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Tekmar 303v or 400 w/Combi Boiler - How will they react?

fiehlsport Member Posts: 13
edited November 2023 in Thermostats and Controls
I'm planning out a Tekmar 400 install on my Laars Mascot FT combi boiler next week. The only lingering question I have is regarding how the '400 will react when the boiler gets a call for DHW. There are no DHW output signals available on the boiler (I think?) to tell the Tekmar that it will be switching off the central heating while it makes hot water, so will the Tekmar 400 behave oddly in any way?

My fear is that it throws an error on the unit during DHW usage + a call for heat, because it will detect the CH loop boiler output temp decreasing while someone takes a shower for 15 minutes... or it will ramp up the requested load on the 0-10v circuit very high so when the DHW call ends, the boiler overshoots a lot.

Anyone have any experience with this particular setup? Are my fears unfounded?

Perhaps finding some sort of DHW wiring in the Laars and connecting it to the DHW Call terminals on the 400 would be wise? Then, it would give it priority, know what's going on, but not actually control anything?



  • fiehlsport
    fiehlsport Member Posts: 13
    edited November 2023
    Well, I've got a Tekmar 303v and a 400 sitting in my dining room right now, and a pile of TN2 thermostats for the 400. Planning to do this next week if it makes sense to. I sent an e-mail to Tekmar support this morning and am hoping they get back to me by the weekend. Looking for them to say something along the lines of... Use the 303 with RoomResponse, use the 400, or neither of these products are ideal for your situation.

    Worst case, I'll use the 303 and turn off RoomResponse, falling back on my standard Laars Outdoor Reset curve. It works decently, I am getting 20-30min cycles out of it right now, but I know it could be better.

    Regarding finding a DHW-call terminal inside of the Laars (Would put me at ease if it had one), there is only wiring from the flow sensor, and it's 5VDC. The Tekmars require a "dry contact" or 24VAC to trigger the DHW mode... so I'm out of luck there.
  • fiehlsport
    fiehlsport Member Posts: 13
    Thought I'd close the loop on this in case anyone in the future stumbles upon this post. No, it's not a good idea to pair a Tekmar 400 or 30Xv controller with RoomResponse to a combi boiler.
    I tried both, and am currently running the 303v, albeit without Room Response. More on that later.

    The 400 is an amazing product, I really wish I could have kept it. As I expected, when there was a call for DHW, the 400 started to ramp up the duty cycle to the boiler, even though the boiler was ignoring it. The CH temps decreased during a shower, for example, so the '400 thought it needed to ramp up the boiler more. By the time someone was done showering, the '400 had the boiler ready to start cranking away at 180F.

    With a combi boiler, there's just no way to make this work correctly.

    The 303V with RoomResponse would behave similarly, even with less "Smarts" and no indoor feedback. A 15-20min shower would cause the 303 to think it's a long cycle on whichever zone needed heat, and end up falling back to a 100% duty cycle call to bring the room up to temp once the DHW call ended and allowed the boiler to switch to CH again.

    I am currently running the 303V with my boiler wired only to the T-T terminals, as I still benefit from a simple zone valve controller with my setup. If I ever disable the DHW on my combi boiler and get a separate tank or on-demand unit, I will hook up the RoomResponse and see how that does again.