pipe sizing question - MJStraw

Mark J Strawcutter

is there anything else in "pumping away" that's not applicable to gravity conversions?

Does the "power purge" from the basement work? Or is there some other way to pipe it for a gravity conversion?

Mark J Strawcutter

on gravity conversion

Thinking about re-piping a gravity conversion to pumping away with bypass (no P/S yet).

Main piping is 2.5in near boiler, boiler itself is 160kbtu in/120kbtu out with 1.25in supply and return tappings. Current near-boiler piping (from initial conversion) is 1.5in.

"rule of thumb" over in Q&A sez take half of mains then one size lower for near-boiler pipting. This gives 1.0in

The "pumping away" book talks about staying "full size" out thru the circulator - I interpret this to mean it should be 1.25in

What would you do?

DanHolohan

Go with

one-inch. Gravity is different.

DanHolohan

Pressure drop

is so low in a gravity system and that's where the smaller near-boiler piping comes in. It gives the pump something to work with.

The power purge would work but you have to think it through when you're adapting the pipe to the existing system. A lot depends on how the radiators are piped. Are they bottom-to-bottom connections? If so, you'll have to feed the system one floor at a time, with the air vents open. Have you looked in the Gravity Hot Water Heating section of the Heating Q&A? There's a lot in there about what to watch out for.

Mark J Strawcutter

how rads are piped

It's pretty much a 2-pipe direct return with a single rad on each "rung" of the "ladder". No instances where supply riser hits a 1st floor rad and continues up to a 2nd floor one above it.

There are two sets of supply/return mains - one for the front of the house and one for the rear (too bad it isn't upstairs and downstairs split :-) I'm planning to come off the circulator discharge to a T and out into each set with a balancing valve on each supply.

Not sure what you mean by bottom-to-bottom. On each rad the supply and return each connect at the bottom, one on each end. If you mean one rad above another with common riser, then no I don't have that.

Been a while since I read the GHW section of the Q&A, but i have read it - hence the question that started this thread.
A review certainly wouldn't hurt, however.

DanHolohan

radiators

Yes, that's what I meant by bottom-to-bottom. You won't be able to power purge the air out of those because the water will just flow across the lower push nipples of each radiator. You'll have to vent them manually.

Boilerpro

Balance valves

I'd also put these on the return. I figure anything that can cause a pressure drop across the system should be piped at the return so the pump can maximize any pressure increase in the system to help with air removal. Zone valves, flo controls etc, if possible on return... plus you will probably get longer life due to lower water temps.

Boilerpro

Mark J Strawcutter

with check valves

on return, couldn't you have trouble with within-the-pipe gravity circulation, especially with the large pipes on a gravity system?

I seem to remember reading where someone ended up putting check valves on both supply and return to avoid this.

Boilerpro

Yep,

B&G shows them on both supply and return....of course they are trying to sell more product. On big pipes I'd put 2 in from the start, but with 3/4 or 1 inch lines I'm not sure if 2 are really necessary. I imagine when you say check valves you mean Flo-control valves...they are a different thing. You could also install themal loops on the piping, but you would still need at least one flo-control with zone pumping.

Boilerpro

Mark J Strawcutter

how are they different?

I was under the (obviously incorrect) assumption that flow-control was just B&G-speak for a check valve.

Boilerpro

A flo-control is a weighted check valve.

It prevents flow of water in both directions, until the circulator turns on and lifts the weighted cylinder off its seat. I understand that its takes a foot or two of pump head to lift the valve open, this is how it prevents gravity flow... gravity flow usually cannot create this much head pressure, so forward flow is prevented. The valve also prevents backflow just like a check valve, to prevent ghost flows created by other pumps in the system running parrellel to its circuit.