Different sized radiators

Mike T., Swampeast MO

Balancing a converted gravity system via the hand valves is a nearly pointless exercise as the flow rates are FAR greater under forced flow and even the little "anti-freeze" hole in the valve will give a decent amount of flow! If you do manage to achieve some sort of balance via the hand valves--no easy feat--changing a single valve in the system will throw off the entire balance!

That said, hand valve replacement isn't too bad. I've replaced at least 50 (with TRVs) and have never harmed a radiator or the piping when simply replacing the valve. (Always use two wrenches of course...)

While you're probably correct that improper sizing of the 2nd floor rads is the main culprit, it could be related to flow as well. The branch piping to the 2nd floor rads should be at least 3/4" and they should NEVER be installed in a series loop. If smaller pipe is used, the head loss will be many times higher than on the ground floor and you won't get good circulation. I've only seen original 1/2" branch piping used once in a gravity system--it fed a small 3rd floor bath. (Never forget that gravity flow increases with elevation.)

TRVs on the ground floor rads could well be the best (and least expensive) option. While the boiler will still have to produce a supply temp adequate for the 2nd floor, the ground floor TRVs will throttle themselves and room temp balance between the floors will be much better--especially if constant circulation and reset are used as well.

Wayco Wayne_2

There are a lot of

old gravity hot water heating systems in Wash DC. A problem I run into all the time is since the original radiators are sized for the low water flow of depending on buoyant water, when circulators are added the water temps needed for these behemoths are relatively low. Sometimes between 140 and 150 F. This in itself is not a problem. The problem comes when homeowners put on an addition and size the new radiators for 180 F. Yesterday I visited a home that had added a second floor sometime in the past. "It's always colder upstairs," the lady said. Downstairs had the big old cast irons beasts. Upstairs had little cast iron rads obviously sized for 180 degree water. If the downstairs rads only need 150 F. water and the thermostat is downstairs, the upstairs will never work properly unless zoned seperately, which this was not. I find myself recommending boxed radiator covers to cut down on the capacity of the big rads, although I hate to cover them up because I like how they look. The radiator valves are 80 years old and have long since seized up so throttling the flow is problematic. What plumber is brave enough to try and change the valves knowing they could damage an old beast and suddenly be responsible for replacing it. (Yikes!) I lay in bed last night imaging some kind of foil faced bubble wrap hood that could go over the top of these old beasts to further cut down on their capacity and further balance them out with the other zone. Is there any other ideas out there? WW

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Big Ed_4

Balance

Zoning and piping the the low temperture radiators to run on a lower temperture will be ideal.

One way to balance if the supply to the radiator is on the bottom is to fill the rad with air and bleed just enough.