Bridging short zones together at tstat-actuator to help with short burns ?

Dave Carpentier

I have a cast iron conventional boiler running always hot 130-150.
From that I have 7 slab zones of various lengths (one per room) mixed down to 110.
No buffer tank at this point.

The situation Im trying to eliminate is like this;
Boiler is at some random temp like 135.
Small zone calls and circ runs, boiler temp drops.
Boiler starts up but zone is now happy and turns off the circ.
Boiler quickly ramps up to 150 without a load and shuts down.

A modcon and buffer tank is in my future, but not now.

So I was thinking of bridging some zones valves together electrically. I was pondering which tstats to use as "masters" in the groups, disconnecting the tstats on the "slave" zones. But then I wondered.. should I just leave all tstats connected but bridge those groups, so that any tstat in a group will cause all in that group to flow (logically, it would be an "OR" circuit) ? It might take a bit of throttling of the flow to balance it out.
From the circ pumps perspective, I would have 3 zones.. the garage (long long loop) as one zone, the 3 bedrooms as a second zone, and the foyer/bathroom/laundry as the third zone.

Does this seem prone to headaches, and I should instead just pick one "master" for each group and disco the other tstats ??

hot_rod

Probably the thermostats in rooms where you sit or spend the most time should be the 3 masters. Wire the others to call on. It sort of depends on tube layout as far as which might group best

It’s a good temporary work around, grouping the small load areas

EBEBRATT-Ed

Why not just do the buffer tank now? You bound to overheat some zones doing that

Dave Carpentier

MikeAmann said:

What control is on the boiler?

I'd have to go down and check the model, but its just a simple aquastat with the TT strapped. Well, not completely strapped.. I use a Ranco to extend the delta down a bit wider (25deg instead of the 10 in the oem controller).

Dave Carpentier

Ed - a buffer would have been great to have, but would have been quite a bit of ripping out iron pipe. I elected to wait until the full rip-out of the boiler and near piping hopefully next summer.

Dave Carpentier

@hot_rod - so you're saying to disco the "slave" room tstats, I think.
You have 3 rooms (ABC) with different loads.
If you put a single tstat in A, rooms B and C could either overheat or underheat.
If you put a tstat in each room and wired them so any/all start the flow to all 3, then any room would never underheat ... but the other 2 could overheat.

Would it be easier to trim flow against overheating, vs over/under ?
Maybe it would be a dog chasing it's tail thing ? (throttle down B, but when it is in control, it overheats C, so throttle that down and then A overheats etc )

hot_rod

Do your manifolds have valves to balance out loops that could cause overheating, if you combine zones? You should be able to do some balancing to even out room temperatures.

You need enough flow to cover the largest load zone, so restricting flow to any overheating areas is an option. Still, it depends somewhat on the loop layout. For example a single loop that goes into two rooms would be hard to balance both rooms individually.

On the front end, the tube layout design, it is helpful to get rooms on their own loops for future balance flexibility.

Are you wired to a relay box, that might make combining zones fairly easy.

What type and brand of zone valves? Unless they are high power consuming valves like gold tops, you should be able to put multiple zone valves on the output in a relay box. The zone valves should indicate Va?

Dave Carpentier

@MikeAmann - Just a basic L8148A. Power in, burner out, and the Ranco on the TT line. (the Ranco sensor is on the outlet pipe just under the cabinet lid).

@hot_rod - Yes, I do have manual valves on the supply ports and they are used as throttles. I had to do that anyhow because of the various loop lengths during concurrent calls. The transformer for 24v is fairly big as I originally had 7 wirsbo MVA plus 7 tstats. Im down to 2 MVA now, as Ive replaced 5 with the thermal actuators.. I think they use less power ?

hot_rod

40Va is typical, if you have that you have plenty to power 5 thermal actuators.

For the past 5 years or more the thermal actuators have been 250mA, previously 800mA. So 7 or more on a 40Va transformer. It should tell you on the actuator.

Make a drawing/ schematic of how it is wired. Make the changes, see if it helps, or go back to the original wiring.

A lot less work and cost to wiring changes compared to piping changes and boiler replacements.

fentonc

I’ve been thinking about this for a similar problem - one way to avoid overheating might be to disconnect the zone valve switches for the “low priority” zones. They would still open (or close) based on the thermostats for their zones, but the boiler/circulator would only run when still-connected zone valves call for heat. A more advanced version of this could use delay-on-make timers on the zone valve switches, so they would eventually call for heat on their own, but would otherwise just run opportunistically. If the still-connected zones are still running semi-frequently then this wouldn’t help much. In the limit you could leave just the zone with the highest duty cycle to call for heat, and let the other zones run as needed while that one is running.

Dave Carpentier

@fentonc Great idea. With my approach, a cold zone would have to wait until the master for that group called, opened the valves for that group and turned the main circ on. With your approach, a cold zone would only have to wait until any other zone in the house called for heat. It can turn the heat off for that zone, but it cant initiate a solo heating zone call. Nice !
All I have to do is disconnect the end-switches from my short low-priority zones !

hot_rod

That is a clever idea @fentonc a design day might be when it could fall short, but an easy enough wiring tweak to try.

Dave Carpentier

A followup. @fentonc 's idea is working great so far.
I disconnected the end switches on my 2 smallest heat load rooms. They both have single exterior wall exposure.
There's no danger to overheating, as those zone valves will close once the tstats are satisfied.
I think the risk to underheating during a cold day are minimized, as the "cold day" implies that other zones will be open (that activate their own end switches). The mass of the slab is key. This would work less well with low-mass heating (rads, etc).

fentonc

Nice! I might try that for my basement zone this winter. It's got fin-tube baseboard, but I care a lot less about temperature swings, and the other zones are usually running for at least several hours a day.