How much effic. is lost due to short cycling

Brain Ross

I recently visited a new heating job done by another contractor because the customers gas bills were high. The job was completed several years ago.

They installed a Weil-Mclean CGi-6, two - hydronic coils when performing as per manufactures lit.(6gpm @ 75,000 btu) with a Smart 40 gal. indirect. Forget that the boiler piping is completely wrong and needs to be redone but the real trouble lies with the size of the heating equip.



After doing a heat loss calc. I found that on a design day each zone only needs 30,000 btu and if both zones are calling the total load is only 60,000 btu and the indirect is prioritized too. So the boiler is roughly 90,000 btu too large.

A buffer tank woud be nice but I would need at least an 80 gal. one. I guess the question 'm leading to is how much short cycling is bad, I try to keep my min. run times to 4 and hour. It seems like replacing the boiler and with a mod./con. would be nice too. But the pay back has got to be there, so if it's not should you just let it cycle away?

Let me know what you guys think, thanks.

CMadatMe

Gotta Love Hydro-Air

So your roughly 35% oversized. Let's start with that savings alone..Is the flowrate of 6gpm giving 75k btus both a/h's or each? How about running a 30 degree delta t and look at the ratings of that a/h at a lower flow rate.  A mod/con would be a good choice for just the fact that no stand by or stack loss as you are seeing here. What you make you use. What will those a/h's put out at 165 degree water at design? This would be your average water temp using a 180 supply temp and a 150 return temp.

Chris S

If you trust Intellidyne...

I installed a Crown Atmospheric cast Iron boiler for my Dad 4 years ago.  Boiler is sized to his load, and emitters.   But....  he burns wood in 2 places in the house making the boiler oversized for when he's burning.  The HW+  which has been in place for just over a year says 30.1%  average savings.  I'd say you could expect that. 

Chris

NRT_Rob

good question

and it's a tough one to answer.



there is a very good white paper out there on the kelley-patterson website claiming 15 to 50% losses are *typical* or *common* for short cycling. He is primarily talking about large, commercial boilers however.



I have been, so far, unable to find any source of info that shows more than a few percent loss on cycling issues though for modern mod/con boilers. For a cast iron one like you're looking at, it would be higher, but it's very hard to say how much higher.



if anyone has any good resources on this though I would love to see them. I have been using buffer tanks but if they are only to reduce wear and tear on boilers I'll probably use them less often in the future.

Brain Ross

reply to short cycling

Exactly, there are few resources out there that discuss the effects (fuel effic.) of short cycling. At some point the boiler is just too big and w buffer tank is really only a band aid. Good design and a buffer will allow for seasonal changes and reduce some short cycling.

But again, my load calc's on a 15 degree day call for an entire load of 60,000 btu and the boiler is 157,000 in and 140,000 out. I can only work with a 30 degree delta T too. Cast iron, power vent, Weil-Mclain CGi-6.

The sytem is 4 years old and the customer will be moving in 9 years. So, its gotta be cost effective too. The solution can not cost more that the savings.

What do think?

CMadatMe

Best Savings

Is to change out the boiler. If this is nat gas are there any local utility rebates offerred? A mod/con would be subject to the 1500 dollar tax credit. Here the local utility gives an additional 1,000 dollar rebate back to the consumer. Those 2 alone are an a immediate payback. Plus the roughly 35% cut in size of the boiler. Safe to say payback could be as little as 2 yrs.