Clark-

"This setup is probably the most efficient way to provide domestic hot water."  An exception to this statement is solar thermal that provides free domestic hot water heating through much of the year.

My solar thermal tank is followed by a 2010 model Tagaki tankless, and it seems to work better when fed with the preheated water.  With the (standard) cold water feed (i.e., when the solar tank is cold), it requires a significant water flow to get hot water.

When considering combining tankless systems in series, verify that the heater is speced to allow hot water at the inlet.  For example, Noritz does not allow it.   

Lee Dodge
http://www.residentialenergylaboratory.com/
in a net-zero energy house

What Lee said, regarding plumbing gas-fired tankless in series with solar. Some older Takagis shut down when the incoming water is over ~ 130F, and won't re-light until it drops to ~120F.

Min-modulation of the tankless also becomes an issue if plumbing them in series with solar making them shut down or not regulate the temp well unless you have the hot water flow really blasting when the solar is less than 15F cooler than the programmed output. The T-K3 throttles down to under 10KBTU/hr output, but even lower would be better in this type of application. Those that are up around 25-30K input at min-mod are a real PITA if plumbed in series- less so if plumbed as a mini-boiler maintaining the solar tank's temp, but that also lowers the collection efficiency of the solar.

Posted By Dana1 on 08 Feb 2011 11:45 AM
What Lee said, regarding plumbing gas-fired tankless in series with solar. Some older Takagis shut down when the incoming water is over ~ 130F, and won't re-light until it drops to ~120F.

Min-modulation of the tankless also becomes an issue if plumbing them in series with solar making them shut down or not regulate the temp well unless you have the hot water flow really blasting when the solar is less than 15F cooler than the programmed output. The T-K3 throttles down to under 10KBTU/hr output, but even lower would be better in this type of application. Those that are up around 25-30K input at min-mod are a real PITA if plumbed in series- less so if plumbed as a mini-boiler maintaining the solar tank's temp, but that also lowers the collection efficiency of the solar.

If my incoming was over 120 I'd want my tankless to shut down! The only thing that might want hotter water is the dishwasher.

Most people who run into the 130F limit on incoming water with Takagis are using them as combi-boilers for DHW + spacing heating, not solar finish-heat. Other vendors have other incoming water temp limits, whether controlled by the heater's internal electronics, or by the warranty... (I'm not sure which is the case with Noritz.) As vendor warrantys go Takagi & Navien tend to have fewer exclusions on how they may be used, but that's not to say they're always well-suited to the particular application. Features such as their behavior at higher incoming water temps aren't always well documented or supported. The 130F incoming water shutdown for TK1s, TK2s and KD20s isn't found anywhere in the operating or installation manuals, but it's been independently "discovered" by several system designers. (I'm not sure if there's a software update that would work around that issue, but the later TK-JRs, and TK3s seem to work fine at higher temps.)

I personally dont know all that much about the different or best types of equiptment for your application, but I do know how you feel about "wasting" energy. Not sure about your state but to help offset the cost you may be able to switch your Energy "Supplier". I am in New York and I have switched suppliers and yes, I have saved money. Good Luck with your project!

If it's winter the water will take longer to heat than in the summer, since the ambient temperature will be lower. The colder the ambient temperature the more heat the pipe will suck out of the water and so the longer it will take. Insulating the pipes does help a little. So I guess, insulating was the right to solution to your problem.