Repiping radiators

Brad White_67

is to know your heat loss, room by room. All hot water boiler sizing should be based on as accurate a heat loss as can be calculated.

Next, know your radiator size in square feet of EDR. There are books available on this site and Burnham has some good general information via their website.

By knowing both of the above and the relationship between them, the maximum hot water temperature you require can be determined. This goes to boiler selection as well. If you are replacing the boiler (you may or may not be, seems like a piping-only question), you might favor a condensing boiler. And if you are reconnecting to an existing conventional boiler, you might consider re-working the near-boiler piping to protect the boiler but still allow outdoor reset of radiator supply water temperatures.

OK.... That all being said: Pipe sizing is all based on flow rate which is based on BTU's per hour (BTUH) required.

With a 20 degree temperature drop (and by my own chart):

<u>Runouts:</u>

1/2" copper will carry 1.5 GPM or 15,000 BTUH

3/4" copper will carry 3.0 GPM or 30,000 BTUH

<u>Mains:</u>

1" copper will carry 5.0 GPM or 50,000 BTUH

1-1/4" copper will carry 9.0 GPM or 90,000 BTUH

1-1/2" copper will carry 18.0 GPM or 180,000 BTUH....stop! House is getting too big! ;)


If you go with a 30 or 40 degree temperature drop, these BTU capacities at the same flow will increase to 150% and 200% respectively. Imagine 30,000 BTUH out of a 1/2" pipe...

Point being, either of these two smaller sizes, even the 1/2", will adequately feed most single radiators. Of course your mains should be sized accordingly....generally never less than 3/4" even if serving one 1/2" radiator runout at the end. Keep main losses down and allow adding another radiator later if you need to. My own practice.

My house is so piped (a mix of 3/4" and 1/2", 50-50) as are others I have designed and installed. My mains are 1-1/4" and 1", all for a 1,500 SF house with a 55,000 BTUH heat loss. This is generous on the pipe side but the runouts work just fine.

Once all is figured out, check your circulator capacity. What now serves the old gravity mains might have a very low head (pressure) requirement. The new copper will still be reasonable at the flow rates noted above, but worth checking. Newer multi-speed circulators for the modest cost they incur are in my opinion one of the best values in hydronic heating.

While you are at this, please consider using Thermostatic Radiator Valves (TRV's) on each radiator. This will go far towards leveling any wide ranges between radiator capacity and room need, will compensate for sunny rooms and make life a grand adventure subject to your active participation.

Mark_7

Repiping radiators

How's the best way to figure out pipe sizes for cast iron radiators.Customer wants to raise pipes in basement,so we are going to take out old 2" and 11/2"iron and replace with copper.System was gravity and when bioler was replaced it was changed to a circulated system.Do we size new pipe to handle the capacity of radiation on each side of house?Info greatly appeciated.

Unknown

forgive me

Forgive me for asking this , markl.. Are u a heating contractor or just a handyman doing whatever is pipes is pipes ?
Those pipes were installed prior to circ pump... and those gravity pipes can be replaced with some homeworks done... by the hydronic heating experts... What will the basement become after those pipes are redone?