Use of tN4 devices in a solar/boiler system

Bill Clark

is heated either from a large hot-water store (solar) or from a modulating boiler - a boiler which includes an ODR control as well as a 0~10V modulating input (Lochinvar KN80). A dual-coil indirect is heated from either source or concurrently from both sources. All radiant loops are fed from a single 3-way modulating valve. This valve is pegged to its hot side (or nearly so) when the boiler is sourced, allowing a boiler ODR control to set the system temperature.

Can I control this beast with a tekmar 422 or 421 (plus tN4 t'stats and zone ctl) together with an RYO ubercontrol which handles details unknown to the tekmar modules? More info...

The Uctl decides whether the system or the indirect has priority, decides which heat source(es) are appropriate. and manages several flows. Importantly, it intercepts boiler signals from the 421/422 (to prevent firing when solar heat is sourced), usurps control of the 3-way valve when valve modulation is inappropriate and filters non-t'stat demands to and from the 421/422. Please assume all of this.

Use of the 421 seems straightforward. The boiler's integrated controls are used for radiant (ODR) and DHW (fixed setpoint). The 421's Boiler Enable goes to the Uctl; that enable propagates to the boiler if radiant heating by boiler is appropriate. The t'stats on the tN4 bus and the mix PID control the 3-way valve for either heat source. Without a boiler sensor, there are no alarms (according to the Data Manual) or shutdowns to get in the way. The worst that can happen is that the PID's integral term maxes while the Uctl has the 3-way valve; I would expect that 421 firmware handles that cleanly. Is this correct?

In the 422 case, the mix PID controls the 3-way valve, and the boiler PID controls the boiler for both radiant (ODR) and DHW (fixed setpoint). I assume that the Mix-1 PID asserts smart demands, including IDR, on the boiler PID when no other demands are present. Is this correct? This design has only one radiant temperature, and so logically needs only one sring of t'stats. Thus, if there is no internal connection between the two PIDs, the 421 is a better fit. One thing I noticed is that I might need one dummy t'stat on the boiler bus to avoid the "Boiler bus open circuit" alarm. Is this the only gotcha? I am a tad afraid of what happens to IDR when the Uctl steals the boiler and the 3-way valve. A lesser issue is the affect on the boiler's PID proper when the boiler 'fails' to fire.

Heartfelt thanks to whomever bears to read this and answer!

Bill Clark

Al Letellier_9

tekmar

not familiar with your setup, but the most direct path to getting your answer is to contact Tekmar directly. They have always been extremely helpful with application questions for their controls.

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hr

Hydronic Mike at tekmar

could figure all that out for you. He lives and breathes that trick stuff.

hot rod

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ALH_4

Simple

I like to keep things simple. Why not use a diverting valve and a 155 Differential Setpoint control to divert the system side of the low loss header ahead of a motorized mixing valve controlling the system outdoor reset? If the solar tank is hotter than the reset supply temp it diverts through the solar tank. Otherwise it runs on the boiler. Yeah the boiler maintains reset temp all the time, but with a sealed combustion boiler and an insulated primary loop that probably isnt a big deal. The boiler handles DHW production perhaps with a second indirect tank for preheat. The Vitodens is set up for this setup by only adding the secondary mixing valve (controlled by the Comfortrol inside the boiler) and the tek 155. I'd run the zones on TRV's and let it go. How much better and simpler can it get with equipment like this?

Sorry, that doesnt really answer your question, but it's the best way I know to integrate solar with one of these boilers. Of course you could use a tekmar mixing valve and control with a different boiler. It's just not quite as simple, and Viessmann's mixing valves are nice.

I should say I have yet to have the chance to do this, though the people I used to work with have done it. I understand it worked out well.

-Andrew