, I could not comply with the annual heating demand requirements because of the parasitic heat loss caused by the cold air coming from the condenser. When I used the Sanden inputs provided by PHIUS, I easily beat the heat demand numbers. However, that did not tell me anything about my DHW demand. All I had to go on was that it was more efficient than using ER or indoor only ASHP. I have been following Matt Bower’s blog for his to be certified PH in Rochester, NY, a colder climate than mine. He recently posted a picture of his Sanden and according to the Energy Guide sticker on the tank, the projected annual consumption in KWH is 1,250! Huh? Less than PH heating demand. Too good to be true. I checked Sanden’s website and found this presentation given by WSU at the the 2015 NAPH Conference.

According to their analysis, the Sanden uses about 5KWH/100 Gallons. 100 gallons is probably much more than a family of 4 uses in a day, unless everyone is taking baths. It also seems to jive with the Energy Guide. This does not include drain water heat recovery, which Matt did install with his unit. Generally the heat recovery manufacturers claim another 30% savings. So, we could be looking at under 3.5KWH/100 gallons. Am I missing something or is this the new reality for cold climates? Does this mean indoor only units can provide a similar benefit is warm climates that are cooling dominated?

It will be interesting to see what Matt’s usage looks like after his first winter and I hope he posts it.

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  1. D Dorsett | | #1

    Heat recovering manufacturer estimates of total HW energy savings are not to be trusted. It depends a LOT on the actual hot water use profile, and the flow rate for continuous flow applications such as showers. NRCAN testing standards are at 2.5gpm at a fairly substantial delta-T between incoming water and drain outflow. Lower flow such as a low flow shower head increases recovery efficiency, but also reduces the total hot water used for showering, making it a lower fraction of the total hot water use. Warmer than tested incoming water temps reduces the heat transfer effectiveness,, but also reduces the total amount of energy needed to heat that water to useful temperatures.

    Bottom line- 30% is likely to be a significant exaggeration, though it's conceivable that fanatic "endless shower" hot water hogs could beat that, if showering represented 3/4 or more of their total hot water use. For most families I suspect 15-20% net energy savings is more realistic.

  2. Jonathan Lawrence CZ 4A New Jersey | | #2


    I suspect you are correct that the actually savings is very dependent on behavior/usage and may be less than expected. My original plan was to install a drain water heat recovery pipe downstream of the water purifier and have it feed the cold water supply as recommended by the manufacturer. I am going to keep the design, but hold off on purchasing and installing one to see what my actual usage is without it first.

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