Hot Topics: Perma-Circ and Outdoor Reset

A plumbing system with valves and meters

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Original Post: Perma-circ and outdoor reset

percher Member

I'd love to get the benefit of some hydronic guru experience on a couple of questions...

Our house has a 20-year-old WM GV-4 Series 2 gas boiler with 2 fin-tube baseboard heating zones and an indirect DHW zone, all run on the boiler's single Taco 007 circulator. There's also a bonus un-valved zone I'll get to in a moment. We've owned the house less than 2 years, our first with hydronic heat, and I've been learning a lot from fixes and upgrades -- replacing the DHW control, the circulator and an air scoop; adding a transformer and wi-fi thermostats; tidying up the 24V wiring. But I'm a little puzzled by the 24/7 circulator setup.

The thermostats in the house and the DHW control are wired to the zone valves, and there's no electrical connection from all that to the boiler. The thermostat input wires on the boiler's UT 1013-10 control are joined together to keep the circ running, and the burner fires up when water temp hits ~160 (it shoots for ~190, usually tops out around 205). With the short (~12') un-valved loop adjacent to the boiler, the circ doesn't deadhead when the 3 zone valves are closed. And that loop helps warm the garage, which, despite having no dedicated radiating units or thermostat, is sometimes the warmest space in the house. The overall system seems to work just fine, though it doesn't seem especially efficient.

So, what's the advantage to running the circ full time in this way? Might it have to do with hard well water (14 gpg; 281 ppm dissolved solids)? Or just an extra defense against freeze-up? The old circulator leaked at the flanges when the system was powered off and cooled down, but that seems an unlikely reason in itself. Ideas?

My other question depends on whether this perma-circ thing ain't broke and needn't be fixed. I found a Tekmar 260 for a good price and want to see what outdoor reset might do for us in the shoulder season (summer). I expect benefits to be somewhat limited by DHW demand and minimum temp for the non-condensing boiler. Tekmar's application sheets seem pretty clear about how I should wire it in for on-demand circulation, but I've not yet found a good description of how to set it up for full time. I thought I saw a post somewhere about providing constant power to the Boiler Demand terminals and wiring a thermostat in series on the Boiler-TT line, but I'm not sure how good that idea is or whether I grasped all the implications. It does sound like there'd need to be more gear (ZVC) in there for the dual zones. If the answer to question 1 was "keep her spinning," any suggestions on how to do it with ODR? Any hot tips on integrating the ODR with the current controls in any scenario would be appreciated, too (e.g., settings for the built-in and backup water temp limit switches).


Steamboy Member

When I started in the heating business in the late 1960's almost all hot water systems utilized cast iron radiation, with either standing radiators or recessed convectors. The idea then was to run the circ pump 24/7 since there were no zones and all rooms were equipped with an almost exact size radiator to suit the heat loss of that room. That is how the systems were engineered by the old timers. The large volume of water in the system from the old large boilers converted from coal to natural gas or oil + the large steel piping and cast iron radiators would benefit from a circ pump running 24/7 as long as the water in the system was above 110 degrees. Remember, cast iron radiators heat by radiation, conduction and convection.

Today, all that is changed with the advent of finned tube radiation, copper tubing and very small boilers. Once the burner shuts off, the water in the system cools rapidly, so there is no need to run the pump for any extended length of time. Everyone will have an opinion on how long the pump should run after the burners shut off and a lot depends on how long that person has been in the business. Old ideas die slowly.

Now, let me add an opinion, I never liked boilers with DHW coils in them since the boiler had to run 24/7, 365 days per year. I felt that was excessive run time on a boiler capable of heating a whole house even during the summer months. It was OK in the winter as long as the house did not overheat but why run it in the summer? Instead, I preferred to have an electric, gas, or oil fired DHW tank for summer or for both summer and winter.

I never used a Tekmar control so I can't give an opinion on the use of that device however outdoor resets work great when needed. I always preferred a simple system that heated well to one loaded with a lot of wasted extras. As far as being on a well, once the boiler itself is filed with water it does not require any more water supply to it since the water in the heating system is in a confined space (the water is trapped and never changes). Only the water in the domestic coil changes. @SPOTT will be along shortly to offer his ideas from his area of the country. Hope this helps.

AllanJ Member

There is no reason to run the circulator 24/7 in any system regardless of boiler size, building size, or radiator style.

An outdoor thermostat may be installed to shut off the circulators when the outside temperature is above a certain point to help save energy when there are randomly interspersed hot days and cold days in spring and fall, without having to manually change the boiler between summer and winter modes multiple times per week.

If a boiler exterior housing were insulated as well as an ordinary water heater, then there is no advantage to having a separate water heater compared with using the boiler for domestic hot water.

A "dummy circulator zone" with a short pipe from the output manifold to the circulator and back to the return manifold is useful to churn the water in a boiler so freshly heated water is percolated through the domestic water heating coil, but this does not need a lengthy loop of pipe without radiators throughout the house..

percher Member

Thanks for the comments!

One more detail, just for fun: the DHW tank is a 41-gal Amtrol Boilermate.

I haven't used the term "continuous circulation," because it seems to suggest what I think Steamboy described, where water is always flowing through the heating loops. It sounds like there are some benefits to that with lower water temps than are available to me with my current boiler and piping. And it does seem rather excessive to run a whole-house-heating boiler just for hot water in the summer (though we can have space heating days scattered through the summer, too). Something I'll have to have a look at when replacement time comes.

As for the well water, indeed, none's getting added on the regular, but I'm sure it's what was used to charge the system initially, and I wondered if there might be long-term effects on even low-oxygen parts like pump and mixing valve. Parts with air/water contact, like the air scoop I replaced, do get mighty crusty.

Not sure what you mean, AllanJ, about the dummy circ zone and the DHW. The little 12-foot un-valved loop just makes its turn out in the open next to the boiler. Seems to just give the pump somewhere to push water when the zone valves (incl. DHW) are closed (and cause the boiler to cool faster than it would otherwise).

spott Member

From your initial statements I don't see the need for constant circ. Your stats go to individual ZV's and then from the ZV's should go to TT on the boiler control or aquastat which would start the burner and pump on demand for heat or hot water.

Constant circ is used in commercial buildings by design but depending on your piping layout serve little to no purpose in residential. They may have had a reason since they had the forsight to put in a bypass loop so the pump would not deadhead causing pump burnout.

As far as ODR's go for residential systems in my opinion they serve very little purpose and little savings. The new boilers today have such low water mass combined with the high intensity heat of the new burners, to me it's just another gadget. A normal 3 section cast iron boiler holds about 12 gals. of water and heats in just a few minutes. How much faster is it going to heat if you cut it by 20-30 deg. In my opinion you will be old and gray like SB & ME before you get your money back.

Commercial buildings are another story. I believe in those settings they are invaluable and should be used.

Pics of your system and controls and wiring would be very helpful to determine what you have.

Just my thoughts, hope it helps a little.

percher Member

Thanks for the thoughts, spott! I agree there must be a reason this thing's set up this way, just wish I knew what it is so I don't make a mistake someone already figured out how to avoid. Maybe it's just for peace of mind when away from home in the winter...? Assuming a circ that's always on won't fail to start...? Definitely a new anxiety I have since parting with forced air is having the heating system itself freeze up.

I do wonder how much I'd really gain from ODR given the boiler temps needed to prevent condensation and make hot water. And you're right, the boiler doesn't take long, only about 2 1/2 minutes, to hit 200 from 160 (with no active call for heat or DHW, anyway).

I'll toss in a pic and a couple of wiring diagrams (the one on the boiler jacket and my off-boiler 24V sketch), to see if they spur any other suggestions. Cheers!

A boiler with pipes attached

Schematic for a boiler system

Hand drawing of a boiler system

Hand drawing of a plumbing system

Steamboy Member

In my original rant, I was just informing you on how the old hot water systems ran when the boiler, piping and radiation held a massive amount of water. With the newer very small boilers, smaller piping and finned tube radiation, there is no need to run the circ pump 24/7. On those old systems the thermostat ran the boiler and the pump ran any time the water temperature was above110-120 degrees. ODR's do work but the amount of savings and the break even point is questionable. My normal work was on schools, hospitals and industry but we did a small amount of smaller units.

percher Member

Maybe the constant circ loop is there to keep the return temp up?

This boiler has an internal mixing valve that was deprecated by WM 15 years ago. A 2004 service bulletin says parts/replacements were no longer available and describes plugging the valve bypass or replacing it with an external one. Maybe our valve failed or someone just didn't trust it and was concerned about return temp...?

spott Member

By the boiler pic you have, ZV's on the return side and your bypass loop is just going from the supply to the return, so it's not there to keep the pipes from freezing because it doesn't circulate through the system. Are you saying that your pump runs 24/7 keeping the boiler running when there is no demand for anything?

On your two taco ZV's, two & three on the ZV's should go back to TT on the boiler control and on the HW, the red two should go back. These wires are what activates the boiler and pump on a call for heat or hot water. That boiler should be a cold start boiler because there is no tankless coil.

percher Member

It's hard to tell from the pic, but the ZVs are actually on the supply side. And indeed, the bypass loop just makes that little trip around the ceiling next to the boiler. I was wondering if it's there mainly to elevate the return temp of a cool slug of water that would come after one or more of the three zones have been shut off for a spell. Boiler protection rather than freeze protection.

And yes, the pump runs continuously regardless of calls for heat or DHW. Room thermostats and the DWH aquastat control the ZVs, water temp controls the burner. No electrical connection between the ZVs or their controls and the boiler control. The ZV wiring you describe is certainly what I would do if I converted it to cold start. Which I'm still trying to decide whether or not to do.

spott Member

On the face of what you said there is no need for constant circulation. There are different reasons for a bypass loop. We used to install them to temper the return water when replacing a high water content boiler with a new low to prevent shocking the boiler and possibly cracking the sections but even then there was no advantage to running the pump 24/7.

The way you are presently wired, your ZV's are wired to only open on a call from the stat and nothing else. They are using the aquastat apparently to operate the boiler and pump. It doesn't matter what side the ZV's are on. The AMTROL is the only INDIRECT that comes complete with its own wiring and doesn't need an aquastat like the other brands do.

The way your bypass is set up, it is not to prevent freezing but most likely to temper the return water to prevent shock.

What is that aquastat being used for in the pic and what do you have for a boiler control where the zv wires would tie into?

percher Member

That strap-on aquastat appears to be a safety backup, wired in series with the internal limit control. It's set at 200, whereas the internal is set at about 185. I think the latter is a Honeywell L4080D1226, with a 30 deg. fixed subtractive differential.

Attached a stock photo of the controller in this boiler.

I was thinking the always-on pump would make the bypass loop more effective at reducing boiler shock. If the pump is off, the bypass will cool down along with the zone loops. Then, when the pump turns on again, hot water will have to make it 12 feet around the bypass before it tempers the return water. But I don't know whether that delay is significant and worth keeping the pump going. Not having the pump on full-time would certainly make adding the Tekmar 260 pretty straightforward.

Regulator chip for a boiler

spott Member

It would make it more effective in reducing shock on a demand for heat or hot water but there is no sense running when nothing is calling.
Just my thoughts.

percher Member

Thanks -- I appreciate the time and consideration!