Air Handler previously fed with 1/2" , any point repiping with 3/4" for the new boiler ?

Dave Carpentier

AH specs say 40Mbh at 130f with it's ~4gpm circ. Heat loss estimate on design day needs 20k. Most of the upper floor heat comes from recirculated air sucked out of the main floor (radiant) rooms as we leave the AH blower always running. On super cold days, the AH does start to draw boiler water to keep up.

Our old CI boiler setup had the AH fed with 1/2" copper (15ish ft each leg). That boiler ran always-hot 130-160.

With the new modcon, I was going to run 3/4" but now Im wondering why not stay 1/2" ? The last 3ft of each pipe at the AH will be 1/2" anyhow (PIA to work in that space , but also the oem stubs out of the AH are 1/2" ). Once you have some 1/2" in there, that restriction is set in stone right ?

The new setup will be a little over 20ft of pipe (each leg) to the AH. Water temp can be any target from the boiler.

mattmia2

.5" is about 1gpm which can move 10,000 btu/hr at 20 delta t. unless you slow the airflow down to match or the ductwork is much better designed than usual, it will be drafty.

Dave Carpentier

Could it be that i've only ever had 10k btu being added to the existing air (from both upstairs and the main floor intakes on ceilings) ? We like house temps of about 68f and it seems okay.

mattmia2

if you drive up the delta you can get more out of it. it depends on where it is actually operating.

btu/h=500 dt gpm.

hot_rod

4 gpm would be a lot for 1/2 copper. 1-1.5 is doable

If it has worked in the past, the 1/2” over that short distance should be fine