Mod-Con boiler and cycling

Gavin

Looking at using the Viessmann Vitodens 200-W-WB2B 105 (100-350MBH) for a large but now well insulated building, with 6 zones. We've designed in a buffer tank (120gal) could do more, but here is the problem:

The mod-con's advantages are clear, but there are still times when you need even less than the minimum output continuously. What I had never heard of (that the rep said) is you "CAN'T cycle the Viessmann mod-con" I'm happy to modulate all winter long, but we have a long spring and fall when I will need minimal amounts of heat, and only on some days. Obviously there is a minimum firing rate (100k in this case) but what's wrong with turning it off for a few hours, or half an hour or whatever and refiring at minimum if that's all you need for that part of the season. And in the summer, until the solar system is up, we only need it to make DHW, about 15-30 minutes fire each morning.

Will it hurt the boiler (or effect efficiency a lot) if it cycles? If so should I try for a day's worth of heat storage or more in the buffer to mitigate this problem?

Thanks in advance for your help!

CMadatMe

Vitodens

DHW is easy fix. Can run it for time periods via a schedule or have a fixed time on and time off. The low end is what the low end is and if yiou don't need the 100k then park the btu's in the buffer. Boiler is only going to fire if the LLH sensor sees that the tank need some btus.



Now being a big Viessmann guy I would look at a different approach. First do I really need the enitre load? Whats the heat loss? Would 2 WB2B45's do the job giving me a low end of 60K? What is my domestic load? Would boiler 1 handle it? If so then boiler 2 is my muscle heating boiler. I could play around with external demand in boiler 2 having that boiler come on off an aquastat of even a Tekmar control via 0-10vdc.



Just a couple of ideas as abviously don't have all the information concerning your project but a really good Viessmann installer that knows the control and capabilities of the boiler should be able to give you multiple options for you to choose. Where is your project?



PS. I can turn that boiler on or off remotely anytime I want using a Vitotrol 300. That includes DHW.

SpeyFitter

Multiple boilers

And do you have to have the Vitodens 200? How about a bank of 3 or 4 Vitodens 100's that modulate from 37,000 to118,000 ? It has the exact same heat exchanger as the 200 (same quality) just less built in control but enough to easily do DHW and one temperature regime. And to boot - you'll have some redundancy as well.  Then you'll have a much nicer downturn, and if you don't have some type of complex zoning with various temperatures but only 1 temperature the Vitodens with it's external pump module and the controller should be sufficient (they also have a staging controller that does up to 8 boilers from KW technologies).  Yes more money possibly and more labour but would better meet your needs me thinks in the long run and be better for the burners and  you could negate the need for a buffer tank as well.

CMadatMe

Don't Know

If I'd use the Versotronic. Would rather use a Tekmar 265 via 0-10 to control the boilers, dhw and system supply temp. Better bang for the buck to control 100's. Good idea to use 3 100's price point comes out close or near the WB2B105 and drops him down to 30k on the low end.

Gavin

Two boilers is better than one?

Wow you guys are a great help, and on Thanksgiving no less!



OK I can see how maybe I'm better off with 2 boilers. One with MOD/CON, the other can be simpler "work horse" or back up boiler, to run when first hits top of it's range; allowing it to shift back to minimum when the second one kicks on.

Say 30-180 m/C and 150 fixed or similar. In theory minimum would be size needed to fire DHW in the summer. Any suggestions for actual models in these ranges?

SpeyFitter

More info required...

Gavin,

I guess we're putting the cart before the horse a little bit and should get some more info before making a final recommendation - I have a feeling Chris and I might be fairly similar in our suggestions but I think some more information now needs to be garnered.

Can you tell us more about the types of emitter(s) you have in your 6 zones. Is it all radiant floors or some other type of emitter like baseboards, or panel rads or?

Also, how much DHW do you need? What type of building is this for, etc. ?

What type of DHW tank do you have - an indirect I presume - existing or are you planning on putting something new?  I presume solar buffer tanks upstream of the indirect or in series or?

CMadatMe

What is the Heat Loss?

Besides the heat loss what are the emmitters, baseboard, rads, radiant, hydro? What is the DHW demand?