5 zone system with small zone valves

Mitchell · May 2010

System is 2-pipe pumped returns. The piping to and after the motorized zone valves is large and correctly sized but they have a welded on cone reducer to connect to the smaller valve. Something like a 3" line reduced to 1" for the valve and quickly back to 3" to feed the zone main. The system runs ok, just wondering if it would run significantly better with full sized valves? They wouldn't be all that easy to replace given the welded together design.

The zones each feed separate large open areas of a church each with their own programmable thermostat, any of which can turn on the boiler. It is rare that all the zones are open at once, except in the morning after the setback, so the boiler is usually pretty oversized for the connected load. We are replacing the back-up boiler soon and they're hoping to make it the new primary. Should we size assuming only some of the zones are connected at once and then kick in the other one if more zones open? Should we make it so that it takes say at least 2 zones open before any boiler fires? It would be really nice to have a separate boiler for each zone but that's not practical.

Any ideas?

The Steam Whisperer (Formerly Boilerpro) · May 2010

ideas,

With two pipe steam it is possible to downsize radiation to the heat load so a smaller boiler can be used. In addition, with mutliple zones, it would be a very good idea to use a modulating input steam boiler. The idea of using a smaller boiler that can handle a couple of zones at a time is good, but could cause heating imbalance if too many zones are open at once. This imbalance issue can be addressed by a steam professional, though, with proper tuning of the system or additional controls.

nicholas bonham-carter · May 2010

church setback

there has been a lot of discussion here about setback, and economy of running. the prevailing theory seems to be that the fuel used coming out of a setback of more than 3 degrees would be the same as maintaining a constant temperature [unless for many days].

the modulating burner, controlled by pressure, would enable the system to compensate for varying zone operation. maybe some sort of thermostatic valves on the radiators would do better than the present setup.--nbc

Jamie Hall · May 2010

It would be much better

if those valves were full port. Reduced port valves on saturated steam can cause a lot of headaches, and may be costing a lot of steam, oddly enough. So the fundamental answer to the first question is yes, it should run significantly better with full port valves.

If you reduce the size of the boiler based on running only certain zones together, you will be limited to that. Your controls will have to be arranged so that they can't all come on together! This wouldn't be a consideration -- except coming out of the setback (most churches I've seen run pretty cold during the weak, then up for the services). Coming out of the setback is going to be a long slow process, as you will have to bring each zone up separately, one or two at a time. Not to say it can't be done, with the right controls.

Mitchell · May 2010

Thanks for all the responses

I'll look into making those valves full port, like I said it's going to require some welding work, but I agree it should make a difference. I really like the idea of using a fully modulating burner. If we do decide on a smaller boiler (limits number of zones at once), it wouldn't be that hard to add controls because all the thermostat and zone valve wiring comes together in the boiler room (relays control the valves, thermostats are just an input signal).

Say we got a boiler sized to operate up to 3 of the 5 zones at once. Does anyone know roughly how much efficiency gain there would be in operating those 3 zones with the correctly sized boiler rather than just down-firing a larger boiler that is sized to handle all the zones at once? Other than coming out of setback it is rare that more than 3 zones call at once, so most of the time there wouldn't be zones waiting for a slot in the 3-zone lineup. I'm just wondering how much more efficient it is to fire a boiler at rated load rater then under-firing a larger one to get the same output.

Unknown · May 2010

Mitchell,,,

You have not said what steam pressure is needed to get to those rads.

Are these mains pitched & sized properly for the EDR load?

Depending on where these ZV`s are in the piping scheme, are they dripped at the inlet?

Quite surprised no-one asked that.

Mitchell · May 2010

piping and pressure

About 2.5 to 3 pounds is enough to get to the last radiator on the longest run. I checked the mains and they're sized correctly except for the valves (hence the need for a little more than 2 psi), and have good pitch to the F&T traps at the ends. There are no water hammer problems ever, all the traps were recently replaced and the zone valves are properly dripped believe it or not so that any condensate than flows toward them is removed via and F&T trap or a thermostatic trap with a cooling leg. The zone valves are about 10' after the take-off from the header for each zone. There have been zone valves as long as anyone knows, so they're not new.

Unknown · May 2010

Mitchell, I am curious because,

The Empire State Building ran a little higher than 2PSI(I`m told),,, what makes your situation so different?

The Steam Whisperer (Formerly Boilerpro) · May 2010

It depends....

If you have a good modulating burner that can keep excess air low through most of its range and a boiler than can scrub heat from gases at low firing rates ( firetube boilers with turbulators are probably the best, but I'd check with others), your firing efficiency will rise at lower inputs. I have found that most boilers tend to fire more efficiently below full load (assuming good excess air control) because there is simply more heat exchanger area for each btu. Many seem to peak out at about 60% of max input, but I have seen a number of old Kewanees steamers firing at very low rates (probably 1/4 of rated input) and have firing efficiencies approaching 86%.

Steamhead · May 2010

Have you measured the pressure

down-stream of the zone valves, to see exactly what pressure the system is getting?

Mitchell · May 2010

pressure drop across zone valves

I haven't measured it but I guess that is really all that matters. It would be good to know before deciding to replace them. By the way, how much fuel savings is there from being able to run say 2psi instead of 3? I know you have to get everthing hotter but that satisfies the thermostat a little faster too.

So it sounds like a boiler sized for the full load (all zones) with a good modulating burner to handle the lighter loads would be the best way to go. Any recommendations? Full load is around 1,000,000 Btu/hr.

The Steam Whisperer (Formerly Boilerpro) · May 2010

Take a look at the Rayes boiler....

I haven't seen one in person yet, but the burner on top concept is exceptional and follows long standing good practice for efficient heat transfer characteristics. http://www.rayesboiler.com/ Typical burners can modulate down to 3 to 1, but I have seen some capable of 11 to 1. Rayes does have a number of burners available with the boiler.

5 zone system with small zone valves

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