My boiler is too small?


  #1  
Old 12-22-08, 08:21 AM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
My boiler is too small?

I put in a new hot water Biasi boiler about 3 years ago. I think it is too small. We have a small cape - about 1300 sq. ft. I have the smallest boiler Biasi makes (B10/3) with a heat capacity of 67. Here is what happens. Only on a very cold day (today it is 16 degrees) - when upstairs and downstairs zones are calling for heat, the boiler can not move past 165 which is about at the low limit. I have the high limit set at 185 (30 diff). If only the downstairs is calling for heat, then the boiler can make it to high limit and shut off. With the downstairs theremostat set at 65, it can't make it there on these cold days - only gets to about 62-63.
This of course is when the indirect hot water zone is NOT calling for heat.

This is not really a problem because we like the house cooler and it does not get this cold for too long (usually), but I wonder if it is hard on the boiler.
There are a couple of things I can think of doing:
1. I could insulate the pipes in the basement, at least where they are close to the outside walls.
2. One fin tube (8' long) goes into our attached garage/converted studio (on the 1st floor loop), which is 50 degrees. This room is still not finished, and the ceiling is uninsulated. I have an auxillary electric baseboard out there set at 50, so when it is cold it comes on. If I were finish insulating the room it should help.
3. I could set the high limit higher - perhaps 190 or 195 degrees. Not sure if making the differential lower would help.

Would any or all of these things help much, or am I just plain undersized? Is it hard on my boiler?

Thanks!
 
  #2  
Old 12-22-08, 11:58 AM
G
Member
Join Date: Oct 2008
Location: USA
Posts: 3,060
Received 23 Likes on 23 Posts
Jacking up the high limit won't help - if the boiler won't make it to 185 deg, then it won't make it to 190 or 195.

Your other two ideas may help some.

Your situation is not "hard on the boiler."
Doug
 
  #3  
Old 12-22-08, 12:43 PM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
Well, it does make to high limit when only one zone is running, so wouldn't the higher temp help to warm that zone better/faster during that cycle?
Also, I noticed that the temp seems to maintain whenever the 2nd zone kicks in. In other words, zone 1 was just running and the temp was about 180. When zone 2 kicked in, it stayed there, so I am assuming that if the temp. was at 190 when another zone kicked in it might also stay there.
Any thoughts on this?
 
  #4  
Old 12-22-08, 04:56 PM
Join Date: Aug 2006
Location: usa
Posts: 902
Likes: 0
Received 0 Likes on 0 Posts
You have it entirely backwards. The HARDER the boiler works, the MORE efficient it is. A boiler is sized to the heatload for the coldest design day of the year. The rest of the year it is oversized. Having an oversized boiler means short cycling and excessive wear on the burner. Running continuously is nirvana. If your home is slightly cool a few days a year, live with it. You're saving serious money.

If your boiler is reaching 165 degrees it probably means the boiler is also not in danger of condensing, since the return temp should stay above 110 degrees. Your return is probably about 20 or 30 degrees less, so it should be well within range.

I'd suggest doing a heatloss calculation for your home. A B3 should be more than adequate for 1300 square feet. My home is 2,000 square feet and my Bias B4 is unfortunately way oversized for my heat loss. Even a small B3 Biasi would be slightly oversized according to my boiler run times over the past several days. The best money you could spend would improving the home's envelope: better windows, more insulation, etc. So long as that boiler is big enough to meet you DHW needs, don't worry about the heating side!!!!

I'd also point out many of us have added expensive outdoor reset controllers, just to keep our supply temps as low possible to meet the homes heating needs. Wanting to run the boiler at 180 degrees when 165 degrees will heat the home is dollar foolish. BTW, my system runs with full ODR, and my heating system heats with 110 degree water when its 19 degrees outside.
Pete
 

Last edited by NJT; 12-22-08 at 05:39 PM. Reason: fixed confusing typo ... weather --> water
  #5  
Old 12-22-08, 05:10 PM
plumbingods's Avatar
Member
Join Date: Apr 2008
Location: Manch-vegas, New Hampshire
Posts: 2,180
Likes: 0
Received 0 Likes on 0 Posts
How about increasing the nozzle size. Many boilers today are coming with multiple firing rates and when the boiler was installed, it could have been installed with the lowest firing rate.
 
  #6  
Old 12-22-08, 05:15 PM
X
Member
Join Date: Nov 2005
Location: USA
Posts: 2,455
Likes: 0
Received 0 Likes on 0 Posts
67,000 BTU/hr output divided by 1300 sf = 52 BTU/hr/sf. That is HUGELY lossy. Improving the building envelope should be priority 1. Through 20.

As other have suggested, two (plus one more) things to try:

1) clean the fin-tube elements. Pull the covers off, and get in there with a stiff bristle brush, paintbrush, or whatever.

2) do a heat loss calculation. It will help you see which rooms are the most 'lossy' and in need of improvement.

3) insulate and air seal 'til you can do no more. The better the building envelope, the less energy it takes to heat it. A good DIY starter guide for this stuff is at

http://www.energystar.gov/ia/partner...e_May_2008.pdf

And starting in 2009, the tax credits for weatherization resume.

federal energy efficiency tax credits
 
  #7  
Old 12-22-08, 05:56 PM
rbeck's Avatar
Member
Join Date: Jul 2007
Location: United States
Posts: 2,643
Likes: 0
Received 42 Likes on 35 Posts
What type of radiation do you have?
 
  #8  
Old 12-22-08, 06:06 PM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
Thanks for the great replies!
Radioconnection: you have made me feel so much better about the boiler size. I was not thinking of it that way, but as you say it, it makes great sense.
In general - yes - I can and will do much more with envelope. I have changed many doors and windows, but I do feel drafts - especially at switch boxes. I need to finish the attached garage. I have some old single pane windows downstairs (which I don't want to replace), and my cellar windows are terrible which I would love to replace. There is cold air blowing through the cracks on those cellar windows.

plumbingods asked about increasing the nozzle size. I have a Riello burner 40/F3 and am using a .5 - 80 degree nozzle as I read off the manual. Can you tell me if I can use a bigger one? What about the limit settings??

xiphias - I have all new fin tube and so they are pretty clean.
I will look at the links you provided - thanks for that.

PS - I have a Riello 40/F4 that I got originally with the boiler - and later found it to be the wrong size (too big) for my boiler. Do you all agree with this? I still have it.
 
  #9  
Old 12-22-08, 06:30 PM
Join Date: Aug 2006
Location: usa
Posts: 902
Likes: 0
Received 0 Likes on 0 Posts
If you are not going above 165 degrees, resetting the hi limit isn't going to make the boiler produce more BTUs! I suspect you are getting closer to the hi limit on DHW calls, yes? My boiler is reaching 175 degrees, and it 17 degrees F here, and it is short cycling (running for too short time periods.)

How cool is the house getting?
 
  #10  
Old 12-22-08, 06:39 PM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
It's 26 degrees right now. Downstairs it is at 65 and upstairs it is at 62, but when I crank up the thermo upstairs, I can usually get it to 70.

I turned the high limit up today to 195, and it seemed to make a difference. When the downstairs zone runs alone, the boiler reaches 195. It is the downstairs that I can't seem to get above 64-65 on the very cold days.
 
  #11  
Old 12-22-08, 06:51 PM
Join Date: Aug 2006
Location: usa
Posts: 902
Likes: 0
Received 0 Likes on 0 Posts
We keep the house at 65 degrees, and it feels warm, but having outdoor reset and constant circulation make a big difference in comfort factor. We invested in new windows and insulation in the walls and attic. Getting rid of heat losses and drafts will improve your comfort. The downstairs is also probably losing heat since the heat will rise to the higher floors, making them warmer and cooling the first floor.
 
  #12  
Old 12-22-08, 07:02 PM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
Were comfortable too. It just makes me worry when I wake up, come downstairs, and see that it is only 61 degrees even though the thermo is set at 64.
 
  #13  
Old 12-22-08, 07:13 PM
Join Date: Aug 2006
Location: usa
Posts: 902
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by tomfmal View Post
Were comfortable too. It just makes me worry when I wake up, come downstairs, and see that it is only 61 degrees even though the thermo is set at 64.
Does the downstairs temperatures ever reach the thermometer setting, say when it is 40 degrees outside?
 
  #14  
Old 12-22-08, 07:20 PM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
Yes, I can easily get the downstairs higher if the outside temp is 40, or a bit less.

Is 195 too high to set the high limit? Any danger of the pipes being too hot in contact with wood (like where it goes through the floors and runs along the joists)?
 

Last edited by tomfmal; 12-22-08 at 07:59 PM.
  #15  
Old 12-23-08, 12:43 AM
plumbingods's Avatar
Member
Join Date: Apr 2008
Location: Manch-vegas, New Hampshire
Posts: 2,180
Likes: 0
Received 0 Likes on 0 Posts
The rule of thumb used to be...

high limit 200*
low limit 180*
Differential of 15-20*

I do not know if this rule of thumb has changed, but you want to circulate 180* water in the zoning pipes or your heating system may not be able to keep up.

Another question is could the thermostat be junk?
 
  #16  
Old 12-24-08, 01:46 PM
T
Member
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 249
Likes: 0
Received 0 Likes on 0 Posts
180 to 200 - seems like that would help me alot. Are there any safety issues with having it that high?
Why are you thinking that the thermostat is junk? It is just a simple mechanical mercury type.
 
  #17  
Old 12-24-08, 02:18 PM
plumbingods's Avatar
Member
Join Date: Apr 2008
Location: Manch-vegas, New Hampshire
Posts: 2,180
Likes: 0
Received 0 Likes on 0 Posts
180-200 is standard where I live with typical oil or gas boilers.
When you get into radiant floor heating and mod-cons, the temps may change a bit.

Thermostats do go bad or there would not be such a huge industry for them. I am thinking yours may be out of calibration, and if it is a Honeywell T-87 round, you will need a special wrench to reset it.

Do you have a GOOD thermometer you can put next to the thermostat to check the accuracy of it?
 
 

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