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Please critique this design
I'm not sure if I've seen this exact design anywhere, but it contains many of the concepts that I learned about on this website and others. The cartoon is below. Here are the system specs:
* Design day heat load 40k BTU/hr (@ 15F outdoor).
* Lochinvar Knight WHN-055
* primary-secondary with closely-spaced tees
* Turbomax 23 reverse-indirect DHW piped as a series buffer in the boiler (secondary) loop, on the boiler supply
* Thermostatic mixing valve for DHW (115F setpoint)
* System (primary) loop has a Grundfos Alpha in delta-P mode, probably running @ 14 gpm when all three TRVs are open
* Each of the three 50-ft baseboard runs will be "zoned" with TRVs, one on each run
* A single indoor-reset thermostat will be set a few degrees above target, used as a high-temp cutoff (i.e. ODR will be the main feedback)
* The boiler pump will be controlled by the boiler (0-10V signal) for constant 35F delta T, probably running between 0.5 and 2.5 gpm
* The boiler will control its supply temperature with outdoor reset (160F on design day, minimum 145F)
* The boiler will sense the Turbomax temperature and treat it as a zone, calling when the tank temperature drops below 145F
* The boiler will disable the system pump whenever the DHW zone calls
* I didn't draw the air separator and Y-strainer, but they will be included
* (side note: there will be a drain water heat recovery system)
I think the advantages of this design are:
* No short cycling, even with 50ft fin tube zones at 110F
* Return temperatures will vary between 110F and 126F (design day), so the boiler will condense most of the season and maybe beat 90% efficiency on design day
* Even heating within a zone: Design day baseboard temperature of 134F has 8F delta T
* Primary/secondary piping satisfies Lochinvar's recommendation
* Only 2 pumps needed
* No expensive controls beyond what's included with the boiler (unless you count the built-in controls on the pumps)
* (un)availability of small, inexpensive, efficient circulators with 0-10V control?
So, what do you all think? Glitch, fix, or something else? Please critique. Note that this is for a retrofit, so the baseboard is a design constant (obviously radiant would be preferable).