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Will I regret this over rated size boiler choice?

04090
04090 Member Posts: 142
Upgrading an old hot water boiler and switching from oil to to gas.  Decided to go with a basic energy efficient model as we don't use a lot of fuel (445 gallons last year) as the low fuel use makes it hard to justify the higher costs of a mod/con .



We might be adding on to the house in a few years, and the contractor suggested a manifold with room to accomodate.  Which is fine.  But he also suggested an oversized Burnham SCG5 system which will accomodate the addition in the future.  All specs indicate a Burnham SCG4 is the perfect size for what we have now.



But we're 50/50 on the addition, and if it's not done will we have regrets with the  SCG5?



The SCG4 is 105,000BTU, the SCG5 is 140,000BTU

Comments

  • Ironman
    Ironman Member Posts: 6,555
    Heat Loss Calculation?

    Has an accurate heat loss calculation been done on your house? Your present boiler may very well have been over-sized. I've hardly ever seen an oil boiler that wasn't over-sized. If all your doing is replacing it with the same size gas boiler, it will just as likely be over-sized. Do a heat heat loss on the present structure and if you have any drawings or design of the proposed addition, do one on that also and then you'll know the size needed.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • 04090
    04090 Member Posts: 142
    It's correct

    The SCG4 is confirmed the correct size for the house.  The SCG5 will accomodate present need and a possible addition which we might build a few years down the road.



    My question is if we proceed with the SCG5, and do not proceed with the addition, how bad will life be with the oversized boiler?
  • 04090
    04090 Member Posts: 142
    It's correct

    The SCG4 is confirmed the correct size for the house.  The SCG5 will accomodate present need and a possible addition which we might build a few years down the road.



    My question is if we proceed with the SCG5, and do not proceed with the addition, how bad will life be with the oversized boiler?
  • Ironman
    Ironman Member Posts: 6,555
    To Answer Your Question...But

    Your life won't be that bad, but the life and efficiency of the boiler will be reduced. How much? Only God knows for sure, but short cycling always equals a shorter life.



    You could add a buffer tank that is sized to meet the difference between the smallest load and the minimum firing rate of the boiler. I've attached a recent thread on this. Or, you could pipe the system to allow for a second boiler if the addition is built. Both of the these options will obliviously add more $$ to the cost than up-sizing the boiler, but would be better in the long run.



    Now for the But... You stated that the 105k btu boiler is the correct size and that you only used 445 gallons of oil last year. I don't doubt your truthfulness or sincerity,  But those figures don't add up.  Are you taking you contractor's word that it is the correct size? The only way to know for sure is with an accurate heat loss calculation by an approved method (such as ACCA manual J). Most contractors don't want to do these or won't learn how. Ask your contractor for the calculation sheets. If he can't produce them, I would recommend you consider one who can.



    Again, I'm not doubting your truthfulness, but I'm trying to help you make the correct choice based on facts, not theories or "guesstimates".



    If you'll give your location (city, state) and the square footage of your house, I can back calculate a close estimate of your heat loss. This is not a substitute for a load calc., but a good means of checking and confirming one. With the 445 gallons used last year, you're coming up about half of  what your boiler is sized for (based on an eastern regional average in boiler territory). If you're using a supplemental heat source, then these numbers would be off also. :)





    http://www.heatinghelp.com/forum-thread/137300/Hydronic-system-buffer-tank-effect-on-modulating-boiler#p1237417
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • 04090
    04090 Member Posts: 142
    I sent you a PM with details

    I sent you a PM with lots of detail. 

    Thanks.
  • lchmb
    lchmb Member Posts: 2,997
    how about

    Using the correct size boiler now and if you go with the addition, put in a space heater, Rinnai or even a gas log (not the most efficient) to gain your extra BTU..
  • 04090
    04090 Member Posts: 142
    edited September 2011
    response

    We talked about supplemental heating systems, and the wife is on track for a having one heating system for the place. 



    Am concerned that the SCG5 will be short cycling until the addition is done, and that might be years down the road, if ever.  Is there a way to throttle it down to the same output as the SCG4 or is it even significant?
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