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previous post on boiler sizing

Jake2010 Member Posts: 24
I had previously posted regarding this issue on the main wall, I just realized that there was a oil heating section, so I am going to re-post, with some additional questions.  Sorry if this may be repetitive for some.

I am planning to replace my 47 year old boiler.  My house is 2,000sf, heat loss is supposedly 50,000btu, 118 feet of baseboard in 2 zones (colonial), all new Anderson windows, 40 gallon Weill McLean indirect, 2.5 baths, 2 adults, 3 young children.  In a few years when my children are older the primary HW draw will be in the am with maybe 5 showers within 1.5 hours (I'm estimating). 

A few contractors have recommend MPO147, Buderus g114ws/4 section, Buderus G125BE/28 (he is the only one that did a heat loss), and one MPO115.  I have been trying to learn allot about boiler sizing as I am primarily changing my boiler to save $$.  These seem that they are a little big for my needs. It is hard going against there recommendations and feeling confident, but something doesn't seem right.  I have been eying the Buderus g115ws/3 or maybe the MPO84. I would use HW priority and OD reset, with mixing valve to increase tank capacity. My questions are;

What size and what brand boiler would you folks recommend?

Do my thoughts/plans sound OK. Am I likely to be OK with HW needs.

If I used the Buderus and did not necessarily plan on using indoor room sensor should I still use the Logomatic or should I use a different controller.

I have heard differing opinions on lining chimney, mine is 8x13 clay lined, 2.5 floors, outside. Should I plan on installing a liner?

If went with the larger boiler, is there a way of knowing how much more inefficient it may be?

Thanks for any help provided.



  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    I think I responded on this before.....

    First.  I would definately line the chimney, or go direct vent.  These smaller boilers will thank you.

    I like all your choices.  I think the MPO 84, 40 gallon existing indirect, DWH priority will work fine.  The 147 is way too big.  I recently remodeled, 2400 (i think) sq feet, 2 baseboard zones, 4 radiant zones, dhw (TT SMART 60), and I have the MPO 115, tekmar controls, od reset, etc.  I think I'm a little oversized.  Your biggest concern is the DHW, and with the low water content of the boiler, it will recover fine.  Most times the boiler doesnt fire for DHW until we're done the first shower.  If you go Buderus, you really should use the Logimatic Control.

    The larger boiler will be a waste of money when you buy it, and a waste of money when you run it. It will short cycle 98% of the season. Bigger is not better.

    There was an error rendering this rich post.

  • meplumber
    meplumber Member Posts: 678
    A couple of questions.

    While you gave us some information, I didn't catch your location. In my area, with a -10 design temp, 2000 sq ft needs the G115/28 or /4 (same boiler) at a minimum.

    The guy that did the heat loss chose that boiler for a reason. If your heat loss is truly 50K, then the 115/3 or 125BE/21 would be the better choice.

    I concur that Logamatic is the only way to go for savings. Line the chimney. No question about it.
  • Jake2010
    Jake2010 Member Posts: 24
    My location is north central CT

    Going by Weil Mclean replacement guide the outdoor design temp is 7. Ironically it is almost 60 deg outside today. The contractor who recommended the g125be/28 had done the heat loss with Slant Fin program. He came out with 50,000btu.  He had initially said a three section would be fine, but when came in with quote he said 4 section.  I believe his reasoning was the HW demand. I messed around with the Weil Mclean Guide and came up with about 45,000btu.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    the Weil Mclean Guide and came up with about 45,000btu.

    I used the W-M guide to estimate the size boiler I was going to need to replace the oil burner that had a 1/2 gallon per hour nozzle in it. Since that boiler always had enough heat, I knew 70,000 BTU/hour would be enough. The W-M guide had a lot of problems for me. It has a lot of tables to compute parts of the heat loss, but none of them matched what I have in my house,  And I could not figure out if I should use their figure for the heat loss from my radiant slab or not. Now for me, it turned out not to matter because in either case, it came up with a heat loss much lower than the smallest W-M Ultra 3 boiler that they make, so I had the smallest one put in. I live in New Jersey where that same guide says the design temperature is 14F, and that seems to be correct. It came up with 36,250 BTU/hour without the slab and 41,450 BTU/ hour with the slab.

    The Slant/Fin program came out with 29,234 BTU/hour. I had to guess some things about that too, but it seems to me to be the more reliable of the two. My house is an 1150 square feet Cape Cod on a radiant slab.

    My hot water heater is a W-M "40 gallon" indirect (actually Triangle Tube, I believe). It is the highest priority load on the boiler and runs about 10 minutes to recover, and does that two or three times a day. So I made no allowance for it in sizing the boiler.
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