Church heating: minisplits vs. gas-fired infrared units?
rhodebump
| Posted in Mechanicals on
Hi,
I am doing a low-budget rehab on a old church near Pittsburgh, PA
The main sanctuary is 3280 SF with 20 Foot ceilings. Previous heating was a steam system, but the boiler is unrepairable.
Max occupancy will be 100 people (since i have no fire sprinklers). I am considering putting 4 mini splits (2 on each wall) that will provide heating and cooling, but I am also thinking about a few of these gas fired infrared heaters for backup.
Putting in a boiler will be at least 20K (it’s the labor to pipe them in), so that’s off the table.
Thanks for your thoughts!
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Replies
Rhode,
There are two issues here:
1. It's impossible to design a heating and cooling system unless you know the design heating load and design cooling load. For more information on this issue, see these two articles:
Saving Energy With Manual J and Manual D
Who Can Perform My Load Calculations?
2. I'm assuming that this building will continue to be used as a church. If so, the code requirements are probably different than they would be for a residence -- commercial codes may apply. That may require the help of an architect.
If code and design requirements allow, I would vote for the all-electric approach. It will inherently better for the environment than gas-burning appliances. Plus, as the grid adds renewable sources of production, the electric powder system will become greener and greener (so to speak). With the gas burner, you are stuck with a particular level of efficiency throughout its service life.
I appreciate the answers. Early on the project, I did pay an architect and HVAC engineering firm to make a recommendation. Unfortunately, the recommendations put the project to almost 2 million dollars, and this church is in a economically distressed area, so no bank will touch this project. At this point, I can only afford to put in something so the building starts to get utilized.
Here is PDF from HVAC consulting.
-sorry, editing this comment. I haven't been able to attach the pdf or write the link here
Max occupancy is 100 people, but what is the minimum occupancy and occupancy duty cycle? Efficiency doesn't matter a whole lot if it's only running 5-10 hours per week, whereas it matters quite a bit if it's running 80+ hours/week.
Right-sized mini-splits shine at keeping a place at a constant temperature 24/7, but really SUCK at recovering from a 20F setback (or completely "off") quickly in the dead of winter to get the place up to temp for services 1-2 times per week. Oversizing natural gas burners by 5-10x for the peak load just to allow a setback to 45F when unoccupied, yet still be able to ramp up to temp in a couple of hours is a common solution for low occupancy duty cycle solutions. But you can't do that with mini-splits. Keeping a church office or meeting room well conditioned 24/7 with a mini-split while the rest of the place is operating in deep setback mode can be useful.
It's impossible to find the optimal solution (or really ANY solution other than repairing the steam system that worked previously) without some sort of heat load calculation. Run an IBR type heat load on the place per Martin's articles. This is going to take some time, and you're going to have to measure or estimate some dimensions, but you can get there using computer spreadsheet tools. If you need help figuring out what U-factors to use, check back with a detailed description of the wall/window/ceiling or roof assembly you're unsure of. Use +7F as (Pittsburgh's 99th percentile temperature bin) as the outside design temperature, 68F as the interior temperature, a 61F temperature difference.