Boiler Circulator Pump question.

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DEWFPO

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First, I want to apologize for the length of this post.

Background :

Our house had a 5 zone hydronic baseboard heating system that uses 5 thermostats with one main Taco circulator pump and one boiler. (a 14 year old Weil-McLain CGa-7-PIDN)

We recently did a remodel and added a 6th zone that is hydronic underfloor heat. The PEX was installed under the subfloor and foil backed insulation was added between the rafters about 3 inches below the PEX.

They installed a separate, smaller Taco circulator pump for the hydronic floor zone(6) only. We added a Taco switching relay to control the circulator pump for Zone 6. The switching relay communicates to the TT wires on the 14 year old Weil-McLain CGa-7-PIDN boiler so that the boiler knows one of the zones is calling for heat and will cycle the gas valve as needed to maintain supply water temp.

The issue I have is :

When zone 6 calls for heat, the circulator pump for Zone 6 comes on and the boiler fires up if needed to maintain temp AND the main circulator pump energizes as well even if none of the 5 other baseboard heat zones calls for heat.

I am concerned that this will wear out the main circulator pump since Zone 6 runs for many hours on end since it is a below floor system.

The boiler only knows that a thermostat is calling for heat and doesn't know that the main pump is not needed for Zone 6. It has no way of knowing.

I can install another switching relay in the circuit that goes to the main circulator pump and wire it such that the main pump only comes on when 1 or more of the baseboard zones comes on, and does not run the main pump if only zone 6 is calling for heat.

My question is : Should the main circulator pump run at the same time as the Zone 6 floor circulator pump when ONLY Zone 6 is calling for heat?

The way Hydronic Floor Zone 6 is plumbed right now is that 1) Hot supply comes directly from the boiler to the mixing valve (which I have set so that the new pump is seeing 120F input supply water), 2) the return leg goes to the cold side of the mixing valve, 3) the blended supply water goes to the new pump, 4) there is a bypass plumbed into the return leg back to the cold (input) side of the boiler BEFORE the mixing valve. (This allows whatever return water that is not reused by the mixing valve to return to the boiler to be reheated.)

Your thoughts are appreciated., DEWFPO
 

Dana

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A couple of points:

A system diagram would be useful here.

The 7-plate CGa is a gia-normous boiler for most homes, even derated for altitude.

Micro-zoning a high mass boiler with low-mass radiation is a recipe for short cycling, high standby loss, and low operational efficiency. What are the typical burn times? (A retrofit heat purging control might be worthwhile here.)
 

jadnashua

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Depending on where the pex radiant is in/under the floor sandwich, would determine what temperature the boiler needed...often, that loop would not want to run as hot as a zone using a radiator. This can get messy.

Without a diagram, can't say. It sound like you could probably fix the control part of this with a wiring change. What you need is a control box that would operate the boiler when either thermostat was activated, but only turn on the needed circulator(s) to get heat to the calling zone. Anything more would be speculation without knowing the current control logic setup.
 

Dana

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The radiant pump is probably on a mixed recirculation loop and not pumping direct through the boiler, and wouldn't likely be able to deliver the minimum flow requirements of the boiler in that configuration (not that it really needs to, if the radiant zone is just sipping heat off the boiler), which may be why the installer opted to run both pumps whenever the the radiant zone was calling for heat (or not.)

But without more information about the system it's hard to say how likely or necessary that really is.

I've seen some real hacks out there with radiant operating off cast iron boilers, including one where the circulator for the radiant zone was on a wall switch near the boiler and running 24/365, with the thermostat for the radiant only wired to control the firing of the boiler. It's a bit hard to figure what some of these folks were thinking! But it's all fix-able.
 

DEWFPO

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Thanks for the reply's. I am attaching a rough diagram/schematic of the existing system.

Cycle times vary because all 5 baseboard legs are of different lengths and baseboard lineal footage. Two of these zones run in cantilevered areas (heat loss) of the house as well.

The boiler has a 220k I/P rating but is derated to 165k at our altitude of 7,100 ft. The house is 2,600 sq. ft. We do hit -20F during the winter occasionally each winter. Sometimes for several days in a row and this is not a tight house.

Zone 6, (the radiant floor zone) does cycle the separate zone circulator with the thermostat for that zone.

My only real concern is does the main circulator pump for all the zones need to come on if only Zone 6 is calling for heat? When zone 6 is calling for heat it is on for hours at time and off for hours at a time.

Thanks, DEWFPO
 

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Dana

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The section of the manifold marked "120F", isn't anything separate, right? The 120F is the setting of the thermostatic mixing valve near the "Floor Pump" in the diagram?

Assuming the main pump isn't presenting a high impedance to the flow, the radiant floor pump should have enough to draw heat from the thermal mass of the boiler without engaging the main pump. With the main pump running whenever the floor pump is running it pressurized the 180F side of thermostatic mixing valve more than if it was just the floor pump running, but it's unlikely that it needs to.

Running the floor pump off just a single zone relay, and using the zone controller for the other zones energize the main pump is probably the right thing to do.

A typical reasonably tight 2x4 framed house with clear glass double panes and at least R19 in the attic will come in with a heat load of 15-20BTU/hr per square foot of conditioned space @ 0F, or 20-25 BTU/hr per square foot @ -20F. If the foundation is insulated and it's reasonably tight it'll be on the low end of the scale, if there's no foundation insulation and it leaks a bit of air it'll be on the high end. Assuming it's on the leakier end of the scale you're probably looking at a design heat load in the neighborhood of 2600' x 25 BTU/hr= 65,000 BTU/hr @ -20F (which is a ridiculous design temp, more than the 99.6th percentile temperature bin for any CO location at your altitude.) You're probably looking at ~50-55,000 BTU/hr at your actual 99% outside design temperature of about -10F, give or take a couple. (Pb-villes design temp is -14F.) That makes the boiler ridiculously oversized at more than 3x the design heat load at the 99th percentile. ASHRAE recommends no more than a 1.4x oversize factor at the 99% outside design temp. Assuming a 55K load @ -10F, the biggest boiler that makes sense to install would have an output of 77K, which more than covers the occasional dip to -20F or below.

You probably don't even have enough radiation to emit 165,000 BTU/hr @ 170F AWT (about 500 BTU per linear foot of fin-tube baseboard, assuming 180F entering water temp, 160F return to the boiler), with all zones calling for heat, but you might have half that. Assuming (without any info) that the radiant zone emits 10,000 BTU/hr, do you have 300 feet of baseboard to deliver the other 155,000 BTU/hr of output?

You can verify the actual heat load using the CGa-7 as the measuring instrument by logging fuel-use against weather data using these methods. This is worth tracking (using wintertime fuel-use only) even if you're not planning to replace the boiler any time soon. Another thing to measure would be the burn times. If it's <5 minutes per burn it's not going to make it's AFUE numbers, if it's <3 minutes it's really slipping over the efficiency edge.
 

DEWFPO

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Thanks for the calc's Dana.

You are correct, the 120F section (Zone 6) is the setting on the mixing valve for the radiant floor heat.

Our home is 2x6 construction with the minimum required wall insulation in 1989. The interior walls in the basement only have a thermal break between them and the cinderblock foundation walls, no insulation at all (the owner was a builder from CA and had no appreciation for -20F). We had R19 in the attic at the time. The house is by no means tight by any shape of the imagination. We have done a lot to air seal the home and add insulation where we can and each thing we have done has made a difference to various degrees. We even had a blower door test.

When we first moved in, with an outside temp of -20F during the night, and the master bedroom thermostat set to 70F in the master bedroom, we would wake up to 64 degrees in the morning. We had additional baseboard lineal footage installed in the coldest rooms and that helped quite a bit but on some nights the heating system could not maintain a set temperature. The original boiler was a 100k (I/P) unit and it ran most of the time on cold nights and still couldn't keep up. We even raised the boiler temp from 180F to 190F.

It's been a while since I measured exactly what the inlet temp is at the first baseboard in any of the zones. One of these zones covers 4 rooms and another zone covers 3 rooms. You can tell the difference in heat output from the baseboards in the first and last room in the zone.

That's when we had the 220(I/P) unit installed and we can now maintain temps in all rooms.

None of the heating pipes in the floor or ceilings are insulated. We now have a total of about 161 lineal ft. of baseboard in the house, of which 64 feet is installed in cantilevered sections of the home.

The shortest burn time I have measured 7 min. 10 secs. but that is so dependent on temps and which zone(s) are calling for heat.

Thanks for your comments, DEWFPO
 

Dana

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With a minimum burn time of 7 minutes it's likely to come close to hitting it's AFUE numbers despite oversizing. It's still worth running fuel use heat load calculations to track system performance and verify sizing issues. Even at an AWT of 200F the ~160' of baseboard would be emitting at most ~120,000 BTU/hr of that 165K output. The radiant floor isn't likely to be emitting more than 20-25 BTU/hr per square foot with only 120F water, distribution losses from the uninsulated pipes are almost certainly less than 10K, and are mostly inside of conditioned space, thus not really "lost", so...

What are the construction details of the thermal break between the empty studwalls and foundation? With no insulation on the foundation that could be a large fraction of the total fuel use. It's pretty common for uninsulated foundations to account for 20%, sometimes more of the total heat loss of a house in cold climates.

How much floor insulation is there in the cantilevered floor areas? Are they reasonably air-sealed?
 

DEWFPO

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The thermal break in the basement walls is just a 1-1/2 inch air gap between the raw concrete foundation and the uninsulated 2x4 basement walls.

The cantilevered sections had either R-13 or R-19 but we've added blown-in, in those areas and air sealed them the best we could. It's difficult to estimate the current R-value in those cantilevered areas now because blown-in around existing batts in those bays is less than ideal and certainly less than perfect.

I do plan to run the fuel use heat load calculations.

Thanks, DEWFPO
 

Dana

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With 1.5" of space between the stud edges & concrete there is enough room to add R10 of HFO-blown closed cell spray polyurethane foam then fill out the rest with batts, if you ever got fed up enough to want to re-do the wallboard.

R10 is sufficient for dew point control on the above-grade sections of foundation wall with a batt-insulated studwall in any US climate zone. HFC blown 2lb foam would run about R9, which is probably going to be good enough, but the HFC245fa blowing agent is a powerful greenhouse gas (~1000x CO2) whereas HFO1234ze is at most a low-single-digit multiplier of CO2 for greenhouse potential. The cost is roughly the same, but the HFO blown goods have only been out there for a few years, and most installers have yet to make the change. (HFC245fa is likely to be banned for this application by international agreement within a few years.)

The IRC code minimum for any location in CO as cool as yours would be the thermal equivalent of R15 continuous insulation, and even with the thermal bridging of the framing undercutting the performance of the batts you'd beat that with some margin. With 1.5" of HFO blown foam and unfaced R13s you'd be pretty close to R20 "whole wall-R" or even a bit over, depending on the actual % framing fraction of the studwall.
 
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