Adding zone relay. Advantage of Taco Expandable?

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COBRA_ESQ

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I am in the planning stages of adding a zone & circulator for new indirect DHW . A couple of years ago the following was installed;

Boiler: Slant/Fin Sentry S-120
Aquastat: Honeywell L8148E
Relay: Taco SR504-EXP running 4 circulator zones

My game plan is to make the indirect zone Zone 4 which will allow me to make it a priority zone. The current loop served by Zone 4 will be moved to a new relay and for discussion purposes will be Zone 5. Taco makes a non-expandable relay (SR501-4) and an expandable relay (SR501-EXP-4) which is almost twice the price. I believe that only if I use the expandable relay will the primary zone circuit over ride Zone 5. This is OK because it is for the basement which very rarely is used. Is there any other advantage to using the expandable model?

Please also see my thread about a bypassed end switch https://terrylove.com/forums/index....-and-t-tv-terminals-jumped-in-aquastat.62279/

Thanks
 
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Dana

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Unless it's a very large or very leaky house the boiler is way oversized for the space heating load, and even more oversized for any one zone. If the basement zone happens to be running and not suppressed during the priority zone call it will only barely affect the domestic hot water performance. There is no need to override that zone. How much radiation is there on that zone anyway?

How much radiation is there on each of the other space heating zones (separately, zone by zone)?
 

Tom Sawyer

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No advantage at all. Buy the cheaper one. For that matter, bug two zones on one relay.
 

COBRA_ESQ

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Dana - by "how much radiation", I assume you mean how many feet of base board. The four zones are 60', 38', 55' and 40'.

Tom - What do you mean by "bug two zones on one relay"
 

COBRA_ESQ

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I bought the house with four zones installed.

I isolate areas that are not being used, such as the basement zone which almost always off. The doors to the bedrooms and two main bath zone are kept closed and the thermostat for that zone is programmed to kick up at night and drop a few degrees during the day . An hour or two before we get up the kitchen/living room zone automatically kicks up and then drops back down until an hour or so before we get home. The last zone, mud room, den, laundry room and third bath is separated from the rest of the house by a door so that zone is kept low unless we will be using those areas.
 

Dana

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Dana - by "how much radiation", I assume you mean how many feet of base board. The four zones are 60', 38', 55' and 40'.

Tom - What do you mean by "bug two zones on one relay"

Here's the fundamental system design problem that crops up when it's cut up into so many zones:

The DOE output of the Sentry S-120 boiler is 101,000 BTU/hr.

Fin tube baseboard puts out about 600 BTU/hr per running foot when operating at an average water temp of 180-190F.

The shortest zone is 38' and only capable of emitting (600 x 38' =) 22, 800 BTU/hr, which less than 1/4 of the boiler output.

Even the longest zone is only 60', and only capable of emitting ~36,000 BTU/hr which is barely over 1/3 of the boiler output.

When serving any one zone the thing will short-cycle like crazy, and unless three zones (say, the 60' + 55' + 40' zones) are simultaneously calling for heat the thing is going to be doing a lot of on/off burn cycling during continuous calls for heat. This puts lot of wear & tear on the boiler, and cuts significantly into it's operational efficiency. With the higher frequency of startup cycling losses and low burn durations it is unlikely to be performing anywhere near it's stated AFUE numbers. Even with the three zones tied to gether it's only good for emitting ~93K, not the full 101K, but the thermal mass of the boiler & system water is high enough that it will still have decent burn lengths, with very few cycles per hour.

Only with all four zones calling for heat at once can the radiation can actually balance the full boiler output, settling in at an average water temp of about 170F.

Retrofitting a heat purging economizer control like the Intellicon 3250 or similar onto the boiler, bypassing it's aquastats can probably cut the the number of burn cycles in half to help some- might even save 15% on the fuel use, but it would be better to combine some zones as well. Since the basement load is probably comparatively low it's fine to let it short cycle on that one a bit for those times that you turn it on, but for the rest of house it's better for the oversized boiler to run it all as a single zone, provided that can be done with reasonable room-to-room temperature balance.

Longer term it's worth thinking about sizing the replacement boiler to the actual heat load of the house. Using mid to late winter gas bills and the exact meter reading dates, and a ZIP code by which we can find the nearest weather station for temperature data we can use the boiler as the measuring instrument for figuring out an upper bound for the true heat load. In most cases it will be under 50,000 BTU/hr @ +15F (a typical Long Island 99% outside design temperature) , and it may easily be under 35,000 BTU/hr. (How big, what vintage is this house?)

If the true heat load is under 35K it's possible to keep the zoning as-is if you right-size the boiler for the load, since even the smallest zone's radiation could then emit well over half the boiler's output. A right-sized modulating condensing boiler could do even better, and run nearly continuous burns (even as the zone calls come & go), once you dial in the outdoor reset curves.

If the boiler is fairly new and not likely to be replaced for more than a decade you might get more mileage out of installing a system buffering "reverse indirect" as the hot water heater instead of a standard indirect, slaving the boiler to the indirect's aquastat, and letting the zones sip from the buffering thermal mass of the reverse indirect. With 101,ooo BTU/hr of output you'll never run out of hot water when you do something like this:

Radiant.jpg


From the boiler controls' point of view it's only "zone" is the reverse-indirect. The controls for the other zones only gates the flow of the heating system water out of the buffer tank into the zone calling for heat. Even a 25-30 gallon Ergomax E23 or Everhot EA-8-50 would add a couple of minutes to the burns during continuous calls for heat, and can deliver over 120 first-hour gallons when it has a ~100K boiler like the S-120 behind it. It's a bit more expensive than a standard indirect, but it improves the boiler's operating efficiency and reduces wear & tear on the boiler by cutting way down on the number of burn cycles, lengthening the burns.
 

COBRA_ESQ

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I think I got it. I need a day to digest.

Might this explain why in the aquastat the thermostat connections (T, TV) are jumped and the end switch connections (X,X) in the relay are not wired, which as I understand it would leave the boiler to cycle on/off unaffected by the zone thermostats, except to the extent that the boiler cools from the loss of heat through the baseboard(s) that are running
 

Dana

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I think I got it. I need a day to digest.

Might this explain why in the aquastat the thermostat connections (T, TV) are jumped and the end switch connections (X,X) in the relay are not wired, which as I understand it would leave the boiler to cycle on/off unaffected by the zone thermostats, except to the extent that the boiler cools from the loss of heat through the baseboard(s) that are running

If the boiler's aquastat controls are jumpered to always seeing a call for heat, it will always be cycling to the high-limit every time it hits the low-limit independently of whether there's a call for heat from a zone thermostat. That maximizes the idling losses, making the net efficiency even worse!

The SR504 zone controller should be what's calling for heat from the boiler, and it should be set up for a "cold start boiler" (unless it has an internal tankless hot water coil that you've been using for hot water- which you won't need after installing an indirect, or reverse-indirect) See the SR-504 instructions.

Has the thing been on all summer? If yes, as a test, turn the basement thermostat way up for a half-hour and time each of the burns to the nearest 5 seconds, and count the number of burns you get in that half-hour with just one zone calling for heat. (Let's measure the baseline short-cycling parameters!)
 

Beads

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Will someone highlight the differences in design between the more common indirect tanks on the one hand and the Ergomax and Everhot on the other?
 

Dana

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With an Everhot/Ergomax the boiler's only "zone" is the tank- the heating zones all sip off the tank, and the boiler doesn't fire until the tank is calling for heat, which only happens after the heating zones or hot water draws have lowered it's temperature to the aquastat's setting. A zone can call for heat for several minutes and the thermostat satisfied without firing the boiler, only drawing down the temp of the tank several degrees. The thermal mass of the tank and the temperature difference between it's high & low temp defines a minimum burn time for the boiler independent of how much a zone might be emitting from it's radiation.

With a standard indirect the tank is it's own zone. The boiler fires whenever any of the heating zone thermostats are calling for heat, halting when it hit's it's high temp limit even if the zone is calling for heat, re-fires when the boiler hit's it's low-limit, repeat, until the thermostat(s) calling for heat is/are satisfied. On many/most systems the indirect is set up as the "priority zone", so that when it's calling for heat all other zone calls are suppressed until the tank's aquastat is satisfied, guaranteeing maximum domestic hot water performance.
 

Beads

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Thanks, Dana. I understand all that. I got the impression from your earlier post that the Ergomax and Everhot are somehow different from other indirect tanks. The only thing that I can think of that might be different is a larger heat exchanger relative to the water volume. I suppose I could look up that info for these two and for some other indirect tanks, but I was looking for some knowledgeable input (and being lazy).

My reasoning rests on the fact that an indirect HX for DHW only needs to transfer a reasonable amount of heat for the DHW with a reserve that can be drawn on. The HX for a reverse scheme needs to transfer enough heat for a similar demand with no reserve to draw on, just like a combination boiler or an instant water heater.
 

Dana

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Yes, the Ergomax and Everhot ARE different from other indirect tanks. A standard indirect has the thermal mass is the potable water, whereas the thermal mass of a reverse indirect is heating system water.

Both have a "reserve to draw on" for the potable water, namely, the stored thermal mass. But with a standard indirect the heating system has no "reserve to draw on", which requires the boiler to fire on every heating zone call. The fact that the boiler is ridiculously oversized for both the whole-house load and INSANELY oversized for the radiation means that the boiler will short-cycle itself into lower efficiency if the heating system doesn't have a reserve thermal mass to draw upon.

The heat exchangers in smaller reverse indirects are not significantly larger than those of standard indirect, but are up-sized for VERY high flow rates (higher than would be delivered by an instant on-demand tankless with a 199,000BTU/hr burner.) The "8" in the model number of the Everhot EA-8-50 is the potable GPM rating at a 90F temperature rise. If you need more than that, (unlikely) they have bigger versions (EA-10-50, EA-20-100, even an EA-50-100). Similarly the "3" of the Ergomax E23 is the number of internal coils, each of which is good for 3 gpm. An E23 can deliver 9-10 gpm continously, and can peak to 12 gpm or so, but they also make 4 coil versions for higher rate draws.) Turbomax is a similar product line in slicker packaging with a model range nearly identical performance model to model as the Ergomax lineup.

The heat exchanger on an E23 or Turbomax 23 has 19.6 square feet of surface area. A typical 30-60 gallon standard indirect will have 15-20 square feet of heat exchanger surface area (take a look at the last page of the SuperStor brochure for heat exchanger surface sizes). An E24 has 26.2 square feet.

There are standard indirect HW heaters and reverse indirects with heat exchangers much larger than that, but at your boiler size you wouldn't necessarily get better hot water heating performance out of them. An E23 or EA-8-50 is pretty optimally sized for a ~100K boiler, delivering about 175-200 gallons/hr of 110F (super-hot bath) water continuously, with enough thermal mass to support much higher flows on an intermittent basis (like a couple of simultaneous 10 minute showers at 4-5 gpm total draw.)
 

COBRA_ESQ

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Has the thing been on all summer? If yes, as a test, turn the basement thermostat way up for a half-hour and time each of the burns to the nearest 5 seconds, and count the number of burns you get in that half-hour with just one zone calling for heat. (Let's measure the baseline short-cycling parameters!)
Ran it for 30 minutes. The first time burn was 4 minutes, the second, just shy of the 30 minute mark, was 2 minute 35 seconds.
 

Dana

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A minimum burn time of only 155 seconds is an efficiency-robbing short-cycle. The initial cycle was a longer as it was initially heating up the thermal mass of the water in the distribution plumbing over a much bigger temperature difference, but even you could guarantee minimum burn times of 4 minutes it would take a measurable efficiency hit with a cast-iron boiler.

With just a heat purging economizer control it would "exercise" the thermal mass of the whole system over larger temperature swings, possibly taking those 155 second burns up to 4 minutes but adding some thermal mass to work with as well (such as a reverse-indirect between the boiler and the zone loops) to get the min- burn times well above 5 minutes would also be in order.
 

COBRA_ESQ

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The boiler is under 3 years old so the reverse indirect seems to be the way to go. I am not sure about combining the zones. How much further benefit would I get pulling out the circulators and totally re-plumbing down to a single zone as compared to keeping the temperature in 80% of the house lower at night and the den/mudroom down when not in use. The house is a long single floor ranch and is easy to section off by closing doors.
 

Dana

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Your efficiency loss due to short cycling is probably on the order of 10%, maybe even 15%. The amount you save by using overnight setbacks is maybe 7-10%, depending on just how cold it actually gets overnight.

Making it a single zone and keeping the rooms constant temp would likely reduce fuel use at least a little bit, since with all into one zone (except for the basement) you'd gain most of the efficiency lost to short-cycling back. It's still ridiculously oversized and won't make it's nameplate AFUE numbers though.

If you add the thermal mass of a central buffer you also gain most of the efficiency back, and it would also allow you to use overnight setbacks or turn zones off with some amount of fuel savings.

The problem you now face is finding a contractor who really understands how to design and optimize these sorts of systems. I gave up looking and re-designed my own system, but I'm an engineer and understood the risks. A heating system designer working for the contractor who installed it bet the contractor $100 that it wouldn't work (and ended up paying.) Most of the system optimization I ended up doing myself, after testing & measuring different aspects. YMMV. (In many ways your mix of radiation is simpler than mine to deal with- I have radiant floors in some zones, high-mass radiators in others, as well as a hydro-air coil in an air handler.)
 
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