Question about outdoor reset

[rdm190]

I just installed a Tekmar 256 yesterday and had a few question about my settings.

My system:

93 year old home, likely a converted gravity-hot water system. Unknown when It was converted. Blown in insulation.
2 zones: 7 cast rads and a baseboard rad on main zone, fin tube convector in an addition on the second zone
Weil-McClain Gold CGa-4-pidn boiler
Taco 3 zone controller, 2 Taco zone valves, Taco circulation pump wired to the zone controller
Tekmar 256 Outdoor Reset
Located outside Philadelphia

I have the factory settings on the 256 at the moment:
Boil design: 160
Boil Max will calc at: 170
Boil min: 140
Outdoor design: 12
Indoor design:68
Differential: Auto

I cycled the boiler a few times yesterday and get a supply ramp up to 150+ before the 256 pulls the plug (It was warm yesterday, so it was calculating a 140 boiler target). This gets my return water to 130+ ish.

Weil-McClain recommends no less than 130 return water, in their factory manual.

I am contemplating having a bypass installed, but that is out of my skill set.

Am I running on safe numbers? Is the bypass a time sensitive necessity? I am new to hydronic heat, do I am learning on the fly.

-Ryan

 

[Dana]

With the high thermal mass of fat gravity feed plumbing & high volume of the radiators you'll definitely need a boiler bypass even if you set the minimum to 170F. With the outdoor reset controller it's even more important.

Odds are pretty good that the rads were big enough to heat the place even with some windows cracked open with 180F boiler output, even before the house was insulated, and better windows than leaky wood sash single panes were installed. With some air sealing and insulation and at least some storm windows over the leaky panes they're probably big enough to heat the house with 130F water, which would be ideal for a condensing boiler and great for comfort, but a problem requiring a bypass protection for cast iron boilers.

A couple of suggestions:

Zone by zone, measure the EDR of the radiators, and measure the convector to estimate it's output at some temperature or other. (You'll need to measure the fins, & height, and find a similar product currently still being manufactured.) Then estimate the output of the fin-tube baseboard at different average water temps.

If you have a heating history on the place (or as you aquire that history), use the wintertime-only fuel use and the 84% nameplate efficiency of CGa-4 to MEASURE the heat load @ +12F. The purpose is to figure out the water temperature requirements for the radiation to emit sufficient design-day BTUs, but you would also then be able to calculate the oversize factor. Like the radiation, the CGa-4 is probably more than 2x oversized for design load with house at it's current insulation levels, even if the basement isn't yet insulated and still leaks a lot of air.

When you know the load/radiation ratio you'll then be able to predict your water temperature requirements at any arbitrary temperature. It's still possible to do ODR on the radiaton water temperatures for maximum comfort even if the temperature requirements are below the 130F minimum EWT of the boiler most of the time (which is probably the case), but the Tekmar 256 wouldn't be the way to do that. It would have been better to run all these numbers BEFORE installing the Tekmar but run them anyway, we'll then know what can still be optimized with the Tekmar and what can't.

 

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[rdm190]

With the high thermal mass of fat gravity feed plumbing & high volume of the radiators you'll definitely need a boiler bypass even if you set the minimum to 170F. With the outdoor reset controller it's even more important.

Odds are pretty good that the rads were big enough to heat the place even with some windows cracked open with 180F boiler output, even before the house was insulated, and better windows than leaky wood sash single panes were installed. With some air sealing and insulation and at least some storm windows over the leaky panes they're probably big enough to heat the house with 130F water, which would be ideal for a condensing boiler and great for comfort, but a problem requiring a bypass protection for cast iron boilers.

A couple of suggestions:

Zone by zone, measure the EDR of the radiators, and measure the convector to estimate it's output at some temperature or other. (You'll need to measure the fins, & height, and find a similar product currently still being manufactured.) Then estimate the output of the fin-tube baseboard at different average water temps.

If you have a heating history on the place (or as you aquire that history), use the wintertime-only fuel use and the 84% nameplate efficiency of CGa-4 to MEASURE the heat load @ +12F. The purpose is to figure out the water temperature requirements for the radiation to emit sufficient design-day BTUs, but you would also then be able to calculate the oversize factor. Like the radiation, the CGa-4 is probably more than 2x oversized for design load with house at it's current insulation levels, even if the basement isn't yet insulated and still leaks a lot of air.

When you know the load/radiation ratio you'll then be able to predict your water temperature requirements at any arbitrary temperature. It's still possible to do ODR on the radiaton water temperatures for maximum comfort even if the temperature requirements are below the 130F minimum EWT of the boiler most of the time (which is probably the case), but the Tekmar 256 wouldn't be the way to do that. It would have been better to run all these numbers BEFORE installing the Tekmar but run them anyway, we'll then know what can still be optimized with the Tekmar and what can't.

I appreciate the input. The windows are new (last decade) vinyl Pella's, so I would agree with you that I have quite the excess of radiation now, compared to when the house was built.

I am asking for some quotes to install a bypass.

When the main zones calls, it frequently is satisfied in < 10 min, especially in warmer weather. This rarely gets the water above 120, when it runs these small short cycles, regardless of the Tekmar (it did this with just the hard set 180 on the aquastat).

This unit has been in place for the better part of a decade. So, how would I know if it has been condensing? The burner tubes look spotless and the pan below just has some debris in it. The tech that serviced it before we moved in had no complaints. What am I looking for if this thing has been doing this for assuming 7 years?

-Ryan

 

[Dana]

The condensation would only rarely be copious enough to drip down to the burner tubes, but the amount of corrosion on the fire side of the heat exchanger plates could be pretty high for a boiler it's age if condensing. It's not impossible for a boiler to be completely toast after 2 seasons if condensing copiously, but it's also not impossible for it to still be functional after 10 years (though the raw combustion efficiency would be well below the nameplate efficiency.) I haven't torn into a CGa-4 myself and can't say for sure, but the fire side of the plates are usually at least partially inspectable with a mirror from below without partially disassembling the jacket (which would be necessary to view it from the top.)

A typical cast iron boiler of this ilk would have plates that look like this on the fire side if burning cleanly without condensation:

If it has been condensing a lot those nipples creating the extra surface area and turbulence can sometimes be corroded down to pointy cones, and the overall appearance of the castings will be more pitted. (This particular picture is from the top, post cleaning, on a boiler that had sooted up.)

If there aren't any indication of bits of rust falling out onto the burners yours probably doesn't look anywhere near as bad as this:

But if it's been condensing it might look like this:

If it's satisfying calls for heat in under 10 minutes, rarely exceeding 120F and never actually hitting 180F, your system is an ideal candidate for a condensing boiler. Are you certain that there isn't already a boiler bypass or system bypass? Some pictures of the near-boiler plumbing could be useful. Putting the bypass-plumbing money into a right-sized condensing boiler might be the better investment if you plan to live there for another decade.

 

[rdm190]

It definitely does not have a bypass.

I cannot justify replacing a 7 year old boiler. I think you are exactly right, a condenser would be perfect, I will plan on that next time.

 

[Dana]

Given that there is well over a decade's worth of life cyle to this boiler, it's worth using a thermostatic mixing valve for the bypass branch rather than just a cheap ball valve. That way you can dial in the return water temp to the near-condensing zone under all operating conditions the average combustion efficiency by a percent or two.

Whether or not the Tekmar 256 will improve efficiency to the extent that a heat purge economizer would is an open question, but it will lower the temperature swings of the radiation, for a potential uptick in comfort. For best efficiency ideally all burns would be 10-minutes or longer under all operating conditions, and it may in fact do that, given the high thermal mass of the water volume. Efficiency will still be pretty good even if under some conditions the burns are as short as 5 minutes, but below that it starts to slide over a cliff.

It's still worth doing all the napkin math on the heat load and the radiation, and keeping it in a file, if your planned tenancy there is likely to be another 15-20 years, or if you plan to add air conditioning. During the shoulder seasons high efficiency heat pumps have a lower operating cost than even a condensing gas boiler. Knowing the heat load with some precision would allow you to decide if/when to change over between heat pump vs. boiler heating.