Extremely Low-Load House

Unknown

that doesn't make any sense at all. If you are firing on a differential, then how does the system get around the physical definition of a BTU?

If you are in fact firing on any differential, then it's as simple as volume of water x density x delta T rise. Add volume, you add mass/buffer capacity when cycling on the differential, and it dissapears when you are "cruising" in modulating mode.

I suspect your viessmann is firing on a different methodology than on differential.. or are you actually saying that your system temp swings 40 degrees under those loads? If so, how can you say the "buffer dissapears"?

Mike T., Swampeast MO

The high (40F+) delta-t appears only in the boiler even though I'm absolutely positive that differential pressure bypass is closed and there is significant flow in the system. The temperature in the huge supply mains stays effectively constant. This only happens when the load on the system is less than half of the "buffer" capacity. (Again, you have to grant me that huge mains are essentially the same as a series-connected buffer in a constantly circulating, fully TRVd system.)

Back to the system in question:

The LOWEST mod-con ouput I can find is from the W-M Ultra, but such is STILL significantly greater than the maintenance load I expect to see at design (-20F) conditions. I've stressed to the homowner that his TRV settings should be mainly "set and forget". NO use of daily setback with only unoccupied rooms set back as low as they find practical.

Unknown

You are not talking about the same Differential as I am.

Most boilers, when cycling, fire on a differential. That is, turn on at X and turn off at X + differential. Your viessmann is a very different animal that can vary flow rates through the boiler, so the 40 degree rise through the boiler could very well happen at a very low flow rate... or a high one.. a typical "differential firing mode" doesn't exist. In fact I bet the viesmann, through flow modulation as well as pulse firing, can maintain a relatively constant outgoing radiant supply water temperature regardless of how the burner is cycling, using the mass of its own heat exchanger to cruise between pulses. right? pretty smart boilers after all. it's just doing its own version of variable speed injection to the secondary circuit, and integrating its burner pulse with its extraction speed...

But for every other boiler in north america (as far as I know), unless you are in modulation mode, the buffer tank will NOT be invisible when the boiler is cycling if you are piped like a LLH through the buffer. Because the boiler will wait to X to turn on, and won't turn off until it hits X + differential or loses the heat demand. Your buffer tank likewise traverses this temperature swing, which is where the buffer capacity is realized.

Believe me.. the boilers have ODR but they aren't necessarily that smart. Set a minimum water temp low enough to never trigger that differential firing under low loads and you'll see what I mean! Not that we've ever done that, cough cough... certainly I've never seen a boiler waiting patiently for the house to fall to 60 to start firing! ahem, moving on...

NOW, your fuel usage numbers may not be indicating that a mod/con is correct on THIS system, if your initial numbers were not "real", as you are indicating here. That's fine: just understand the mod/con will hold its efficiency (that is, don't assume it will be less efficient) in your estimation, with a buffer tank, which will not be invisible, perhaps unless you use a viessmann.

Otherwise, buffer tanks would be completely useless with modcons, since when you actually need one (i.e. get significantly below min mod) it would stop being effective??? Silly!

jp_2

40F differentail?

only appears in the boiler???????????

where does this energy disappear to after the boiler?

hot_rod

Thanks PS

did you find the table online?

hot rod

maine ken_3

Mike, My camp is almost about 40mins north of Washburn. We routinely see -30*. I have been out in -30* at camp and five miles away it was only -20* and vice/ versa. This region enjoys extremely localized weather. Even tight envelopes don't appear to be too tight in the winter winds in this region. Good luck with whatever you recommend and end up utilyzing. Computer programs and design experience sometimes doesn't equal "being there".

scott markle_2

vito differential

Mike, your mean supply return delt not firing differential correct?

Unknown

I think he's primary/secondary and the viessmann varies flow through the boiler.

So outgoing water is hotter than SWT in the secondary circuit, and "blends in".. just like variable speed injection.

PS_2

Hard Copy

No - I have hardcopies of Engineering Weather data from the National Climatic Data Center at work (i'm an energy engineer). ASHRAE makes you buy their data, unfortunately.