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Does anyone have experience of adding a "pre- warming" system to a tankless hot water unit to reduce cost/ increase efficiency?

I'm in a northern climate, house is small, one bathroom with a 3"high enclosed crawlspace. The crawlspace is normally around 60-70 degrees, the water from the street comes in at around 40 degrees in the winter. My best choice (convenience, no need for venting, install price, space) of an electric tankless efficiency is impacted by the temp of the incoming water. I'm thinking of adding a crawlspace tank prior to the tankless unit to allow the incoming water temp to warm up for "free" in the crawlspace- thinking that a tank of about 40 gallons or 100 ' of pex in the warm crawlspace would warm up several degrees on its own and help reduce my electric /water bills. Anyone tried this?

Asked by stu m
Posted Wed, 12/04/2013 - 23:44

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2 Answers

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1.
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Stu,
Q. "Anyone tried this?"

A. Yes, it's been done frequently. Of course, there is no such thing as free heat. Introducing cold water into an uninsulated tank in your crawl space will lower the temperature of your crawl space; this may or may not increase the amount of energy used by your furnace or boiler (your space heating appliance), depending on a variety of factors.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 12/05/2013 - 05:55

2.
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A three inch crawlspace? That's got to be one thin pancake of a tempering tank! :-)

From an efficiency point of view, Martin has it right- the tempering tank is being heated by your heating system, at whatever system efficiency it has. A tempering tank DOES give you more capacity with an under-powered electric tankless, but from a raw BTU savings it's only buying you the slightest amount of energy in summertime cooling, and nothing in the winter.

A mere 100' of PEX is at most a couple of gallons, and a fairly lousy heat exchanger. A 40 gallon uninsulated tank could provide some capacity benefit if it's designed to stratify, with the incoming water at the bottom the exiting water at the top.

If you have three FEET of crawlspace rather than three inches, and take showers rather than baths, you can get a significant capacity & energy savings boost out of a drainwater heat exchanger. A 4"x24" or 4"x 30" (if it fits) heat exchanger can buy you about a 30-40% energy return/boost if plumbed to feed both the hot water heater AND the cold side of the shower at typical shower flows:

http://oee.nrcan.gc.ca/residential/personal/retrofit/13302

That means a 15kw tankless would be delivering ~20kw performance, which is about what you need to run a continuous shower at 2gpm with 35F incoming water. That'll be about a 25-30% savings on power used for showers, but not on other draw. It won't buy you anything on batch draws like tub or washer fills, since can only put the heat into the incoming water stream when the drain is flowing simultaneously with the potable flow.

Answered by Dana Dorsett
Posted Thu, 12/05/2013 - 11:49

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