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design temp vs record low

Steamhead Member Posts: 16,728
the house is a bit colder. This is a choice you could offer the owner on a boiler replacement- size the boiler to the regularly expected low temp, or the record low temp?

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  • R. Kalia
    R. Kalia Member Posts: 349
    what if my boiler can't keep up?

    Why is the design temperature higher than the record low temperature in a given area?

    In our area the design temp is 0F, but very rarely we've seen -20F hold for significant periods (up to a couple of days), which is 30% colder in degree-day terms and so would appear to need a 30% bigger boiler.

    These days the trend is to NOT oversize boilers or furnaces...what happens if a boiler is perfectly sized based on the design temperature, and then a cold wave hits?
  • Someone asked

    that @ an IBR School 20 years ago. The instructor walked over to the coat rack, pulled out his sweater & put it on.

  • Ken_15
    Ken_15 Member Posts: 34

    In this region we have many developers that hire discount installers. They pay for volume not quality. We get many calls during cold spells complaining that "the heat doesn't work". When we get to the home we discover that the home wasn't sized for design days, never mind "exra cold".

    We just got called to a new house, 3500sq/ft with all radiant heat and a 40g Super Store attached to a 71,000 btu boiler. It was 0 outside and her house was only at 58. The boiler hadn't gone over 125 degrees in two days. I asked the HO if she ever has hot water. She told me that she has bath tubs that she has never used.

  • R. Kalia
    R. Kalia Member Posts: 349

    I remember another guy who put on a sweater. He lost his job over it. Jimmy Carter---a good man. I don't think it would work any better for a heating contractor. You can't sell a heating system that doesn't work satisfactorily in all weather.

    So what's the point of a design temperature? I do realize that most systems are oversized even after a heat loss calculation, so maybe it doesn't matter, but why would the powers that be choose a design temperature higher than the lowest possible temperature?
  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Things that heat loss calcs don't take into consideration...

    Like internal gains (3.14 btu/watt), solar gain, bodily gain (500 btuh/body/hr @ rest) AND the flywheel effect of all the internal mass within the home. These are things that "buffer" the typical heat loss calc and allow the home to remain relatively comfortable even during dips into the "coldest weather in a hunnert years" scenarios.

    My own home is a prime example. It was calced to use ~ 80,000 btuH at design condition. It actually uses about 1/4th of that per hour, at design condition.

    As contractors, we MUST perform due dilligence and perform a heat loss calculation, but I have to ask you, when was the last time you saw a properly sized heating appliance running at 100% of capacity, even at design condition???

    We have a lot more going in our direction than we think.:-)

    Just my $0.02 worth.

  • hr
    hr Member Posts: 6,106
    A line has to be drawn somewhere

    Really that can be addressed by the person doing the heatloss calc and design. By raising the indoor or outdoor design when doing the math.

    I like to use 70 as an indoor, but sometimes for older homeowners I bump to 72.

    Actually the room temperature number on the thermostat on the wall has little to do with "complete comfort". The bigger picture needs to concern mean radiant temperature, humidity, drafts, etc. That's where radiant heat gets it's praise by warming the walls and objects. Even 72° can feel cold if the walls and surfaces are cold!

    Equipment wise, the newest modulating equipment can solve this dilema. A 10- 20 % oversize in the boiler is not as big of a deal when the unit can "turndown" based on demand. The Munchkin, among others, for an example, really shines in this arena. No longer is a bit of oversizing "taboo" :)

    Generally speaking the extra btus can be delivered by the floor, ceiling, wall loops, if a few extra degrees of surface temperature is not objectionable for those days beyond design.

    We like to design around 80-82° surface temperature with most design software. Bumping a couple 2-4 degrees will add the extra btus by about 2 btu's per square foot for every degree difference between floor surface and air temperature.

    This should be the buffer need, unless the original design is way off. Kinda hard to do with the fudge factor built into most design software (other than input error :)

    So the extra horsepower is conviently "locked up" inside modulating equipment already. Unleash as needed!

    hot rod

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  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Don't assume the heating sytem is at fault

    just because a home is not heating properly. If the home is not performing as it should from things such as improperly installed insulation, air leakage from things such as recessed lighting, poor quality housewraps, etc.,
    it also won't heat.

    A well built 3500 sq ft home quite possibly should heat with only 71,000 btu/hr. Thats works out to about 20 btu/sq ft...easily within reach of good design and construction practices with conventional techniques, especially with radiant. I heat my own 2800 sq ft 1906 home with about 60,000 output. I have an energy hog 45,000 sq ft church running with only 960,000 output, of which about a third is for pick up capacity. It's been consistantly in the single digits above and below with little sun and everyone is staying warm.

  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542

    the coldest temperatures only occur less than 1% of the time, and it grossly oversizes the heat plant, causing short cycle seasonal deficiency syndrome. Basically short cycle inefficiency. That used to be the case where you were dealing with a one spped fire breating dragon. Nowadays, with all the perfect modulation capabilites we have today, a fuel savings of 30 to 50% is possible with properly sized, properly applied modulating, condensing equipment.

    If you want to design your system for "worse case scenario", then go for it. Just make sure the consumer knows what you're doing and remember, you may be bidding against me...:-)

    Oh, and regards President Carter, what he did may have gotten him fired, but he saved billions of barrels of fuel doing what he did. That's undeniable.

    Some people call it "Euro Cave" temperatures. Most people call it conservation.


  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479
    Good point BP

    I did a job about 10 years ago. did a heat load installed a forced air propane furnace. The homeowner called the first cold weather and said the furnace wasn't keeping up and his fuel bill was very high. He said his nephews HVAC class had done his house as an example heat loss and bottom line I had put in too small of a furnace. The question begged, why the high fuel bill then? I drove up to his house in Martinsburg WV, and luckily it had snowed the night before, because as I drove up his street it was quite obvious what was happening. All of his neighbors roofs were covered with snow like frosting on a cake. His roof was free and clear, the snow melted off. The GC had not insulated well. He couldn't get the GC to do anything so he hired an insulating contractor who set it right and the same furnace is chugging along today carrying the load and keeping things efficient. WW

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  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Yep, we always get blamed

    even if it is not our fault. First radiant floor I did was that way. I did a manual calc and the manufacturer's designer did a calc, we were within 2% of each other. When it got real cold the radiant couldn't keep up. I found out the siding contractor had removed the original siding from the home (about 100 years old) and then wrapped it with a perforated house wrap and vinyl siding. I was told the original sideing was to stay in place. I believe this opened up the balloon frame walls to all sorts of air movement (like a chimney) cooling the first floor walls as air moved through the fiberglass. I added baseboard (at my cost) to 2 rooms and it heats now, however, it is using about 80,000 btu/hr at design instead of the calculated 60. The siding contractor was never made to correct his problem. Some thermal imagining would really come in handy for jobs like that.

  • Ken_15
    Ken_15 Member Posts: 34

    I guess i also should have stated that the Super Store was set on priority. The 71,000 cannot keep up with the tank. So with the settings on priority, the radiant heat shuts down. With the priority off, the floor still doesn't get enough every time the tank calls.
  • R. Kalia
    R. Kalia Member Posts: 349
    modulation and oversizing

    A couple of people have said in this thread that oversizing is OK with modulating boilers, but I don't agree.

    The Munchkin, for example, modulates only by a factor of 3. So if I put a 199,000 BTU Munchkin in a house that needs 150,000 in a cold wave and 110,000 on a design day, then for the entire spring and fall and even on some winter days, the Munchkin will be running entirely on low (66,000) and controlling output by going on and off, which kind of defeats the whole point of having a modulating boiler in the first place.

    I have heard that the Munchkin floor models will go to 5-fold modulation at some point, but I don't know when.
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