what size boiler would you use? Burnham MPO

bob eck

contractor has a job old farm house that has a heat loss of 96,000 BTU. Will have three heating zones only.

Has a outside design temp of 9*F to 70*F inside. How often does it get to design outside temp in a year?

Wants to install a Burnham MPO115 with a 85,000 BTU net rating will be three heating zones using Grundfos UPS1558FC circulators pumping away from the boiler and expansion tank. Will be using a Taco 4 zone circulator relay that has one priority zone.

I asked him if he is going to use the MPO115 boiler ask home owner what zone are they in most of the time when inside during the heating season and make that the priority zone.

Question would you use the MPO 115 at 85,000 Net BTU or go with the MPO 147 net BTU 112,000 just to be safe.

I know Burnham has the new MPO IQ boiler with outdoor reset as a option. I would use the MPO IQ 147 with outdoor reset control to be safe.

Thoughts. 

Brad White

Confidence Level

Good questions to ask.

Firstly, what is the location? You ask how often it gets below 9 degrees and that is location-specific. Also, I do not know what "bin" that 9 degrees was taken, 97.5%, 99% or 99.6%. (See the thread "About Design Day Temperatures" nearby.)



The second is the one area of greatest discussion, error or guessing: Infiltration. This could be half of your heat loss if the place is leaky, or one quarter of a very large number if the place is both leaky and not insulated well.



In other words, your presumed  96 MBH may have an infiltration loss of 40 MBH (pretty awful!) or 24 MBH (still not so great).  But it may mean that if your infiltration is under-stated you could be in trouble and if over-stated, you may really be able to use the smaller size.



 If the place has fireplace chimneys, that is a huge factor. If unused, even worse.



Building mass- if frame, not so much, but if stone, that helps absorb swings. A few factors to ask about.



A blower door test and good interpretation of the results is one place to start. Or you can try a number of infiltration methods and triangulate them.



The crack method ("Preferred by Plumbers!" (tm), the CFM per SF of surface method, the air change method by number of exposed walls method and oh a few others. All have their merits.



So it all gets down to confidence level in your calculations. This is not specific to the MPO, just the calculation method.  If too close to call, can you use the larger MPO and down-fire it as an option?

heatmiser

larger is better?????

i would want to use the larger boiler and slightly down-fire if needed.at least you would have the extra btu capacity if needed.what if they decided to add an indirect tank etc. in the future????

Ron Jr._3

I'd go with the 115

If heatloss calcs are basically the same ( I used Slant Fin for my home ) , I think the numbers are very liberal . My own boiler is almost exactly matched to what my house needs on design day ( maybe 10 percent more than needed ) . Measured my oil use in a 24 hour period , with temps lower than my design day numbers that whole time . It ran just 7 hours ( should run 24 hours continuous if it's matched to the load ) . I'm not sure of Burnham's policy , but generally a boiler can be upfired a little if needs be .



The MPO IQ is a good choice . Here's a 5 section we installed last week . No outdoor reset though . Customer was not convinced of the benefits . The great thing is the option is there and very easy to upgrade .

Jason_13

Do not upfire

You use the DOE output which is 101,000 so that is close also being zoned it builds in an automatic bumper.

You cannot upfire boilers beyond the manufcaturers input.

icesailor

Upfire/Downfire:

It's better to buy the bigger boiler and down fire it than to get the smaller boiler and find you need more. You can't upfire boilers. I should clarify that. You can do anything you want. It may not be recommended or allowed but who is looking?