OK to oversize a modulating boiler?

Old 03-17-10, 05:04 PM
Thread Starter
Join Date: Oct 2005
Location: worker
Posts: 130
Likes: 0
Received 0 Likes on 0 Posts
OK to oversize a modulating boiler?

Sales rep recommended a Weil McLain Ultra 80 (71K output). My design hour is about 38K for space heating, plus some DHW production. Boiler seems bigger than I need. The rep said that because this modulates down to 14K output, there was no reason not to use this.

Everyone seems to emphasize not oversizing - is that still true if the boiler can modulate down this far? Any downside? Am I better off with a smaller boiler, like a Triangle Tube Solo 60?
Old 03-17-10, 07:47 PM
Join Date: Mar 2010
Location: MD
Posts: 113
Likes: 0
Received 0 Likes on 0 Posts
You Dont want to oversize your boiler, You need to know your Max and min Btu rating this is Very important in a modulating burner they call it turn down. The higher turner down the longer the boiler will stay on line without cycling off.
Thats the name of the game Keep the burner on line as long as possible with as high efficiency rating as possible.
So lets say your design is 40,000 btus let say on best you hit design conditions 30 days a year on those 30 days how many hours of those days. Every thing else is partial capacity or less then 40,000 btu load demand. this looks like a better selection for a boiler Peerless® PUREFIRE®.
I have worked on both boilers and they are both reputable companies with good products, just that one doesnt fit your load requirements as efficiently as the other.
PF-50 look at that its max input 50,000 net (46,000) min 16,000
Looks about perfect to me.
Keep in mind you want to be able to utilize the full modulation range if possible to maximize your turndown
Old 03-17-10, 08:50 PM
rbeck's Avatar
Join Date: Jul 2007
Location: United States
Posts: 2,646
Likes: 0
Received 44 Likes on 36 Posts
Here is a link to look at about oversizing a mod con.
Click the Hot Water Boiler Menu and than click on the link about "what happens when oversizing a mod/con"
Technical Menu
Just another slant on it.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: