Prim/Sec??

Dave_22

I have a question about a Prim/Sec. circuit like the one listed below. It is said to pipe the lower temp. circuits further down the loop because of lower temps. This is due to the other zone returns mixing back in the primary. What if the upstream primary zones are off? Wouldn't the lower temp zones then see the full boiler temps? Is a mixing valve usually installed on the low temp zones for this type of situation? thanks ahead for any responses

TGO_26

you still need

mixing devices for the low temp zones. The mixing is figured with the high temp zones running, the avalable water temp for the lower temp zones is calculated using the formulas shown. The high temp is taken off first so you get the required temp for indirects, fin tube baseboard, fan coils or anything else that requires high water temps.
Hope this helps

hot_rod

A 3 way

thermostatic mix valve would be my choice for a small low temperature load. It maintains a constant output regardless of the temperature it sees in the primary loop.

The middle load in your pic looks to be a panel radiator. Not sure how hot you would want to run them. 180 may be a bit warm for safety. Probably be best to mix that down also.

With a little repipe and proper emitter selection & sizing you could mix the high temperature via reset control and let the lower loads "float along" with 3 way manual mix valves.

hot rod

Dave_22

Thanks, I understand why the high temp zones are first- just a little fuzzy on what happened if the high temp zones were off and low temp were on. It seems like the concensus that you still need mixing valves. I like the idea of manual valves and letting them "ride" with outdoor reset controlling the Prim. temp. Thanks again to all.

hydronicsmike

a Modulating Mixing Device...

...will give you the temperature you need, regardless of the boiler water temperature if operated by an outdoor reset control. When sizing the mixing device, always keep the temperature drop in mind as you go down the primary/secondary arrangement. IE.: 1st primary/secondary take off boiler temp = 180°F, 2nd = 170°F, 3rd = 160°F.

If you prioritize your primary/secondary take-offs based on design requirements, you generally end up with a smaller mixing device. Anything I design is done that way and never had any problems.

In my opinion, a thermostatic mixing valve is NOT a hydronic heating item. It’s for Domestic Hot Water.
The thermostatic mixing valve always gives you the same water temperature regardless of outdoor temperature. The heatloss of the building or zone is related to outdoor temperature change. If the thermostatic valve is set, it is set to give you the water temperature required at the coldest day of the year, which is when the heating load is at 100%. When it gets warmer outside, the heatloss goes down and therefore the heat requirement. If you still supply the same water temperature during milder loads as you supply during the worst case scenario, you will end up with temperature swings in the space and expansion noises in the system. Especially with Fin-tube Convectors (Baseboard). Shorter zone On times and longer Off times of the zone are the cause.

I like Siggy's idea about the manual mixing valves and letting them ride along with the rest on the boiler(s), but does it give you any boiler protection if you need it?

So far, I haven't had any boiler problems caused by thermal shock or condensation, but I stay away from manual mixing devices or thermostatically controlled ones.
If you use the 'riding reset method', I suggest you do that after (in series) with the first MOTORIZED mixing device. At least then we can get boiler protection by modulating the mixing device as needed.

Mike

PS.: Please ignore my spelling and grammar......those immigrants, I tell you. ;-)

Jed_2

Proportional Secondary Mixing

Mike, I think Siggy's point was that you could use proportional mixing off an already controlled mixing device, which has boiler protection built in as a protection limit. And there are limits to the amount of proportional secondary mixing you can effectively employ.

The example above, I believe, was simply to provide the necessary engineering tools to be able to calculate the temp drop across a primary circuit, at design conditions, using a 4-way motorized mixing valve, which would have sensors controlling it's modulation. I believe in Siggy's designs, and his efforts to provide the necessary tools. But you have to study them, and understand what it is he is saying. If the 4-way mixing valve in the example was not motorized, it would perform as a proportional mixing valve, only if there was boiler reset(still with boiler protection limits), and it would have to be set DURING the coldest time of the year to be proportionally accurate.

Questions and more questions, but the example above would not jeopardize the boiler as drawn. The 4-way is shown as a motorized modulating valve, and would have sensors to control its travel.

Jed

P.S. In the example above, I would not call that low temp zone a "small" load relative to the total system load

Dave_22

I guess what I was trying to get at is, for example:

If zone 2 is spec'd out to have 149.6 inlet water- what happens when it is on and zone 1 is off? Now it would have the full 170 deg. boiler water. Same for zone 3 if 1 and 2 were off. I guess this is what confuses me on a system like this. You could use mixing valves, but then I would think you wouldn't even have to pipe it with closely spaced tee's in the given arrangement of the example. I do like the idea of having everything off a common supply returning to a common return. Then put in manual mixing valves with an outdoor reset and let them "ride" with the primary temp. I plan on using a fully condensing boiler, so i'm not too concerned about return temps. Thanks for all the "schooling"- I can't think of a better place to figure it all out!!

Mark J Strawcutter

without the closely spaced Ts

you'd have flow in the secondary even if the secondary circ was off.

Mark

hydronicsmike

right,

I agree. I just wanted to stress the point that one mixing device should be motorized before you do the fixed stepdown reset with manual valves.

I strongly believe in Siggy's methods and strategies. He's a mentor. I don't think there have been very many articles that he wrote that I didn't read.

Its good that so many great topics are discussed here. Am very excited about WETSTOCK when all this is discussed at round tables.

Mike

Arthur

Prim/Sec

The returns should always go back into the main AFTER the last flow has been taken out, This way the flows to each circuit is not cooled by the returns coming back from the previous circuit.

PJO_3

That wouldn't be primary/secondary...

Arthur,

What you described is not primary/secondary piping...I believe the secondary circuit would definitely affect the primary (and vice-versa) if you did that.

Having closely spaced tees in the secondary connections to the primary cicuit keeps the two piping "systems" uncoupled from each other. Yes, the temperature will lower through the primary this way when all zones are calling for heat...that's why the lower temp. zones are later on in the series.

What the heck am I doing awake right now here on the east coast of the states, you may ask? I am searching for some last-minute details as Glenn (my tech) and I rip out my own near-boiler piping this morning to do...primary/secondary piping with an Ergomax to feed my radiant and other fun stuff. That's for another thread.

Take Care, PJO

Dave_22

How about a system that looks something like this????

Mark J Strawcutter

cross-over

what about it?

still primary/secondary, still closely spaced Ts, cross-over bridges give you same temp primary water at each secondary.

Mark

Dave_22

Yes, I think this setup with manual mixing valves on lower temp loops "riding" the Primary on outdoor reset is a decent setup. Any thoughts??

Arthur

Primary/Sec

Obviously your definition of Primary/Sec is different to mine. Don't see the point of putting the flow n returns close together before the next take off as you feed cooler water to each next circuit. Cann't see how the sec circuit will affect the primary circuit as the pump is circulating the water past both connection points at near enough to the same pressure.
Over here we've always done this way and never had any
problems .Actually if Dave continued the loop around to the right in a complete circle you would have the same setup we use.Never had a problem with this setup.
As far as 3 way valve go these are used with weather/temperature compensating controller so that the water temperature trys to equate with the heating requirements of the building.Several different brands around Honeywell,Landis & Gyr, Staafa, etc