Formula or Online Calculator for BTU’s n
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Calculating loads is not particularly easy and not exacting. It’s a lot more than a simple formula. And it takes some educated guesses on the tightness of the construction, for example. 88k v 100k btu is not that big a diff. Just did one with proposals that ranged from <30 to 120!
Ask each the basis of their load calc. Some use the software/formulas and some use experience and/or rules-of-thumb.
Replies
Try this:
http://www.hydronicnetwork.net/hydronic/hea/flashpro.html
Here is a pro HVAC software. You can get for home use for $49.
http://www.hvac-calc.com/main.asp
It is based on manual J. Ask the different HVAC contractors for a copy of their manual J calculations.
Bill, is that program the same one or close, to the one at http://www.mrhvac.com/manualjshort.htm ? PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
I have not used the HVAC-calc so I don't know now close they are. But don't think that they are the same (except they both are working to manual J).
MrHVAC has there own commerical version, http://www.mrhvac.com/products/software/elc/elc.htm
Heat loss is not going to be super precise. Has a lot to do with the construction details, particularly how airtight the final structure is. That said, you can probably get close.
Try this:
http://www.slantfin.com/he/
"some use experience and/or rules-of-thumb"
The rule of thumb I like is a very tight, very well-insulated house needs about 0.1 BTU/hr/sq ft/degree F. Which I used for heat load in my own house. To wit: 0.1 x 1700 sq ft x 110F (70 inside minus -40F outside) = 18,700 BTU/hour during worst case weather. I actually acheived a factor of 0.07 BTU/hr/sq ft/degree F by building REALLY tight. The sq ft are total square footage of the living space, e.g. 1000 downstairs + 700 upstairs = 1700 sq ft in my house.
Continuing that rule of thumb: use a factor of 0.3 BTU/hr/sq ft/degree F for typical new construction. Code-required insulation, average workmanship, decent windows, etc. To use a lower number, you need to be using specific technologies and techniques to make for a better insulated and (more important) a tighter house.
So a typical, new 2,000 sq ft house with a design temp of 0F would need 0.3 x 2,000 x 70 = 63,000 BTU/hour net from the furnace.
A factor of 1.0 would be for a early 70's home - some insulation, leaky but you can't actually fly a kite inside.
A factor of 3.0 would be used for a 100-year-old house - single pane glass, no insulation, leaky as all get-out.
With a 30-fold range in these factors (and that is appropriate - houses do vary by that much), the naunces come in by deciding how much to bump them up or down for the particular conditions: windows, sheathing, insulation, etc. And that is where the experience comes in.
Continuing rule of thumb:
1600 sg ft house
Low-E argon filled panes
12" (settled) cellulose over ceiling rock, 24"C trusses
ceiling recep boxes taped to poly VB
All window and door framing-to-jamb gaps foamed and taped
2X6/16"C (2X8 on North wall) walls with FG batts installed with care with 1/2" foil faced iso on interior, taped at seams, 1/2" drywall
Conventional recepticle boxes sealed on back (around wires and over un-used clamp holes) with electricians putty, taped on front to foil faced iso
North side buffered with bedrooms, laundry, mudroom on that side
3/4s of North wall earth bermed ~ 3 ft up from sill plate
Double entry exterior door to mudroom to kitchen
Living areas with South exposure to MN latitude
FG Batts and poly VB in/on 2x8 PTW crawl space and cellar foundation walls
Would you consider these details would get close to or below your 0.1 factor?
Edited 3/8/2004 2:43:38 PM ET by johnnyd
I like the attention to the north-side details - wider wall, buffer rooms. I like the arctic entry ("airlock") both for the reduced leakage 23.9 hours a day but also to prevent that -20F air spilling in directly from a single door opening.
I assume you got the foam strip down between the bottom plates and the concrete. I'd say a factor of 0.12.
I like to allow cross-ventilation of all sun-exposed rooms and the house as a whole through modest sized windows. And vertical "cross ventilation" as well to use the chimney effect instead of A/C in the summer. And, of course, code-dictated egress windows in the bedrooms. But those egress and cross-ventilation windows can overlap. After spec'ing good quality windows to acheive the above, I'd make the rest fized-pane.
I'd go lower is you sprayed urethane on the sheathing all over. Could be just an inch ($.60/sq ft) and then fill the rest of the bay with FG, carefully as you say. Then, with your attention to the electrical and lighting penetrations, you're getting towards 0.08.
Now the biggest remaining hole is the flue pipes for the HWH and the furnace. If those are direct-vent (and/or the same unit) and you really watch the details on the dryer vent, range hood, bath fans, etc you get down to 0.06-0.07.David Thomas Overlooking Cook Inlet in Kenai, Alaska
40883.8 in reply to 40883.7
Sounds like I'm doing OK...
I like to allow cross-ventilation of all sun-exposed rooms and the house as a whole through modest sized windows. Got it. And vertical "cross ventilation" as well to use the chimney effect instead of A/C in the summer. And, of course, code-dictated egress windows in the bedrooms. Got em. But those egress and cross-ventilation windows can overlap. You bet. After spec'ing good quality windows to acheive the above, I'd make the rest fized-pane. 24 X 36 awnings on the north, 36X 48 double hungs elswhere, plus three big fixed pane double glass pictures salvaged from "Big Blue" IBM building in Rochester...figuring R 1.5 for those. Oh, and the ceiling over the south-facing great room is gable, with 2X6 rafters..half of that roof only has R-19, the other half is vallied by additions and has the 12" cellulose.
Now the biggest remaining hole is the flue pipes for the HWH and the furnace. If those are direct-vent (and/or the same unit) Probably dual-fuel-rate electric boiler (3.5 cent a KWH) for RFH with DV propane fired HWH through heat exchanger as back-up through details on the dryer vent, range hood, bath fans, etc you get down to 0.06-0.07.
I'm going to have a pro do a room-by-room heat loss analysis...need rules of thumb like this to keep him honest.
Thanks.
That all sounds good except maybe those double-hung windows. The modern ones do an okay job but they aren't the best sealing windows.
If you had done the spray foam, then you get so tight, you need mechanical ventilation such as a heat recovery ventilator (HRV).
With electric rates like that, what part of the Pacific NW are you in?David Thomas Overlooking Cook Inlet in Kenai, Alaska
Actually I'm in SE Minnesota, electric utilities (coal fired for the most part) here push 3.5 - 3.7 cents a KWH, with the right to shut you down during periods of peak demand. You have two meters, one that measures the heating/HW load at 3.X cents, the other at regular domestic rates...8.7 cents here. They control the switch via radio.
In order to qualify, you need to have an automatic alternate source of heat, like a propane space heater, or HWH that switches over to heat when they shut you off. Some high mass systems qualify for the dual fuel rate, as they expect the slab to have enough risidual heat to outlast a peak period shut down. I think baseboard resistance that's on domestic also qualifies. That might be the cheapest up front way (and maybe long term) to go, as baseboard resistance units are cheap, and you could keep them set at minimum temps so they might not even cut in during a few hours of peak demand shut down.
We'll also have a small wood stove for ambiance and backup heat in the living area.
I think 3.5 cent KWH electricity is roughly equivalent to 75 cent/gallon propane....a price we haven't even been close to for awhile.
Payback to the utility is a larger, more dependable load. BIL is on this, says only time he gets shut down is during first cold snap as they ramp up generating. He has all baseboard resitance on dual fuel, and conventional propane HVAC for backup.
Anyway, I sized the service entrance to this house to handle a heating load, to keep my options open.
>Some high mass systems qualify for the dual fuel rate, as they expect the slab to have enough residual heat to outlast a peak period shut down.
That's a wonderfully enlightened view! Glad to see it being recognized. We have similar rates off-peak, but there's no shut-down capability, just on and off peak times. Saves about 100/m.
Here (just outside Kansas City) are biggest load is AC and we have seasonal rates.
The first 600kwh is 6.93 for both winter and summer. For 8 months of non-summer the next 400 kWh is 3.74 and all above that is 3.1. Now that is if you have primary electric heat. It is about 10% higher if you don't.
The sumemr base rate is the same, but then rates for higher usage increase rather than decrease.
Just talked to them about dual fuel...they require 6 - 8 inches of SAND underneath the slab, on top of the insulation, to provide a larger heat sink.
Also came up with an idea to kill two birds with one stone:
Install a 30K BTU/hr electric boiler (they have modulating boilers) as the primary radiant floor heat (RFH) source, and circulate its output through a heat exchanger in a propane water heater (like the Bradford-White Combi-Core). Under normal winter circumstances, the electric boiler would be consuming 3.5 cent/KWH electricity, heating the heating fluid to ~120 degrees for the radiant floor, and also indirectly heating domestic hot water. When the demand for both domestic HW and RFH hot water is great, the propane would kick in to pick up the slack. It also would automatically kick in if peak demand shut the current to to the boiler, and satisfy the requirement for an automatic back up.
So I would be able to heat both domestic and RFH water with relatively cheap BTUs during the winter and also take advantage of quick recovery during a long hot shower...and just shut off the electric boiler (or flow to the RFH system) during the summer and use propane when the off peak rate goes up to 5 cents/KWH. Depending of course on what the price of propane is at any given time.
This would also leave me open to potentially heating domestic hot water with solar panels during the summer.
What do you think?
>>"qualify for the dual fuel rate"
>"That's a wonderfully enlightened view!"
Yeah, took a while to get here on a residential level. Germany has had radio, remote-controlled water heater for 30 years. And time-of-use pricing. My F-I-L wrote his Economics PhD thesis on demand pricing at a time when US phone calls were priced the same regardless of time of day. And if the lines got busy mid-day, they just built more lines.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
I think some of these hvac calc programs ignore/can not handle open cathedral ceiling and 1 1/2 rooms like cathederal ceilings in homes where there are two units
John, Give the utility room(washer/dryer) its own air supply if you can.
Otherwise, it will suck out side air in where ever it has to, and exhaust conditioned air. Often, a duct to the attic, with filter, will allow warmer air than outside air to be used. If dryer is gas fired, you have some specific codes to follow for confined space appliances. With a super tight house like David's, you have to be very careful to provide enough combustion air and avoid back drafting.
I am working on a design now that will have the air lock foyer, east wall buffered with non heated storage/cedar room, walled to non conditioned utility room.
Closed cell poly foam sprayed in all walls with fg behind that and exterior of sheathing will be 3/4" foil foam board. Attic will also get 1" foam spray. Foyer, stairs to 2nd story and all upstairs will be isolated from rest of house with french doors so that up hvac system not needed unless company staying with us.All hvac, duct work to be in conditioned space and air tight. Advise either returns in rooms that will have doors closed, like bedrooms, or bridge vent to a room or hall that has the return. That way, you won't be pressuring a room with no air escape except to the outside, if using forced air system.
Being in mid-Atlantic,summer humidity a problem, so refrigerator will have condensate drain, and shell formed by cabinents around the dish washer and built in ovens will have low volume exhaust system.
Shower drain on master shower will have a GFX heat capture pipe.
Hope some of this helpful to you. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Paul
>>If dryer is gas fired, you have some specific codes to follow for confined space appliances. With a super tight house like David's, you have to be very careful to provide enough combustion air and avoid back drafting.
I've never been able to find how to include dryer requirements (I figure either gas or elect will draw air from the house and contribute to neg pressure) in coming up ith the air supply requirements? Is it a manufacturer spec?
Or is a straight BTU calc - at least for the gas?
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Bob, the water heater installation manual and our local inspector both say to total the Btus in the confined space of all the gas utilizing equipment in that space. Then you go to the chart for supply air source and whether horizontal or vertical supply duct. Typical is 1 sq inch per 4000 Btu direct vent to outdoors, two vents, one high (12" from ceiling, other low, 12" from floor). Each must supply the sq inches required, not a total of the two, and grille, louver interference must be accounted for. Horizontal supply is double in size.
Now the weird part is what you alluded to, the cfm of the dryer, and in my case, the cfm of the WH power vent. When both running, there is a lot of neg pressure in the room. With the dryer operating, you could expect pilot flue gas to be pulled back through the dryer flue, but with the sq. inches satisfied, there does not seem to be a problem.
So, bottom line, total Btus rule. I think a house with multiple dryers in a confined space with a standing pilot WH could have a problem with back draft. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Drugrep,
I bought the HVAC program linked in Bill Hartmann's post last week and it's great.It gives you good background on equipment sizing and can help size your duct runs too.I did a heat loss/gain on my own house, and then had two contractors that sell Carrier and Trane products over for estimates.They were both "rule of thumb" B.S. artists.Do yourself a favor and go to the HVAC Talk Forum before you buy.
Barry