Constant Circulation again

Pwall

I have been reading a lot here and on the web about constant circulation and there seem to be a few things people just skip over when talking about this type of setup and of course it's where my biggest questions arise. Assuming a mix of fin tube baseboards and panel radiators and a conventional oil fired boiler.
Sorry about the rambling questions and comments.

Is primary/secondary piping the best piping layout for this type of setup or just pumping straight through the boiler going to work as well and be simpler as long as it's a low head loss boiler?
Do you run a fairly wide differential, say 20-30 degrees? As a means to limit short cycling.
Is a manifold system with TRV's a better way to control flow or would a single loop with TRV's work as well? Keeping in mind the temp drop at the end of the loop when sizing.
The OAR controls the water temp, the TRV's control the heat output of the emitters, the pump speeds up and down assuming a variable speed pressure regulated pump. You jumper the TT and use the WWSD to enable or disable the pump through some sort of control interface.
Would you run the indirect water heater as priority or let it's pump cycle off and on as demanded by the aquastat with the boiler connected to override the OAR while the indirect is calling forcing the TRV's to regulate the hot flow into the radiators.
I'm assuming piping insulation is a must to limit heat loss while water is just flowing around bypassing various things not needing that heat right now.
There is always the comment about this is how they have done it for years in Europe but finding piping info and control ideas seem to be hard to find, unless I'm not asking the right questions of Google, and suggestions?

EBEBRATT-Ed

If I had my way everything would be primary/secondary. With a CI boiler I would try and keep the TD around 20 deg. I would pipe the indirect as a separate zone and put no heat in the heating loop for the TRVS to regulate unless there is a call for heat. Flowchecks should prevent this or zone valves. Weather the indirect needs to be priority depends on dhw load, boiler size etc can it keep up without priority

hot_rod

I think the primary goal of P/S piping was to provide a hydraulic disconnect for various circulators in the system. And to allow heat takeoff from a line anywhere you applied closely spaced tees.

i think Gils original intent was for large buildings where a single pipe loop could run around the space allowing connections at each heat emitter. Or heat input at tees.

It came into wide acceptance in residential hydronics when Munchkins arrived, as they really needed a dedicated circulator, in some sizes a high head circ.

If you have a wide open, low pressure drop boiler like a cast iron steel. electric, tank type, or some of the fire tubes, really no need to use P/S.

Keep in mind with P/S or a hydro sep, you have temperature blending going on based on flow rates. The boiler could run up to 180 for instance, but the distribution may not ever see that temperature depending on A and B flow rates.

Or you could blend the opposite and return warmer water to the boiler, lowering efficiencies.

This handy Excel that a wallie created calculates that blend condition, if you know some flow rates and input temperature.
It applies to P/S closely spaced tees as well as separators, called the mixed temperature formula.

In some cases IF the A and B side flow rates are exactly the same, boiler SWT and system supply temperature would be identical, no mixing going on in the sep.

Pwall

My biggest concern I would guess is circulating low volumes of water through a relatively low volume but low flow resistant boiler.
If the water cools down rapidly then the boiler is going to be firing frequently in short bursts trying to maintain a target temp. The emitters won't care but I can see fuel consumption would be high.
It seems that heating a buffer tank would be better but those can get pricey.
Wouldn't it defeat the purpose of TRVs and constant circ to not provide any heat to regulate?

hot_rod

yeah, ideally your modulation would screw down to those low load conditions. You would want to look at the smallest load, determine how often, if ever, that one load would be calling, see how it matches.

Ramp delay also helps, by keeping the boiler at low fire for determined periods.

Also the reset control on the boiler would also lower SWT on milder days and help eliminate cycling.

It always comes down to what type of cycling you are comfortable with, a buffer could be designed to any criteria as far as that goes. Fire once a day with a multi hundred gallon buffer.

The load(s) are rarely static, they can change minute by minute.

TRVs, and an accurately sized heat source, reset combined can get you real close to constant circulation.

A tank type heater, like HTP products, modulation, Alpha circulator and TRVs on panel rads and some radiant would be a nice system. or a mod con with separate buffer and build your own high volume condensor.

Many ways to skin that cat.