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Infiltration HTM Options for Old House

user-7827017 | Posted in General Questions on

I know this has been covered in past posts, but I guess I would like smart peoples’ take on a best next step.


I am considering new mechanicals in my large old (late-19th-c.) house and thought it would be smart to do a Manual J. I have done a fair bit of insulating and air sealing and am hopefully in a different place than when the previous owners installed their Big Ass Furnace.


Anyway, my question: after checking and double-checking, that infiltration HTM is clearly doing a LOT of work. And it’s based on scenarios that probably don’t apply to me. I’d like a more accurate number. From past posts, it sounds like my options are:


-Get a blower door test and adjust by another factor, which itself sounds fuzzy. (And given that I am nowhere near passive-house tightness, the error will matter more.)

-Do a workaround, such as an existing energy usage calculator somebody linked to at Borst Engineering


Is there a third option I’m missing? How would folks recommend proceeding? 


(Sidebar: at the risk of opening this question up too much, do the downsides of oversizing decrease with variable-drive mechanicals?)


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  1. orange_cat | | #1

    Re your sidebar: if you are in cooling dominated climate oversizing is not great. If you are in heating dominating climate, oversizing - it is argued - is not such a problem. See this:

    and a lot more here:

    (But larger heat pumps are more expensive and - may or may not be important - louder. A lot louder).

  2. user-7827017 | | #2

    Thanks for the references, I'll take a look. I deliberately left a ton of details out of the original post to keep the question general, but in case it's relevant: I'm definitely in a heating-dominated climate. Zone 4A

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