Oversize ModCon Boiler to operate at lo-fire (more efficient?)

Mike T., Swampeast MO

That is a nice little presentation. Considering it comes from the DOE, perhaps they'll finally do something about AFUE ratings that seem to penalize modulating/condensing boilers for their very efficiency. (Or is that reward other furnaces & boilers for their inefficiencies?)

D107

Pro told me prefers to oversize munchkin/buderus

This goes against what I have read on the Wall. I believe steamhead always says to get the boiler closest to heat requirements. (Though I understand operating at lo-fire might be more efficient that hi-fire.) Comments?

Same pro also told me he adds btu load of indirect HWH into boiler sizing; also seems against Wall advice. With indirect you start out with a tank full of hot water; setting for domestic priority would seem to merely delay a heat cycle a bit or lower the water temp of supply.

Thanks,

David

S Ebels

Pro?

Did he actually do a heat loss? That helps to sort the Pro's from the wannabe's.

I anticipate a chorus of boo's from the rest of the Wall directed toward his advice to you. I have never added the DHW into the heating load unless it's a commercial application. (IE: DHW load is nearly constant.)

Get that thing sized for your heating load and prioritize your DHW. Simple as that.

jerry scharf_3

therre was a thread about this recently

David,

The concensus was that it mugt be a slight theoretical possibility, but almost any other factor would swamp it. The arguement is that with a higher output unit, the HX is larger and the exchange is better. With a condensing unit, the final efficeincy is set by the secondary (condensing) side of the hx. Since this is so sensative to the return water temperature, the general feeling was that the advantage was undocumetned and at best very low.

Competing against that was the fact that the shoulder season performace would be worse. With the ability to throttle lower on a smaller unit, there are fewer on/off cycles. Though these start/stop cycles are better for a mod/con unit than an atmosperic boiler, the costs are still there.

No vendor has ever come out and said this, and you would think it would be to their advantage to sell larger units for more money. Some people have tried to interpret some Viessmann graphs in various ways, but without the real data and collection methodology behind it I question the accuracy of this.

So given there is no agreed support for this, and the manufacturers universally say to size to heat loss, I think the rational position is to listen to the vendors.

jerry

D107

Feedback sounds true for both atmospheric and CON

boilers. Also confirms what I think Mike T or Steamhead wrote on another thread that MODCON boilers 'like' to be pushed to their limits. Thanks guys.

No the contractor didn't do a heat loss--I'd expect to pay for that. I'm in the middle of measuring rooms myself and at some point will give the raw data to a PRO. Ideally I guess you seal and insulate your house first, so you don't end up oversizing.

This contractor is a real nice guy, experienced, and spent alot of time answering questions but here's what I like about the people on the Wall: most of you never stop questioning, investigating, checking with other PROS, expanding the knowledge base. For example this contractor had at least one bad experience with pumping away and it led him to believe that only under certain ideal conditions would that work, rather than thinking he could have made a minor mistake that threw the whole thing off.

David

Mike T., Swampeast MO

A very well-placed source did tell me that for otherwise identical conditions there is a known and very slight decline in efficiency with increased modulating output. But he was adamant that the boiler should never be oversized to compensate.

The limit that modulating/condensing boilers seem to "like" to be pushed to is finding the lowest possible supply temperature to meet the desired comfort level. The same source insists that most American systems using these boilers are operated at WAY too high of a reset curve.

Cliff Brady

DOE take on Partial Load

See page 14 of this great promtion of condensing boilers.