How much work is the heat load calc

Discussion in 'HVAC Heating & Cooling' started by Soapm, Feb 5, 2014.

  1. Soapm

    Soapm New Member

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    We've brought in a few companies to get estimates on using split systems to heat our sanctuaries and so far none has done any calculations. They look at the two furnaces currently heating the sanctuary and start telling us what products they have that can match that capacity.

    I want to make sure we don't buy more than what's needed since that would be a giant waste and from what a read, less effective.
  2. houptee

    houptee Member

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  3. Dana

    Dana In the trades

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    The SlantFin tool is an I=B=R spreadsheet type, with GENEROUS U-factors on the construction type. In a residential app it will usually overestimate the load by about 25-35%.

    For sanctuary buildings that are often unoccupied with deep temperature setbacks it's usually a good idea to oversize by 50-100% to shorten the temperature recovery ramp to something reasonable during the coldest weather. If it's to be kept at temp or set back to only ~5F below the normal temp you can size it dead-on (or even 10% under) the Slant-Fin calculated heat load at the 99% outside design temp and never be cold.

    If the heat pumps have resistance-electric coils as auxilliary heating and you're using deep setback strategies for more than 12 hours/day, size the heat pump output (before the coils kick on) for the heat load at the mean January temperture, or between the January mean daily low and mean temp. It'll always run the resistance heaters for part of the recovery ramp, but once it's up to temp the heat pump carries the load, as well as nearly 100% of the load at the deep setback temp.
  4. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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    When I bought my Knight Boiler the supplier did the Heat Calc for $75.00 - with engineering documents and a stamp. This fee was deducted from the price of the unit at purchase time.

    No brainer....
  5. Dana

    Dana In the trades

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    In commercial & industrial spaces the heat load calc is just the starting point- usage patterns actually matter, and differ quite a bit from residential applicaitons. Air source heat pumps add even more complexity to the mix (extended temperature output tables, determining the crossover points for kicking on the auxilliary heating strips, etc.) Sizing a heat pump is never a fully straightforward thing the way it is with heating boilers, even for residential applications.

    A $75 heat load calc is often worth less than what's charged, (even in residential applications) since that sum doesn't come close to covering the amount of time it takes to actually do it right in a fully Manual-J compliant manner (or even I=B=R).
  6. Tom Sawyer

    Tom Sawyer In the Trades

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    The easiest thing to do is look at the current fuel usage and K factor which is probably what the two contractors both have done. If I can get an accurate use and K there's no way I'm spending a few hours to do a manual J, especially on a commercial, non-full occupied use structure.
  7. Dana

    Dana In the trades

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    There's a reason why Manual-J only really applies to residential apps, eh? ;-)
  8. jacobsond

    jacobsond DIY Junior Member

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    I would think a heat load calculation is the 1st thing. Any heating or cooling guy worth having would do that before he recommended a unit. Typically the old system would have been over sized because bigger is better right? Well from what I have read that is not the case in the heating and cooling biz. Correctly sized system will save you money and work good. Over sized will cost extra $$$ and may not work good. Undersized and your cold (winter) or hot (summer)insist on a heat load calculation or move on to the next guy. For the money it costs for a new system maybe you can pay for one yourself.
  9. Tom Sawyer

    Tom Sawyer In the Trades

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    Because this structure is a church, 90% of their heating and cooling loads are for one day a week. I would believe that they either have a set back thermostat or someone cranks the heat back during the rest of the week (cooling in the summer) So we're really not all that interested in maximum efficiency although we don't want to grossly oversize the equipment either. What they need is a balance between operating cost and being able to either recover heat fast or cool the place down fast. Sized for daily occupancy both the heating and cooling systems would be sized for the daily, occupied load without deep set backs but that requires running the systems 24 hrs a day at temperature. Here we need reasonable operating costs but we really need rapid recovery. If the system is oversized its really only going to run at max temperature for a few hours a week. I personally have screwed myself in this same scenario. In my zeal to prove that a local church's equipment was grossly oversized, when they needed a new furnace I did the heat loss and naturally I was proved correct so I sold them two Ruud 90++ condensing furnaces to replace the single huge Lennox beast. All things being the same, they set the thermostats back to 50 during the week and lo and behold, in order to get the church up to 70 for Sunday sermon, they had to crank the thermostat back up on Thursday. Not happy customers but.........the lord forgivith and so did they after I added another furnace and A coil to the mechanical room at...... Cost :) No good deed goeth unpunished
    Last edited: Feb 6, 2014
  10. Soapm

    Soapm New Member

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    The guy that came out today took the time to explain a lot of this to me. I appreciated his education which will make a huge difference when I go before the congregation. He said the building is usually empty and comfort isn't as important to an empty building. However, Sunday comes, you got heavy traffic with the doors opening and closing them a room full of 98.6 degree's people. Add to that the fact that the sanctuary is open to the rest of the building which is on a separate boiler so that makes makes exact calcs a moving target.

    Then he suggested we stay with the existing furnaces since heat pumps are really good to 30 degree's outside temp and said the electric that kicks in at 30 degree's is more expensive than running the old furnaces. He suggested we use the splits for summer cooling only and not get the heat pump option. Does that sound about right?

    Then he said a big unit designed for multiple heads with one head will be about $4K and about $1K per additional head. Is that about right? This is no heat, just cooling...

    He's kind of talking himself out of a job because as soon as he left we got a loud squeal from one of the furnaces. The bearings went out on one of the blower cages and set us back $275.00... The temp today was -2 degree's so what else could we do but go with the first quote...??? I don't think he get's it, we want to replace those 1983 furnaces AND get cooling if we can work it into the solution...
    Last edited: Feb 6, 2014
  11. Tom Sawyer

    Tom Sawyer In the Trades

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    From a reliability stand point it might be a good idea to replace the furnaces but again, even though they are 30 years old, they really don't get a whole lot of use so yes, blower bearings were an issue and someone that knows their stuff should go through them and repair or replace worn parts but unless the heat exchangers are cracked those furnaces are not a whole lot worse or different efficiency wise than brand new ones of the same type would be. I do like the idea of leaving them and going with mini-splits for the AC though. Unless the church plans on running them full time though, like the furnace they will need to be oversized to rapidly cool the area's just prior to use.
  12. Dana

    Dana In the trades

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    There are plenty of heat pumps that have significant output and decent efficiency at temps down to +5F (and ductless heat pumps good to -15F). The outdoor temperature at which the resistance heaters kick in is NOT fixed at 30F, it's a design/sizing decision.

    Your description of the heat pumps said "split systems" which refers to ducted split units with air handlers, but when you talk about " ...big unit designed for multiple heads...) it sounds like a reference to ductless mini-splits. Which is it?

    If the latter, look into the just upgraded/released Mitsubishi M-series, with many models good down to -13F and comes in a wide variety of multi-split options, if that's really called for. eg. The MXZ-8B48NA is a 4-ton (nominal cooling) unit that can handle up to 8 heads or cassettes of mixed sizes & types. The PUZ-HA36NHA is a pretty-good 3-tonner, good for 38,000BTU/hr @ +5F, and about 30K @ -13F.

    Rooms that are doored-off from the main sanctuary don't all necessarily need a separate head- if it doesn't have design load of at least 5000BTU/hr it arguably should NOT get it's own. Mini-ducted cassettes can split output between adjacent pairs of rooms. Some rooms may need to be heated with something else if the installation is too awkward or the load is too small. (Radiant cove heaters are a good option for temperature balancing rooms not served by the ductless system.)

    Replacing the antiques with condensing gas furnaces would be cheaper up front and may even be cheaper to operate, but don't provide air conditioning, and may not be worth the expense if the existing ~78% efficiency burners have any life left in them.

    If you install ductless units for cooling it's worth the upcharge for units that both heat & cool, even if the heat output doesn't fully meet the load. During the shoulder seasons when the temps are in the 40s, heating with ductless systems can be even more efficient than with ground source heat pumps, and cheaper than heating with condensing gas. As an optimizing control strategy setting the thermostats for the gas furnaces several degrees below that of the ductless lets the ductless carry the load until until it just plain can't.

    Without any information on the sizing of the existing gas-burners, fuel use against heating degree day, or usage pattern information it's hard to get particular about what could/could-not work. Typical heating + cooling single head high-efficiency ductless units that operate in cold climates runs about $3-3.5K/ton, all-in turnkey price. (The Fujitsu XLTH series and the Mitsubishi "FH" series are good to look at for up to 1.5 ton single-head units.) A 3 ton multi might run $5-6K for the compressor unit, $1-2K per cassette or head (depending on exactly which cassettes/heads are used.)
    Last edited: Feb 7, 2014
  13. Soapm

    Soapm New Member

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    Aurora, CO
    Fujitsu was out today and recommended four of these http://www.fujitsugeneral.com/wallmountedRLF.htm

    The sanctuary is 63 feet by 43 feet with a sloped ceiling that peaks at 20 feet. It holds 225 bodies though it's been a long time since it's been full. He said we'd be comfortable down to about 20 degrees with two of these mounted in the front and two mounted in the rear, all four up high. He did say the heat pumps would work down to 0 degrees so I guess the 20 degrees is when the effectiveness begins to taper off???

    Currently we have two natural gas Lennox G8, the highest capacity they came. I know the guy said over 200K but not sure what it was but he did say it was the biggest for that model. We'll keep these but they'll only be needed on extremely cold days. My zip is 80010.

    Any experience or reviews on this Fujitsu model? Does this sound like a viable solution?
    Last edited: Feb 25, 2014
  14. Dana

    Dana In the trades

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    Your link isn't working for me- how about just the model numbers?

    The output capacity starts falling off below 60F, but it's not entirely linear, and the question becomes how much capacity does it have relative to your heat load at the 99% outside design temperature. Different mini-splits have different output characteristics as the temperatures fall off, but many are still delivering about 2:1 efficiency at full speed at 0F and below. Whether it has the capacity to cover full heat load at 0F is a separate question, which requires some sort of heat loss analysis. Some heat pumps have no specified output rating at 0F, even though they don't stop working, others are specified down to -15F or below.

    Heat load is not a function of the square feet of conditioned space- it's all about the exterior surface areas of different construction or material types, and the temperature difference between the indoors & outdoors. The 99% temperature bin for your zip code is about 0F, so if the system is sized EXACTLY for the full load at for 0F will means you won't lose ground except during the pre-dawn hours of the coldest nights of the decade. Turning on the lights and putting even 30 live bodies in the pews takes quite a bit off the the load that the heat pump has to carry.

    If you want to run a quick & dirty I=B=R type load calc on it we'll need to know the total amount of wall area (with construction type and R-values), window area & type, roof area and type, as well as the total door area & type so we can come up with reasonable U-factors.

    A typical single-pane window with no-storms loses about 1 BTU/ per hour per square-foot per degree difference, so with an interior temp of 70F and an outdoor temp of 0F you have a 70F difference for 70F x U1= 70 BTU/hr per square foot of window. Add clear storm windows and it's half that.

    Solid 2" doors run about 0.5 BTU/hr-degree-foot.

    A 2x4 studwall with fiberglass loses about 0.1 BTU/hr-degree-foot. A 2x6 studwall with fiberglass loses about 0.077 BTU/hr-degree-foot. A brick-veneered CMU wall with no insulation runs about 0.3-0.5 BTU/hr-degree-foot, but with an inch of foam in the cavity between brick & CMU it's in the ~0.14 range.

    A 2x6 t&g cathedral ceiling with no insulation with composite shingles and no snow load runs about 0.6 BTU/hr-degree-foot. With 2" of exterior foam under the shingles that falls to about 0.1.

    Full basement, or slab-on-grade?

    Only count the walls where the exterior is the great outdoors, not partition walls to other conditioned spaces.
  15. Soapm

    Soapm New Member

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    Last edited: Feb 25, 2014
  16. Soapm

    Soapm New Member

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    This was an eye opening response. The sanctuary has 3 exterior walls, the windows are single pane and very drafty and the exterior walls are brick with nothing inside except paint (that I know of). You can see brick pattern on the inside.

    However, I don't think a more detailed analysis is needed since you gave me much of what I was asking. This system, if we get enough tonnage, will do well down to the point where the furnace will have to help. At that point we care less about energy efficiency and more about keeping warm anyway.

    We do plan to get new windows down the road so it sounds like things will be that much better when we do. It seems we'll automatically get less furnace use and the windows will begin paying for themselves from day one. My other concern is the guy initially indicated 12 tons were needed (3 tons per head) then I notice he his statement changed to 10 tons when I think he realized his heads were 2.5 tons each. Not sure how big a deal that is or not.

    I know there are companies that make 4 ton multi-head units and I was interested in the difference in price since that would allow us to pump 3 tons per head into the sanctuary and use the other ton in the back end of the building. That part that currently won't be conditioned by these units. I guess I will have to try and find another manufacturer in our area which seems to be tougher than one might think, we called 3 Fujitsu reps to find one that would actually price us a Fujitsu unit. The others wanted to install traditional furnaces with AC attached and the other wanted to install more swamp coolers.

    I hate that everything is outsourced now, it was nice when you could call companies like Fujitsu and they would send someone out. Now all you have are authorized service agents which who knows what they're up to??? Or in our case, what they're trying to sell you??? Sounded to me like left over inventory in the warehouse.
    Last edited: Feb 25, 2014
  17. Dana

    Dana In the trades

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    "...I don't think a more detailed analysis is needed..."

    Think again, or you'll spend way too much, and end up with another oversized less-than satisfactory solution. The time to do the analysis is NOW, before you've signed contracts on heating systems and windows, etc.

    The thickness of the brick matters for assigning a U-factor. If it's a 12" thick triple-wythe (with no ventilation cavity behind the outer wythe) you're looking at about U0.42. If it's 12" thick cavity wall it's more like U0.32. If it's thicker than that it's even lower.

    New windows have a payback time on energy savings somewhere between 50 years and never. Drafty single-panes that can be re-weatherstripped & tightened up are worth repairing, then installing exterior low-E storm windows over. Hard-coat low-E glazing adds upfront cost, but cuts the payback period in half due to the higher performance. Typical payback in residential applications is on the order of five years, but in a building that's only brought fully up to temp on weekends it'll be more like ten. An exterior low-E storm over a reworked wood-sash single pane double hung performs at about U0.33-U0.36, and is usually less than half the cost of an equivalent-performance replacement window. Low-E storms can be either fixed or operable, but the fixed windows will always be air-tight. There are multiple vendors- if you can't find any through local window contractors, Larson distributes through the big box store home-center chains.

    Twelve tons of compressor is an insanely high overestimate of what this place likely needs to cover the load, even if it's a crappy one that falls off a capacity cliff at 25F. Assuming it's only delivering 6000 BTU/ton of compressor @ 0F (instead of 11,000-15000/ton like a high-efficiency mini-split) that's still 72,000BTU/hr, or about 25 BTU per square foot. That might be realistic after tightening up the windows and adding storms.

    Assuming you have something like 12' walls, and the common-wall to the other space is the 43' and a very bright 25% window/floor ratio, concentrated on the 63' walls, that' means you have ~650 square feet of U1 window, and about 1000' of U0.4 wall (worst-casing it), and ~2900' of ~U0.4 roof (assuming 2x planking and a couple layers of shingles.) So at a 70F delta that's:

    Windows: U1 x 70F x 650'= 45,500 BTU/hr

    Walls: U0.4 x 70F x 1000'= 28,000 BTU/hr

    Roof: U0.4 x 70F x 2900' = 81,200 BTU/hr

    Add it up and it's 154,700 BTU/hr, which is probably how they come up with 10-12 tons. At 35F outside/70F inside it's half that.

    With low-E storms the window losses drop to ~16K...

    ...the next time it needs re-roofing, putting a layer of 2.5" nailbase EPS (panels of half-inch OSB laminated to 2" of EPS insulation- there are many vendors) under the shingles would cut the roof losses to 18K...

    ...and your total load would be on the order of 62K, cutting your heating bill by more than half.

    Even 4" nailbase roof insulation might still be cost effective, which would cut the total roof losses down to the 12K range and would reduce ice-damming/icicle issues by quite a bit. Talk to a couple of commercial roofing contractors, and get budgetary quotes at different thicknesses.

    It's worth making a plan, keeping the functional gas-burners going as you execute the plan, but size the heat pump system only for the "after" picture rather than for the load on the building in it's current state, then apply the savings on the mechanical systems to things like storm windows and insulation. If you do it in reverse order you end up spending too much up front, and the systems will run less efficiently.
  18. Soapm

    Soapm New Member

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    Aurora, CO
    Thanks again, you’re right and taking time now will save us headaches down the road but this is way out of my area of expertise and some of the things you say are somewhat confusing (to me).

    The brick wall are 8’” thick. They’re cinder block bricks and from what I can tell the same brick is the both inside and outside wall.

    I like that idea, storm windows has to be a lot cheaper than replacing all the windows and it will keep the inside of the sanctuary looking the same. I also like boosting the insulation foam on the roof but replacing the roof would be years down the road.

    The sanctuary is 63’ x 43’ and sits on a full basement. There are 7 current furnace ducts and the one large duct is the cold air return. There are 17 windows and all are 22” x 51”. Here are some pictures that give you a better idea how the sanctuary looks.

    IMG-20140226-00027.jpg

    These are the windows which are 22" x 51". There are 17 total...

    IMG-20140226-00028.jpg

    Sanctuary looking from back to front. This is the 63'

    IMG-20140226-00029.jpg

    Sanctuary looking toward rear including overflow seating. You can also see the swamp cooler duct high up the wall...

    IMG-20140226-00030.jpg

    From the north looking at the south wall. This is the 43'

    IMG-20140226-00031.jpg

    From the south wall looking at the north wall
    Last edited: Feb 27, 2014
  19. Soapm

    Soapm New Member

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    Location:
    Aurora, CO
    IMG-20140226-00032.jpg

    This is the ceiling so you can see how it looks and you can see where one of the swamp coolers was cut into the ceiling. Why they cut into that wood I don't know.

    IMG-20140226-00033.jpg

    This is the one cold air return which as you can see is located in the back corner of the sanctuary and has a chair sitting over it lol

    IMG-20140226-00034.jpg IMG-20140226-00035.jpg

    There are 7 of these heat ducts located through out the sanctuary

    IMG-20140226-00036.jpg

    These are the doors exiting the sactuary
  20. Soapm

    Soapm New Member

    Messages:
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    Location:
    Aurora, CO
    IMG-20140226-00037.jpg

    This is the foyer showing the main entry doors and the stairs leading to the sanctuary main doors

    IMG-20140226-00038.jpg

    My back is toward the main doors showing the sanctuary entry and the stairs to the basement

    IMG-20140226-00039.jpg

    This is the fellowship hall which is under the sanctuary so you can see it's a full, finished basement

    IMG-20140226-00040.jpg

    Fellowship hall showing the stairs that lead to the front of the church. These stairs exit in the front of the church just behind the pulpit

    IMG-20140226-00041.jpg

    There are three of these forced air ducts in the basement...

    Do you still think 10 to 12 tons is overkill?
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