Gravity near boiler piping question

Russ_10

I have a one floor gravity system I'm going to convert. I've read all of Dan's books, so I'm aware of the half the biggest pipe, then drop a size, pump away, etc. I'm going to post my final design on here to let the experts critique it.

I'm planning to do constant circulation, with TRVs. My question is how to hook up to my existing pipes, because I currently have two loops.

I assume I cannot tie one of the returns into the other supply due to the fact that return water will be the supply for the other loop. Can I just T the two supplies together and T the two returns together, then put the circulator "between" them. Or, should I do some sort of primary secondary pumping. The issue is that I don't know the best way to make my continuous loop without the Ts. I also don't know how successful it will be to pump into a T.

I've attached crude drawings to show what I'm talking about. The actual setup will be much more complex, with bypass, etc. but this will get this point across I think.

A second question, if a differential pressure bypass is used to allow for the TRVs, will another bypass also be required for temperature concerns?

Thanks

Mike T., Swampeast MO

The "t-setup" is the way I've always seen it done.

Note however that, as drawn, you show the tee in "bullhead" fashion. Many will say that "flow is unpredictable" with a bullheaded tee and that probably won't "split the flow in half" as it look like it would...

For that reason, many people often install one supply as the branch, the other as the straight flow path. Then for the return make certain that the side whose supply comes from the tee branch comes through the straight path and vice-versa.

With TRVs however it won't matter in the least how you pipe the tees.

Primary/secondary (or low-loss header) and need for system bypass will depend on your boiler and how it's controlled. With a conventional boiler that needs low temperature protection, probably the most cost-effective method is a ESBE type TV thermostatic bypass valve. It will keep all or nearly all of the flow going back to the boiler until it goes above 140F and will then let just enough flow through the system to prevent return temp from falling below 140. Unless required by the specific boiler, primary/secondary is not needed. You'll still need the differential pressure bypass (installed after the thermostatic bypass). It's possible to use a differential pressure bypass valve alone (you can adjust for some constant system bypass) but return temp protection will be far from assured. Older, oversized cast iron boilers connected to gravity systems seem immune to problems with low return temp, but they certainly won't operate very efficiently... With a newer and more appropriate sized conventional boiler, I believe I'd opt for the positive return temp protection afforded by the ESBE type TV.

If gas is your fuel, for efficiency sake you really should use a mod-con with a gravity conversion--virtually a no-brainer when you have TRVs... Such a system really is ideal for use with a Vitodens by Viessmann. In most cases it will be able to directly drive the system via the internal circulator and all you'll need in the way of "extras" is the differential pressure bypass valve.

John Ketterman

Water takes the path of least resistance. For gravity loops, the head is so small anyway that this may not be a problem. But if one is for upstairs and one for downstairs, you have bigger pipes going downstairs than going upstairs, which works well for gravity but is the exact opposite of what you need for forced flow. So if you connect them in parallel (tees), downstairs may get too hot. So you may need to balance the two loops with valves and flow restrictors.

Russ_10

Burnham boiler

I'll be getting a Burnham boiler, though I haven't gotten prices or decided on a model yet. I was looking at the Revolution II, which is a cast iron condensing boiler, which I think has a built in primary/secondary piping and injection control.

Also, they've just released a CHG model which is a Mod/Con with an aluminum HEX. I don't know a whole lot about either other than their product literature.

The house is 71000-83000 BTU heat loss using the Slant/Fin program, depending on how I define the current attic and wall insulation, which is somewhere in between the two probably.

And, the two loops just split the house in half, it's only one story house, and not zoned, just split in half. The current boiler looks identical to the gravity drawings in the Q&A section, with the two pipes out of the boiler.