ballpark heat loss calc?

phillysteve

We are currently switching from fin tube to Runtal radiators and our contractor calculated the heat loss per room at between 13 and 25 btuh per square foot.  We live just north of Philadelphia and we have found ballpark heat loss estimates online of between 35 and 40 btuh per sqft.  We don't want to second-guess the heating contractor, but his estimates seem really low -- especially since it is an older house without great insulation.  Last thing we want to do is not install enough radiation!  One other data-point is that there are relatively low ceilings (8') in the house -- is this enough to account for the significantly lower heat loss he is calculating?



One example is a 14' by 14' bedroom with two exterior walls (R-11 insulation) and two modest-sized double-paned windows.  He calculated 4,050 btuh which is only 20.7 btuh per square foot.  Based on our ballpark estimates, the room 'should' have a heat loss of approximately 7,000.  This is consistent with the amount of slant-fin in the room (12 linear feet) which at ~600 btuh per linear foot would provide 7,200 btuh of heating capacity. 



Am I right to be concerned about this or am I over-thinking it?  Thank you!!

Thorp Thomas

I'd say he's right on the mark for your area.

I've done thousands of heat losses over the years and a lot of people think I'm way under. I tell them, if I am I'll come back and put the next size larger unit for free. I've never yet had to exchange anything.



Keep in mind that you are going from a convection baseboard to a radiant panel. In convection system you heat the air that heats the room from the ceiling down. In radiant system you heat the objects in the room that heat the air.



The Radiant View - Radiators heat what they see; if the radiator is looking at a window or a glass door or the back of the couch or the cabinet it will be pretty much ineffective in heating the room. The lamelles on the back of the panels do more than just quiet the panels down and catch dust. They also act as a convector to augment the heat production.



I use a lot Runtals since they are made in my back yard (sort-a-speak) Ward Hill MA. 20 miles away. My 100 year old house is heated with Runtals and I love the radiant effect.

You've made a good choice, trust your heating guy, he's right on.

 

BTW: disregard what Runtal says about BTU/sq.ft, the best way is a detailed room by room load calculation. If you go by sq.ft. it makes a difference whether it's a greenhouse or a meat locker (windows).  



 

Greg Maxwell

Rads

I would use 25-35 btu on rads. Also, look at what you would need for rads to heat your home with 140 average degree water. It will mean spending more on the rads, but the payback will be huge. 

CMadatMe

Math Doesn't Lie

I just did the room and figured no heat above with 50 degree heat below and it came out to 4,494. Parameters were 70 degree setpoint and 0 degree outside temp with 8' ceilings.



As for panel rads water temp designed to overcome the heat loss dictates btu's per sqft. Low water temp larger panel. Higher water temp smaller panel. All that matters is overcoming the heat loss.

Just designed for panel rads in 8 apartments with a 140 degree water temp and not even close to 25 a square is needed to over come the heat loss.

Greg Maxwell

Rads

25-35 btu per sq ft will give you the load of the room, then calculate the rad output based on the lower water temp to match that load, and you will have comfort, even on the coldest days. Also, by putting thermostatic valves on the rads, and adjusting them to your comfort, you will have a system that will be hard to beat.

CMadatMe

The Load of the Room

Is given to me by my heat loss. I size my rad using the proper water temp correction factors.



If my heat loss is 1000 btu's and I'm designing using 140 degree water temp my correction factor is 2.35.



2.35 x 1000 = 2,350. I pick the rad that will produce 2,350 btu's @ 180 degree water.