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How efficient are commercial water heaters when used in a single residence?

sheropod2 | Posted in Mechanicals on

Commercial water heaters receive a Thermal Efficiency, rather than an AFUE rating. TE does not convert to AFUE as the testing protocols are different. Thermal Efficiency is a snapshot reading at steady state and AFUE is a seasonal rating that takes into account cycling and other losses. If a commercial water heater was tested using the AFUE protocol, the AFUE would be substantially lower than the TE figure. Here are the questions… First, is it assumed that a commercial unit in a commercial setting is under almost constant demand, and therefore the cycling losses are inconsequential? Second, if units like the Polaris are suitable to use in both settings, should they be required to be tested to establish both a TE and an AFUE? I’m a HERS Rater and I see a lot of these units being used but the application in a single family residence looks like a poor choice if the desired effect is to install a high efficiency boiler. Can someone fill me fill me in?

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  1. Expert Member
    Dana Dorsett | | #1

    Are you talking about it's AFUE efficiency when used in a strictly space heating application, or it's EF efficiency when used as a water heater, or it's efficiency when used as a combi-heater.

    When used in a space heating application with the tank & distribution plumbing fully inside conditioned space and return water temps from radiation well into the condensing zone they can be about as efficient as modulating condensing boilers right-sized for their radiation and load.

    When a house is micro-zoned to where individual zone radiation can't emit the minimum modulated output of the mod-con boiler it loses quite a bit to cycling, and a hot water heater can be a BETTER solution.

    A Polaris isn't a particularly great water heater for space heating applications due to the narrowness of the temperature control band and non-modulating burner, and it can suffer losses to short-cycling despite the inherent thermal mass. HTP's water heaters do a bit better due to the modulating burners. For residential combit apps the Phoenix Light duty is pretty reasonable, since at min-fire it's down at ~24,000 BTU/hr- out, but it can ramp up to 72-73K out under domestic hot water loads. At high fire is enough to manage a 24/365 shower if the space heating loads are low enough, but even with the heat load the buffering thermal mass can manage normal showering times without any problems.

    AFUE is pretty crappy protocol for what happens with zoned/micro systems with low mass radiation. The system designer has a far bigger influence on the as-used efficiency of the equipment than how it fares in an AFUE test, and modulation has it's limits. Sometimes the right answer really IS a water heater. Single family home heat loads have been shrinking, and the zone counts rising. Modulation at the low fire end hasn't been keeping up in the boiler world. Many homes have design heat loads under 20,000 BTU/hr, and average heating season loads under 8000 BTU/hr, with the average loads of individual zones a small fraction thereof. Short-cycling a mod-con into an early grave and lower efficiency or increasing the size of the radiation in every small zone to suppress short cycling isn't really the right thing to do for those houses. Thermal mass can be the answer, and by the time you've added 30-50 gallons of buffer tank you've basically re-invented the condensing tank hot water heater.

  2. sheropod2 | | #2

    Thanks, Dana! I'll be rereading your answer again (and again). You've packed it with a lot of information and I'm new at this. Mechanical systems are fascinating to me and I've just started working with a mechanical contractor. He needs help and I need knowledge! Specifically, I'm talking about when it's being used as a combination heater. Combi boilers and boilers with a side arm, and for that matter any unit classified as residential is rated for efficiency with the AFUE protocol. If the unit is classified as commercial the Thermal Efficiency protocol is used. The only thing I can figure out is that a commercial unit rarely turns off because numerous people or apartments or business are creating a pretty constant demand, unlike units used in single family residential settings. Both combi boilers and boilers with side arm tanks that are classified as residential are tested to establish AFUE. It's clear that the AFUE applies to boiler only, regardless of whether the energy used is for heat or DHW. As a HERS rater I want to correctly model the mechanical equipment, but that requires an accurate AFUE, which is only obtainable by testing it with the AFUE protocol. Thermal Efficiency can be converted to an "Equivalent Energy Factor" which, according to RESNET, is what you enter as the AFUE value. For a Polaris, that takes the TE of 95 and converts it to and EEF of .80, which is then used as the AFUE. Convoluted and not accurate, and it bugs me! Although I have my own opinion about Polaris units, my goal is to get the most accurate HERS score I can for my clients. But when a salesman has a customer believing that Polaris units are "95% efficient," It's apples to oranges, and it's misleading. It's a LOT of money for something that could have been a better deal on future energy bills. Now, please do me a huge favor and tell me if I'm out in left field. I depend on the people who know more than I do to keep me on the straight and narrow. Now I'm going back for a reread. Thanks again!

  3. Dana1 | | #3

    A well designed system based on a Polaris that doesn't short-cycle will have an as-used AFUE pretty close to it's steady state thermal efficiency. The Polaris (all water heaters, really) are MUCH better insulated than most boilers, with much lower standby losses. If the space heating load is more than half the burner output of the Polaris, it's AFUE will be less than 0.5% lower than it's steady state, again assuming it's not short-cycling.

    Commercial water heaters are only rated on their thermal efficiency since the actual daily volumes and draw profiles are unknown, not because the are presumed to be high. Many commercial water heaters only get high volume use between Friday night and Sunday AM. Residential water heater draws have a more predictable range of daily draws & draw profiles, and it's low enough that standby losses really matter. Adding a space heating load to a water heater dramatically increases the duty cycle of the burner, bringing it's as-used efficiency much closer to it's steady-state efficiency. AFUE testing presumes 1.7x oversizing of the burner to the peak load, and about a 30-35% average duty cycle, and most cast-iron boilers test with an AFUE nearly identical to the steady state efficiency, and the jacket losses from the poorly insulated casings are ignored, because the test presumes the boiler to be inside of conditioned space.

    The standby losses of a Polaris are no worse than that of in indirect + boiler when operating in water-heating-only mode, and probably better than most mod con + indirect installations. From an AFUE point of view it's also better than most boilers, as long as the 99% design heating load is a significant fraction of the burner output.

  4. sheropod2 | | #4

    Dana,I feel like I've died and gone to Heaven! I'm the only HERS rater in town and am quite isolated from discussion with other professionals since they are all pretty far away and darned busy. Thanks so much for the REALLY through explanation and all the other information related to it. I had no idea when I started training for HERS how much building science and mechanical would draw me in. It's like Christmas every morning and there's a pony under the tree! And one more thing... Taos, NM is a ski area where death cookies are not permitted. I'm grateful - I grew up skiing in Michigan where they raise bumper crops every winter.

  5. Expert Member
    Dana Dorsett | | #5

    It's been awhile since I've been there, but there was one glorious week when I was downright hooked on hiking & skinning the Kachina Ridge for freshies!

    Those were markedly different conditions than skinning for turns at the local hill a few weeks ago, when death cookies might have been an IMPROVEMENT! :-)

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