Gravity conversion

DaveGateway

I have a gravity system that was converted to circulation about 20 years ago. The install at that time was improper. I'd like to reuse the existing piping with a primary/secondary system and TRV's. 1. Is this a good way to go? I've researched the controls end of things and it looks like an injection pump setup would work. I would tie the 2 large (2-1/2") supply and return loops together as a primary loop and put an injection pump in to feed it. 2. Are TRV's a good way to control rooms and would the cause a flow problem (all radiators are paralled to the basement supply and return mains). It looks like Tekmar makes a nice control unit to take care of outdoor reset,DHW,etc.
I did 2 heat loss calc.s and took the EDR measurements and they came out within 4000 btu's of each other. Thanks for any comments.

Mike T., Swampeast MO

Sounds like

you have a good handle on the situation.

Don't forget to size down the gravity mains to give the circulator something to "push against" lest it work its little heart out. The rule of thumb is ½ the main size minus one size farther so you'd use 1" with 2½" mains.

TRVs essentially eliminate balance problems in gravity conversions and achieve their tightest control when circulation is constant with the valves installed on all radiators. The only flow problem with TRVs is that during warm weather they can all be closed and you have to pipe in bypass (a differential pressure bypass valve is good) to prevent dead-heading the circulator.

Remember that some conventional boilers have primary-secondary built in and that most condensing boilers eliminate the need altogether. Many condensing models have really slick "reset" built-in as well. These factors can help somewhat with the high initial cost of condensing boilers as you don't have anything else to buy.

Joel_3

yes

getting towards constant circ with outdoor reset is the way to go. i've done a few with tekmar and they work well but only if the boiler is a couple years old. If it's 20 plus we would recomend to junk it and then I'd use a Viessmann which is made for large water content systems and has all the controlls you need from 1 source.