Multiple Condensing Boilers

CMadatMe

Here are 3 Viessmann Vitodens 200 B2HA-150s 113-530 Btu/hr ouput on a Viessmann Rack System and common vented in a store front down in Manhattan that I went down for finally start up. What surprised me is how they are running. The listed low end of the modulation rate for a single boiler is 113,000 Btu/hr in the literature but the cascade says other wise. Boiler 1 did come up to 113,000 but then drop down to 55 and boiler 2 came up to 55 then the both dropped and all 3 ended up cruising along at 41,000 Btu/hr which is still the min of in total 113,000. Temp outside was 32-33 degrees. At first I thought ok, each one is 410 but not because the control showed boiler one at 113 before system started to cascade so the display shows output in 3 digits. Couldn't even hear the boilers, had to take the covers off. Anyone else run into this where the btu/hr output is significantly less then what's given in the literature and TDM?

NYplumber

Chris, could it be low gas pressure and the lamba control dialing back the output?

CMadatMe

Gas is fine. Each boiler has a low pressure gas switch. They would be shutting boilers down under a low pressure situation.

kcopp

My guess is that the low end if it runs by itself is 113K.... If you run them cascaded they gain a higher turndown ratio.
Just a WAG.

CMadatMe

kcopp said:

My guess is that the low end if it runs by itself is 113K.... If you run them cascaded they gain a higher turndown ratio.
Just a WAG.

That was my logic as well. If that's the case then why can't I have it with a single boiler. Would seem like something they'd want to promote instead of hide.

Just look at the B2HB19,26,35 its all the same boiler chassis and parts. Nothing different except for the coding card that tells the boiler what it is and how to fire. Just give me one boiler from 12,000-125,000 and let me cap off the high end of the modulation rate. Heck really don't even need to do that because boiler is only going to go there if the system demands it. Give me that and open up the coding for controlling a variable speed boiler and system pump and we'd have a home run.

CMadatMe

So in cascade mode the boilers will turn down below the 113,000 btu/hr min modulation rate stated in all the manuals. The minimum modulation rate is the 113,000 spread across the # of boilers in a cascade. So 2 boilers the min would be 56 with 3 the min would be 37 and 4 the min would be 28. That's been verified.

What I don't get is that if I can do this on mult then why can't I do it on a single boiler? You'd think Viessmann would give us a true minimum modulation rate based on the boiler capability not a number they pluck out of the air.

This only makes me think that there is no reason in the world that there needs to be a B2HB19, 26 or 35. The BHB35 could in theory cover that full range. Same goes for having a B2HA45 or 57 and then the B2HA80,88, 100, 112. You basically only have 3 boilers here not 9!