Adjust gas flow valve down for a boiler?

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Nathan Davis

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I recently add an Aquastat 3200 and outside sensor to my Utica 200K Btu MGB boiler, and also replaced my SINGLE zone of 3" diameter hot water distribution pipes with a 3 zone system of 1-1/4 PEX pipe. This is a non-condensing boiler rated at 81% efficiency with a DOE rating of 165K Btu. The remodeled 3-zone system heats the house much more evenly than the previous single zone system. I really like the results. In a previous post, you helped me to figure out that the boiler is almost double size of what is needed.

I live in Springfield MO. Today, it is 18 degrees F outside. The boiler comes on for 5 minutes and then shuts off for 1-1/2 minutes. So that probably confirms that the boiler is larger than needed.

I'm wondering if the overall efficiency might be improved if I slightly close down the gas supply valve to reduce the size of the flame in the boiler? Conceivably, that would eliminate the 1-1/2 minute shutoff that occurs every 5 minutes.

The water temperature is set for 180. But when it gets cold like today, I also notice that the boiler steps up with water temperature in 10 degree increments until it reaches 210 F. I'm worried--that seems a little too close to 220. Should I install a shutoff to keep it below 200F?
 
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Dana

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Downfiring the boiler isn't going to buy you much, and might not be practical to do with that particular boiler. The Hydrostat 3200 will keep doing the right thing optimizing the use of the thermal mass in the system, assuming it's been set up correctly.

According to the manual, the Hydrostat 3200's default HI LIMIT setting is 190F, so if it's stepping up to 210F somebody must have changed it. have you determined that the house can't be kept up to temp at 190F?

Five minute burns aren't a serious efficiency issue, but ~10 burns per hour might be cutting into it a bit. Is it doing the 5-on/1.5 off cycles a LOT, or just occasionally? What is the low limit set to?
 

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Downfiring the boiler isn't going to buy you much, and might not be practical to do with that particular boiler. The Hydrostat 3200 will keep doing the right thing optimizing the use of the thermal mass in the system, assuming it's been set up correctly.

According to the manual, the Hydrostat 3200's default HI LIMIT setting is 190F, so if it's stepping up to 210F somebody must have changed it. have you determined that the house can't be kept up to temp at 190F?

Five minute burns aren't a serious efficiency issue, but ~10 burns per hour might be cutting into it a bit. Is it doing the 5-on/1.5 off cycles a LOT, or just occasionally? What is the low limit set to?

Low temp is set at 170. high temp is set at 180, and economy is set at 2.

My understanding is that the system increases the water temp by 10 degrees for each 30 minutes that it fails to meet the demand temp, which we set at 65F. It's a large house, so it takes about 90 minutes to reach 65F after sitting at 61F overnight, or after sitting at 62F after we return from work. So I expect that it's stepping up the water temp by 10 degree increments until it reaches 210F, due to not being able to reach the demand temp of 65F quickly. That is, it would set the water temp to 190F after 30 minutes, 200F after 60 minutes, and 210F after 90 minutes.

What do you think?
 

Dana

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Why is the low temp set so high? Are you heating the domestic hot water with an embedded coil or something?

>"The remodeled 3-zone system..."

>"...economy is set at 2.

In a 3 zone system with the ECONOMY set to 2 it may confuse algorithms designed to "learn" the system. See on page 6:

"The Economy Feature is factory set for a 1 zone heating system. To adjust, turn the ECONOMY Dial until the number displayed equals the number of heating zones."

If the third zone is an indirect water heater, ECONOMY should be set to 2. If all three zones are space heating zones it should be set to 3.

If it's not heating domestic hot water with either an embedded coil or an indirect water heater, see how it behaves with the low-limit set to either 140F or OFF, and ECONOMY set to 3.

It doesn't take very long at all for the boiler to heat up it's own internal thermal mass to whatever the target output temperature is (less than two minute to rise by 100F, say 80F after a long idle to 180F.) A thermostat setback of any substantial length (even a couple of hours) will result in system water temperatures well under 140F, no matter where the boiler's low limit is set. But keeping the boiler at a lower temperature when idling will reduce standby losses to the boiler room. If it's idling at 170F those standby losses are quite substantial.

If it still takes forever to recover from setbacks the high thermal mass of the radiation on system and the thermal mass of the brick is probably limiting the recovery speed. If that's an issue, running it with the optional outdoor reset kit and NOT using overnight setbacks may prove more satisfactory and may even be more efficient (though that would be rare.)

I believe the Hydrostat's target temperature is based on the temperature at which the prior calls for heat satisfied the thermostat and how long that took. It may or may not be bumping it up by 10F every half-hour (they don't share that level of detail on their algorithms) but it very likely backs off on the target temperature to something below the programmed HIGH LIMIT temperature whenever calls for heat get satisfied quickly.
 

Nathan Davis

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I like your insights.

Although everything is now designed as a 3 zone system, the 3rd zone is the basement and I have it set so that it will come on whenever either of the other two zones come on. So it is running more like a 2 zone system. I did that because we use the basement a lot, and so it feels really nice when it stays warm.

I am not running a water heater off the boiler.

The low temp is set at 170F only because that's where it was set at the factory. I didn't know what to set it at. So I'll turn it down to 140F, and that may even reduce my heating bill a bit. Or should I set it even lower than that?
 

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If the third "zone" is always on whenever another zone is calling for heat it's not really a separate zone, and ECONOMY would rightly be set to 2. If it is being controlled by it's own thermostat it would the setting should be 3.

Firing at 140F has no risk of corrosive condensation on the boiler's heat exchanger plates, and very low risk of flue condensation, which is something to be mindful of with oversized boilers operating at a low duty cycle if vented into a masonry chimney. If the boiler temp is too low during firing acidic gas exhaust will be taken up as adsorb in masonry, degrading the mortar. (If the flue has a metal liner this is less of an issue.) Most gas fired boilers can be safely cold-started, but should not be run for extended periods at temperatures below 130F or the acidic exhaust condensing on the boiler plates can cause rapid corrosion. With a masonry lined chimney set it to 140F.

With a metal lined chimney go ahead and cold-start it- turn the low limit to OFF, but pay attention to the entering water temperature at the boiler. In high mass radiation systems there is usually a system bypass or boiler bypass branch in the near-boiler plumbing to prevent the entering water temperature at the boiler from dwelling in the condensing zone for extended times as the system water is coming up to temperatures. This bypass branch mixes direct boiler output with the returning water just ahead of where the water is going into the boiler.
 

Nathan Davis

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If the third "zone" is always on whenever another zone is calling for heat it's not really a separate zone, and ECONOMY would rightly be set to 2. If it is being controlled by it's own thermostat it would the setting should be 3.

Firing at 140F has no risk of corrosive condensation on the boiler's heat exchanger plates, and very low risk of flue condensation, which is something to be mindful of with oversized boilers operating at a low duty cycle if vented into a masonry chimney. If the boiler temp is too low during firing acidic gas exhaust will be taken up as adsorb in masonry, degrading the mortar. (If the flue has a metal liner this is less of an issue.) Most gas fired boilers can be safely cold-started, but should not be run for extended periods at temperatures below 130F or the acidic exhaust condensing on the boiler plates can cause rapid corrosion. With a masonry lined chimney set it to 140F.

With a metal lined chimney go ahead and cold-start it- turn the low limit to OFF, but pay attention to the entering water temperature at the boiler. In high mass radiation systems there is usually a system bypass or boiler bypass branch in the near-boiler plumbing to prevent the entering water temperature at the boiler from dwelling in the condensing zone for extended times as the system water is coming up to temperatures. This bypass branch mixes direct boiler output with the returning water just ahead of where the water is going into the boiler.
Thanks for the info.
Mine is indeed a masonry flue, so I just finished resetting the low temp to 140F (on Saturday, outside temp of 20F). However, I now notice that I cannot get the water temp to go above 175F, or below 155F, no matter the setting. That is, I have it set at the highest setting and at 140, but it stays between 175F and 155F. So I'm wondering if the controls are messed up. I discount that because it's hard to believe that both would be messed up.
On Sunday, I checked again, outside temp is 34F. During on/off cycles, the water Low temp now shows 140F, high temp goes up to 165F even though it is set to the highest setting. So it appears that the system adjusts the low and high settings based on the outside temp.

Additionally, I've been wondering if I should replace the two manual radiator valves (always in the ON position) on the 3rd floor, and replace them with thermostat valves? Currently, the 3rd floor radiators are also connected to the 2nd floor zone. We rarely use the 3rd floor which has 900 sq ft of additional bedrooms. So I may be able to reduce a little of our heating bill if I replace the manual valves with automatic ones? It would cost about $145 to install two Danford automatic valves, but I'm unsure if the payback would be worth it since our annual heating bill is only about $1200/year for the entire 5800 sq ft house (the 3rd floor is about 1/6 of the house). I could manually shut them almost off, but that still wouldn't regulate the temperature to a constant low setting.

And, should I consider a different brand of automatic valve?

Alternatively, I could install another pex loop from the boiler up to the 3rd floor, and install a thermostat for that zone. That would cost about $200. Then the 3rd floor would have it's own zone. I'm strongly leaning toward this option. If I choose this option, I would probably use a pex loop for the supply line, but leave the return lines connected to the second floor zone, as it has been plumbed since the house was built 100 years ago. My dilemma is that I'd need to know which side of each radiator is the supply and which is the return. I've noticed that some of the radiators have the valve on the supply side and some on the return side, so it's hard to know which side is the supply. I guess that I could put a pressure gage on each line to find out if it is a supply line or return line?
 
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