1 Large BTU compressor vs. 3 smaller ones

I’m in the process of gut renovating a stand alone brick house. An engineer has designed a HVAC system for me, where the first floor should have a 48,000 BTU compressor (mini split system). The contractor is telling me that I should use instead 3 smaller compressors for the same space (which in total would add up to 60,000 BTU power). Does anyone have any experience with this? In addition to AC, this will be our only heat source, since we are getting rid of the alternative systems, and I don’t want to freeze in the winter! Thank you

sverkat

in Renovation 11 years and 9 months ago

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brucef | 11 years and 9 months ago

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There are multiple conflicting issues mixed up in one question. A loud bell went off in my mind when you slipped in that you hope to utilize these mini-splits for your only heating. My knowledge in these matters is a few years behind the times, but new promises of efficient heating from these systems is to my conservative field experience wishful thinking. I have been told the new uber-expensive Citi-Multis can heat as long as the property is highly insulated. Free standing means cold escapes four ways plus up, (unlike a middle row house). Our experience is that heat pump “heat” is technically effective in the shoulder seasons, but strangely ineffective in the winter. We tell our customers and show them our thermo-couple gauges, saying that we have raised the temperature of the air 15 degrees. Wohoo, but it is unlike hot air off a radiator. Another intermixed issue (see Smokychimp’s reply of last week) regards the dunnage/permitting aspects of NYC DOB. Multiple wall hung or ground mounted condensers is more likely to trundle along under the radar than a 5 ton High SEER condenser. I don’t often agree with Gennady, but he has hit the nail on the head this time. Masterbuilder reflects our experience with ME’s that we can talk to.

steam_man | 11 years and 9 months ago

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OK…I think your post is a little misleading as I doubt your 1st Floor alone requires a 48 kBTU condenser. More than likely your entire house requires that condenser. That being the case your contractor might be making the case for redundancy as in, if the condenser craps out at least you will have service on other floors/areas/zones. Now then, if he or she is sizing the system over 48 kBTU just because they are looking to install 3 condensers and you have faith that the engineer’s design load is correct then your contractor doesn’t know what the hell they are doing. If in fact they are telling you the load calc is incorrect and they believe you need 60 kBTU and they have a good cause for that, then there may be reason to consider it. Given my experience, I’m going with the guess that they are not too sharp on the design. Always make your condenser work harder… If they can’t tell you why or they don’t know that to start, throw the bums out. -Steam_man

masterbuilder | 11 years and 9 months ago

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Frankly, 9 out of 10 architect/engineers will humbly ask my mechanical contractor for field advice. We’ve even been asked to go through floor plans room by room and help ascertain if enough BTUs have been assigned and/or whether different makes or models might be more suitable or cost effective. A collaborative environment in construction is a healthy environment.

ellenlourie | 11 years and 9 months ago

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Most COMPETENT contractors will question and challenge a licensed professional such as an engineer. Engineers today are cut and paste masters of overbuild, worrying more about liability than proper system design. First one outdoor unit with 3 indoor units cost more than 3 separate outdoor units with 3 indoor units. Also if one compressor will stop working whole system will stop. If one of smaller units stop, then you still will have 2 working units.

arch007 | 11 years and 9 months ago

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if ea comp could be used for a separate zone i think that’s better http://expediter-nyc.com

thetinkerswagon | 11 years and 9 months ago

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go with what Pig three says and bear this in mind: most COMPETENT contractors will not question nor challenge a licensed professional such as an engineer unless they are pretty damn certain or feel pretty strong about something. I would also ask another contractor. sometimes you can lay both scenarios out to a third party and the third party will be able to give you the pros and cons of each whereas the individuals who originally state one big/three small might be too stuck in their ways to entertain something different. Keep in mind that now may not be an easy time to ask AC contractors academic questions, but maybe someone will be willing. Steve

pigthree | 11 years and 9 months ago

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talk to other HVAC professionals and say you have two engineer recs. See what they have to say about each opinion. Ask for some references from your engineer for past HVAC systems he/she has engineered. Same with your HVAC contractor. After you have gathered some more information, ask your engineer about the contractors proposed system. What are his/her thoughts?