pump on gravity system

Mike T., Swampeast MO

Here's a good link for sizing circulators in gravity conversions (from "Hot Tech Topics" here at Heating Help).

http://www.heatinghelp.com/newsletter.cfm?Id=125

Questions:

How many heated floors?

Has the underheated space <I>always</I> been underheated?

Which floor has the underheated area?

Does the underheated area seem to have proportionately sized radiation (given room size/exposure). Any evidence of removed rads?

17 GPM can carry a LOT of heat. The problem with gravity conversions is that with each floor above the boiler, the head loss was intentionally nearly doubled. The hot water wanted to rise to the highest level in the system and had to be forced to flow through the lower levels. This is the exact opposite when you originally pipe for forced flow.

Because the head loss in the piping is so slight, you have MUCH more water than flowed under gravity to overcome this numerically tiny but relatively huge difference in head loss between the floors.

If the owners are ready for an "update" sell them on the COMFORT and EFFICIENCY of TRVs. If they're ready for a serious update, sell them on a Vitodens.

My own gravity conversion (TRVs added) is working perfectly with only the little built-in variable-speed Grundfoss circulator. Old boiler had 230 mbh input. System is being served with only about 4 gpm!

harvey

what size pump?

We were called out on a hot water gravity system that had been converted to forced flow years ago. The owner said the place didn't heat evenly, one area was always under temp. He said he had vented the air from all rads. All piping exposed and nothing looked a problem. Dunham boiler 280,000 input 226,000 DOE. Grundfos UP15-42F. Looks like a standard install. He also wants to add two panel or CIBB rads to two baths that have electric htrs. Should the pump size be increased? This pump can only move 17gpm MAX. Should we move up to a UP26-64 30gpm MAX. Any thoughts?

PG

Ball Valves

Why not start by installing and adjusting flow with ball valves?

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greg_7

Maybe there is too much flow.

greg_7

Maybe there is too much flow.

jim lockard

Are we

Circulating or suckulating where is the pump in relationship to the compression tank. perhaps moving the pump so that it is "pumping away" may be the start of reworking your old gravity system.

harvey

interesting

Thanks for the reply, Mike. The link you gave shows the opposite of what you did in your house. Your place is 230,000 btu with 4 GPM and the chart shows 30 GPM.?????
The area that is low on heat is the front area both first and second floor. I was going to install balancing valves on the two risers off the boiler to direct the flow. Now, what about the pump? Am I mis-understanding you? BTW it's suckulating(pump on the return)

Mike T., Swampeast MO

It's the TRVs (thermostatic radiator valves) that reduce the flow requirement by introducing restriction. With a straight gravity conversion you need that extra flow to overcome the "backwards" gravity sizing. Even after the TRVs were installed I kept the B&G 100 that previously moved about 35 gpm through the system. With the Vitodens and the TRVs, flow is as low (or even lower) than when it operated under gravity.

soot_seeker_2

Related question

My house has what I believe to be a gravity system that was converted to forced flow. The old cast iron boiler was replaced with a Vitodens 200 as part of a rear addition/reno where several rads were added. Total count is now 14 rads (3 basement + 6 main + 5 second) Majority of the rads are cast iron with a couple of Stelrads on the top floor. The pump is a UP 15-42F. The issue we're having is that one of the Stelrads on the top floor of the two storey house is not getting any heat unless I completely close off some of the other rads. I've restricted the flow to the other rads as much as I can but I can't seem to get hot water to reach it. The rad is part of one of the original loops. The piping was extended to move the rad to another location and the elbows and turns that were added surely contribute to the extra friction. Should I consider moving to a stronger pump, like the UPS15-58F Super Brute or the UP 26-64F?

Steamhead (in transit)

Need to know

1- how many square feet EDR of radiation is on this system- not just the boiler's rating, and

2- whether any radiators were moved around and reconnected to the mains with undersized piping.

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Steamhead (in transit)

If the piping to the Stelrads

was reduced in size from the original, that's your problem. Those pipe sizes must be maintained for proper circulation. If the pipe connections at the Stelrads are smaller than the original piping to the original radiators, you'll have to replace the Stelrads with something that will allow proper flow into and out of it.

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