Tankless Adviser: Firing rate is only a secondary factor on the raw combustion efficiency, and setpoint temp is third-order factor for tankless units. Incoming water temp is primary, and it's ALWAYS going to be well into the condensing range in DHW mode, but can be much warmer/less efficient in space heating applications with high return-water temps.
In DHW mode whether it's at min-mod or full-fire a condensing tankless will always be north of 95% steady-state, whether the output temp is set to 40C or if it's set to 60C. There will always be condensing going on in part of the heat exchanger that contains water under 50C. But flue purges on short-draws eat into efficiency considerably, since every purge extracts the same amount of heat out of the the heat exchanger whether you just took a 20 minute shower, or whether you just rinsed your hands. On the former that loss is a negligible fraction, on the latter it's a good chunk of the total burn. The US DOE EF test over-rates tankless units relative to tanks, since the use profile is all long draws.
I'm curious about the test data on the mini-buffered Naviens- is that Centre for Energy and the Environment available online?
Set-point can play a very important role based on the design of a tankless water heater. Some tankless water heater manufacturers are known to use bypass valves (controlled or fixed) to assist with heat exchanger protection and temperature stability. If the bypass is controlled, setpoint will determine if the bypass is partially opened, fully open, or fully closed. If bypass remains fully closed, then set-point as you say becomes less relevant.
- Agree on efficiency drops on space heating mode due to high return water temperature being too close to or depending on application above dew point.
-Problem with the Navien tankless water heating units with buffer tanks is that the bufffer tank's recirculation pump when activated mixes heated water with cold water and increases inlet water temperature. I have not looked at the pump curve in the units, but since tankless water heater manufacturers typically require 2 GPM flow through the heat exchanger in recirculation mode, I can't imagine that they would recirculate the buffer tank with anything less. Here again, the set-point will affect the efficiency of the unit as the higher the setpoint, the higher the temperature at which water in the buffer tank will be recirculated into incoming cold water line and hence into the water heater. If Navien used Takagi as a reference point, then the buffer tank will always be recirculated on hot water demand, and as such, the inlet water temperature will always be greater than the city supply. I can not confirm this, as I have not done any tests on Navien units, but I am very familiar with Takagi.
- A problem with the Navien CH combi boilers, is that it is a boiler first and a water heater second. This means that it essentially operates as a boiler and hence the domestic hot water does not flow through the heat exchanger. As a result, even in domestic hot water mode, the system will be hard pressed to condense. This is partially why the Navien combi-boiler has lost its energy star rating (visit Navien's website to find out more about their energy star rating for combi boilers). Here again, setpoint will be a factor, as the return water temperature on the boiler side will be influenced based on closest approach temperature of the flat plate heat exchanger employed for domestic hot water production.