Manual J: How Much Oversizing?

Constantin

... as the US market grows in size and the scope of competitors increases as well, I think we'll see more innovative technologies released by all competitiors. Viessmann has a number of neat technologies that are already available in much of the world and as their base of installers grows over here, we may see more of them...

A Merry Christmas to you and keep in mind that you're better informed than most consumers when it comes to your heating needs.. when the time comes to make a heating-related decision, you'll make the right one, I'm sure. as for Manual-J, I am awaiting the end of the first season to determine by just how much Manual-J, 7th Ed. overestimated or underestimated our heating needs.

They are on to the 8th Ed. now, which allegedly tracks more variables, thus increasing complexity, while also reducing the alleged 25% padding. My guess is that a simple way to determine just what the padding is would be to construct a 10' cubic house and to compare the calculated Manual-J result for the home to the results that a manual calculation would offer.

My guess is that the divergence will be pronounced in the area of windows and outside influences, such as the shade from trees, actual vs. calculated infiltration, etc.

gasfolk

Mike T.'s work is truly impressive,

and many on The Wall have also referred to Manual J oversizing.

Can anyone offer other, real-world examples (including rough numbers) where Manual J significantly under-estimated (or over-estimated) heat loss.

Who benefits/loses when Manual J is way off? Is this less of a problem than it appears? Not a problem at all?

Thanks,

gf

gasfolk

Vitodens 300?

This thread seems to be less interesting than I thought, so I'll sign off by saying...

THANK YOU to The Wall and Wallies for all the good advice.

My wife LOVES the radiant floor heat under the kitchen (from below, with heat transfer plates) and in the bathroom floor and shower walls. Heating the house is more uniform and MUCH more efficient since tightening the envelope.

We now understand that the near-boiler piping needs to be redone because 1) "closely-spaced tees" are separated by pumps and check valves, 2) the system circulator pumps into the expansion tank, 3) all of the pumps are oversized (but now throttled), and 4) the boiler is way oversized (280 MBH output for a load we generously estimate at about 50 MBH). Three very nice contractors, using good, MJ7 or MJ8 software, want to install a 110 to 120 MBH boiler--anything less and we (Mr and Mrs homeowner) take responsibility.

In our dreams, a Vitodens 300...but only when MJ9 supports the numbers. MJ10? MJ11? Oh well.

Very best holidays to The Wall and all of the Wallies.

gf

Unknown

this is why I like to build my own calculators. They may not be exact, but nothing is, and at least I get to decide what my safety factors should be.

gasfolk

Thanks for your reply, Rob.

Oversizing seems primarily a concern of geeky homeowners like me--not very sexy for installers. The occasional contractor with guts can flout Manual J. Can't blame the others for not wanting to risk a no-heat callback.

Too bad for geothermal. My house would have geothermal right now if not for months going in circles over sizing. Sub out the well drilling, and most of the PROs on The Wall would kick butt at the hydronics and controls of the heat exchanger and low-temperature distribution system. If system size and costs drop by half, geothermal becomes competetive. The hydrogeology in my area is nearly ideal. Couldn't PROs make a decent profit installing more geothermal?

Without feedback, there is no incentive for ACCA to address this. This thread was a feeble (regrets) attempt to start collecting examples of significant oversizing with MJ8 (somebody should do it). Unfortunately, many boilers are so oversized to begin with, MJ8 looks like a huge improvement.

OK, the Vitodens 300 isn't available in the US, but if it ever arrives...it might just meet my needs.

Good holidays.

gf