heat loss house, sizing boiler

[simonswb6]

im not an expert in heat loss. I am trying to work with an older plumbing outfit to properly size a boiler. The house was a modular and the heat loss was calculated for us based u factors of the windows/doors/walls etc. the total UA for the house is 412. I took one course in heat transfer a long time ago. from the cob webs, Q=UA*delt T. for my area this yields around 42,000 btu/hr with a 15% FC. this seams really low as the smallest boiler i could find was around 85,000 btu/hr? This house has the energy package installed and an HRV unit since the house is so tight, however, the Q of 42,000btu/hr seems really low. am i missing somthing? do i need to include additional factors when sizing a boiler?

 

[simonswb6]

42,000 btu/hr with a 15% FS (Factor of Safty) not 15% FC

 

[quark]

There may be a calculation mistake. 85% of 42000 btu/hr is 35700 btu/hr and if UxA is 412, dT should be 35700/412 = 86.65F (if you chose proper units). If 86.65F is correct, then you should furnish further details like the ambient temperatures, qty. of fresh air, building type and floor area to arrive at a ball park figure.

 

[moltenmetal]

Get yourself a copy of the Weil McLain Radiant Explorer sizing software- it's available for free via request on their website. It will allow you to easily do a room by room analysis based on wall, window and ceiling/roof construction, including an important parameter I'm not sure is in your calc: air infiltration.

 

[simonswb6]

The calculation to get to 42000 was the following
Q=AU*delta T
AU=412
Delta T for my region was 90deg f
Q=412*90
Q=~37,000 btu/hr
i then added 15% Factor of Safty and which came out to be ~42000 btu/hr

i had not added the air infiltration in. There will be heat loss due to the ventilation system. however the HRV unit will also warm the incoming air so my assumption was that it will be small. I have downloaded the software from Weil McLain. i will have to investigate more about heat loss from air infiltration.

 

[moltenmetal]

Yep, you will that. Unless you have extraordinarily tight construction, you will have air infiltration and it will become a significant heatloss factor in your design. A HRV system will reduce the infiltration losses but will not eliminate them.

 

[FredRosse]

Heat loss in a house can be nominally small, however the heating boiler is generally sized to accommodate additional loads. Air infiltration is a significant factor, even without door opening and traffic in/out of the house. Additionally, if kids are coming in and out many times during a cold season (for example, playing in the snow) the heat load on the boiler goes way up.

Extremes of weather also play here, for example, how about the cold day when the wind is blowing strong, the house owner still wants to be warm enough. How about the day you enter the cold house on return from your vacation, do you mind if the heating system takes all day to restore a comfortable temperature, or do you want the heating system to work in a shorter time period?

A good analogy is the modern automobile, the maximum speed limit is around 65 MPH, but virtually any car you buy today can go well over 100 MPH.

By over sizing the heating system, the supplier sells a bigger machine, hence a bigger profit. The oversized machine never brings complaints on account of its inability to heat the house, it just cycles on/off according to the thermostat. An installer will generally prefer to err on the “safe” side, less callbacks from customers is the result. These are the practical reasons to have an "oversized" heating system.

I had a very well monitored house heating system in a new central Pennsylvania house, 3000 square feet size. Average wintertime heat load was carried 98% of the time with a 22,000 BTU/hr heating machine. The backup heater was rated at 70,000 BTU/hr, and was always capable of heating the house during the rare times the smaller heater could not keep up.

 

[mauner]

What you should also be considering is operating time and nozzle tip sizing.

Most heating systems don't run 24 hours a day. During the winter months, I generally see 10 to 12 hours of operation a day. If you raise the temperature of the thermostat, most consumers want to see a fast response. Especially when using programmable thermostats.

Oil fired units are rated with a certain tip size. A 125,000 Btu/hr unit should have a 1.0 gph tip. To lower the rating to 85,000 Btu/hr, you could put in a 0.75 gph nozzle. Similar methods are used to adjust the rating of a furnace using propane and natural gas burners.

The advantage to a larger unit is response time, not running all the time, and it allows for future growth if considering an addition to the house.

 

[IRstuff]

Heat loss is simply a make up quantity. If you had only capacity to make up the heat loss, heating the house from a cold start would take forever, because every BTU in would get sucked right back out again.

The apparent excess capacity is what allows you get the house up to temperature in a reasonable amount of time.

TTFN