Gravity conversion - doing it right?

Andrew Stern

If anyone feels qualified to answer, I'd appreciate some advise on converting my 1928 gravity (former coal, now gas) boiler (the size of a small Buick)to forced circulation using an "acquired" boiler that sizes about right for my 3-storey building heat loss and rads. I intend to tie my 2 system feeds and two returns together to run to the new boiler

I've done my share of homework on thre topic (heat load calcs, lots of reading and calling around to mfg reps). The step-up I'd like to go with (which I think is the recommeded system to save some costs)is a primary/secondary system with an outside setback t-stat, but I have some questions.

1. I think I'd like to use a TECO 007-F5 for the system loop (secondary) based on sizing discussions and advice I've read, but am not sure how to size the primary circulator. How much flow should it need. I'd think the head requirement should be very slight.

2. Is a return by-pass loop within the primary loop still needed to assure tempering of the return water to raise the return temp to the boiler, or would you need to provide some flow in the primary loop when the secondary circulator kicks in. I'm really confused about this.

3. How does a setback t-stat know what temp to set the boiler to for a given outside temp. Is there a fixed relationship or do you have to calculate it somehow.

4. I plan to use two flow control valve on the system loop to prevent gravity circ till the pump kicks in, and lots of isolation valves to allow changeouts. Can anyone recommend a mfg/model for the flow control valves and type of isolation valves to use.

5. I'd like to size my expansion tank but can't figure the best way to determine the amount of water in the system other than draining it into 5 gallon buckets (whew) and subtracting for my estimate ofg whats in my boiler (???). Anyone know a better way. Of course all piping is concealed (except for the basement).

Sorry for the long post. This site's great.

Andy

Big Ed

Gravity

1.. A Low volume on the primary , I would use a Grunfos..Would have three speeds for adjustment.

2. A boiler by pass is not needed and the secondary pump would be just about always running with a out door reset.

3. the setback would be in the out door resets control.Should have a timer or clock to follow a drop in the heat curve.A thermostat is only use as a high limit. A clock thermostat cound be used on secondary zones. The main or largest load -zone ,you will follow it's heat curve in the reset's program..The secondary or smaller heat load zones slave off the mains heat curve....Hope I did not confuss you on this one.

4. Good ball valves for Isolation and B&G or Taco for the flow checks..

5. Sure way .......first shot do the 5 gallon bucket thingy and bump up one size...or start of with two #30's on a small two story gravity, Two #60's on a large three story and add on a extra tank from the same location if you get more then 10# on temperature rise from cold boiler to it's limit.