[RE-wrenches] SDHW and Tankless Heaters

[Bill Loesch]

Hi Peter, Darryl, Dean, et al,

You have good info in searching for a tankless machine that is a temperature modulated machine. In the old days of only natural draft tankless you could choose between flow modulated and temperature modulated (and perhaps other mechanisms of which I am not aware). Today, while there are still natural draft tankless water heaters readily available which exhibit, in the words of one of the manufacturers, "mission critical reliability", the larger capacity units are primarily power vented tankless water heaters. All of the "big five" in the tankless arena that market to North America (Bosch, Noritz, Rinnai, Paloma, and Takagi) offer power vented machines. IMHO, the only commonality between natural draft tankless and power vented tankless are they both use gas as a fuel source. Full stop. Venting and the requirement for electric power are the biggest differences.

To the best of my knowledge, all current production power vented machines utilize an inlet water temperature sensor (as well as an outlet water temperature sensor) and would therefore be suitable for use with preheated water. The rub comes when your preheat water only needs a bit more heat. Since none of the tankless available today can modulate to zero there is a (sometimes) noticeable step when the "backup" tankless heater adds it's minimum contribution. Since both the Vaughn Sepco and Rheem/Ruud heat exchange tanks both offer electric elements as standard (no additional cost) as do off-the-shelf electric tank water heaters, the use of the electric elements as backup did not cost you any more in initial expense. Unfortunately, when the preheat isn't sufficient you are using electric to heat the entire (usually large) tank and you are blessed with standby losses that are costing you out of pocket money. The tankless and preheat are really a perfect match in so far as heating only when (and as much as) necessary albeit the step or perhaps better described as the stumble when/if only a bit of additional heat is necessary and the obvious detractor - the cost of the tankless when the electric tank elements are already in place. 

A high turn down ratio is commendable but the absolute low end of burner performance is perhaps one way to choose preheat backup tankless. The use of code compatible thermostatic mixing valve(s) (e.g. Honeywell AM-1 series) is one more practical tool. The more stuff that goes into the system, the more possibility there is for a failure. 

One aspect of the tankless that all too often is overlooked is the opportunity to put the machine close to the load. If you have widely separated loads then one solution is to put in two smaller machines-one close to each load. Thereby eliminating the wait or need for any recirc system. The practice of putting the new power vented tankless where the old tank was or on the inside of an outside wall (in freezing climates) simply to minimize the venting issues demonstrates to me a lack of good design/planning/understanding.

Apologies for the tardy reply.

Caveat emptor, YMMV, Sincerely,

Bill Loesch
Solar 1 - Saint Louis Solar

  ----- Original Message ----- 
  From: Peter Parrish 
  To: 'RE-wrenches' 
  Sent: Sunday, December 07, 2008 5:02 PM
  Subject: [RE-wrenches] SDHW and Tankless Heaters


  We do a few SDHW systems and only for customers that purchase a PV system from us and want SDHW as well. We want to recommend a tankless heater where it makes sense (very old gas hot water heater or we need the space to swap out the existing gas water heater for the solar storage tank, new construction, etc.).



  I went to a seminar on the subject a few years ago and one of the key issues was choosing a tankless heater that "modulates on input water temperature". The only one I was aware of at the time was a small Bosch system with ½ Cu lines. Are there big fellows that "modulates on input water temperature" and could someone recommend one?  



  - Peter

  Peter T. Parrish, President
  California Solar Engineering, Inc.
  820 Cynthia Ave., Los Angeles, CA 90065
  Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
  CA Lic. 854779, NABCEP Cert. 031806-26
  peter.parrish at calsolareng.com 



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