[the punisher]

Hello everyone,
I was hoping i could get some clarification if my boiler is operating correctly.

Background:

• 2 zones: Radiant heated floors in Kitchen + Stelrad Radiators on the rest of the house.
• HydroTherm Gas Fired Boiler
• Cold water Pressure at about 15psi. ( boiler in finished basement, 2 storeys high home ) At max operating heat of 170F, pressure at about 19-20psi.
• Tstat on boiler set a maximum heat of 170F.

This issue only applies to the radiators zone:

So let's say i set the thermostat in the hall way to call for heat, I set it to a really high temp so that it forces the boiler to be constantly be on. The boiler will turn on, once it hits high limit of 170F, it turns off, cools down and fires back up. I understand this operation and it seems normal, however my issue is that these cycles are 2-3 minutes, so the boiler is constantly turning off and on, until room temperature is reached.

The tstat on the boiler only has a setting for a high limit, and I assume the differential for when it fires back up is hard coded.
My guess here, is that the boiler is heating up to high limit really fast, but also cooling down really fast, hence the on and off.

While looking at the temperature gauge during this operation, it warrants my assumption above, boiler fires up, temperature rises, hits limit, turns off, water temperature rapidly drops, boiler turns on.

The circulation pump for this zone has 3 speed settings. It was on the highest setting, changed it to speed setting number 2, this helped on the off time of the boiler as it was taking longer to cool down, however at this setting, the boiler heats up faster and shuts off faster then before.

Now this could be normal operation, just hate hearing the boiler turning on and off so much, and am not sure if it is impacting my gas bill in any way.

Any advice given is much appreciated.
Thanks

 

[beenthere]

May want to put a second operating limit on it, and set it to a differential of 30 or even 40 degrees.

Low mass boilers tend to cycle a lot.

 

[the punisher]

Are you suggesting replacing the aqua stat with on that has the option to set a differential instead of just a high limit with a hard coded differential ??

 

[Focused2]

Operating control is basically your temperature controller. The controller may have a differential setting to help keep the short cycling down.
Operator control has a set point that you set it to, it may have a built in differential or adjustable differential.
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Hi-limit is as the name implies. It is the maximum temperature allowable limit you want to let the boiler attain. Normally your Hi-Limits have a reset button to warn you that it tripped. Designed as a safety controller.
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Are you adjusting a different temperature controller in the controlled space? Does this controller turn on your pump for that space? Does the controller have a differential? If low voltage t-stat does it have a heat anticipator on it?
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Help us out with an image of what you have , model #'s allow us to read the pdf file on the component and let you know how it was intended to work.
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Joe

 

[beenthere]

Are you suggesting replacing the aqua stat with on that has the option to set a differential instead of just a high limit with a hard coded differential ??

Not replace, add one. Then set the current one 10 degrees higher then the oine you add.

 

[the punisher]

Operating control is basically your temperature controller. The controller may have a differential setting to help keep the short cycling down.
Operator control has a set point that you set it to, it may have a built in differential or adjustable differential.
-
Hi-limit is as the name implies. It is the maximum temperature allowable limit you want to let the boiler attain. Normally your Hi-Limits have a reset button to warn you that it tripped. Designed as a safety controller.
-
Are you adjusting a different temperature controller in the controlled space? Does this controller turn on your pump for that space? Does the controller have a differential? If low voltage t-stat does it have a heat anticipator on it?
-
Help us out with an image of what you have , model #'s allow us to read the pdf file on the component and let you know how it was intended to work.
-
Joe

Thank you, for the reply. The current AquatStat that is a attached to the boiler is a Honeywell L4006A. It only has an option to set a HI-Limit. Only 1 dial, there is no option to set a Lo-Limit or a Differential setting. This aquatstat does not in anyway control the the circulator pumps, those are being turned on by a separate module, that controls the different Zones if the floor or hallways thermostats call for heat.

I have completely closed off the radiant floor heating zone, by closing off the valves and turning off the thermostat. So right now the only zone operating is the Radiators zone. It's like the boiler is heating up way to fast. It can go from 140F to 170F in 2 minutes. Water circulation is not a problem, pump turns on and rads get hot quick fast.

The boiler is an HydroTherm HC-125.

 

[the punisher]

Not replace, add one. Then set the current one 10 degrees higher then the oine you add.

I am not sure that is possible. The aquastat is attached to the boiler. I cant just add one to it??

 

[the punisher]

AquaStat is Honeywell L4006A1868. Cant i just replace this with one that has a option to set a differential setting ?

 

[beenthere]

Yes you can just replace it.

I was referring to adding a strap on aquastat to the supply pipe.

 

[the punisher]

Thank you, I have ordered a new aquastat that allows for a differential setting from 5F -30F

 

[Nestor_Kelebay]

Hot water heating systems work better if you add an "outdoor reset control" and use your existing aquastat as a high limit.

The outdoor reset control adjusts the boiler water temperature according to the outdoor temperature. The cold it gets outside, the higher the boiler water temperature. Most hot water heating systems use an outdoor reset control. You then set your existing aquastat to about 200 deg. F so that if the water temperature exceeds that setting, it closes the gas valve to shut the boiler down.

Your short cycling might be due to your thermostat's "heat anticipator" setting. Think about it. Your radiators are full of hot water. If the thermostat were to wait until it sensed the correct temperature before turning the boiler off, the hot water in your radiators would continue to convect heat into the room until you overshot that temperature setting. So, thermostats will have a small 24 VAC electric heater in them that fools the thermostat into thinking that it's warmer in the room than it really is. That causes the thermostat to shut the boiler down early so that there's less overshoot of the thermostat temperature setting.

Also, your radiators are located around the perimeter of your house (or zone), but the thermostat is centrally located in the house or zone. So, by the time the thermostat senses the temperature setting, it's warmer than that everywhere else in the house. This is another reason why you need the heat anticipator to shut the boiler off early so as to have less overshoot of the thermostat temperature setting.

Download the manufacturer's literature that came with your thermostat and find out how to adjust your thermostat's heat anticipator. The more heat it generates, the shorter the cycling on your heating system will be. The less heat it generates, the longer the heating cycles on your boiler will be.

Talk to any heating contractor about installing an outdoor reset control to regulate the boiler water temperature according to the outdoor temperature. In that case, your boiler circulating pump will run continually and the boiler will only fire to keep the boiler water temperature just hot enough to keep your house warm.

 

[user_12345a]

In a properly designed hydronic system, shouldn't the burner be able to run continuously during a call for heat?

The rads should be able to dissipate the heat produced by the boiler at design water temperature, otherwise the boiler is oversized, correct?

 

[beenthere]

In a properly designed hydronic system, shouldn't the burner be able to run continuously during a call for heat?

The rads should be able to dissipate the heat produced by the boiler at design water temperature, otherwise the boiler is oversized, correct?

The boiler is over sized 99% of the year. Only when the outdoor temp is at design temp, and both zones are calling for heat, can it be the proper size.

The OP's rads are designed to give off heat quicker then cast iron. But they hold less water per BTU output then cast iron rads. So it will effect boiler on and off operation.

A 5°F differential will short cycle any boiler.

 

[user_12345a]

What does it have to do with outdoor temp? seems only indoor temp and the rads.

Cycling on and off like that can't be efficient, especially with a natural draft unit.

 

[supers05]

Outdoor rests assume heat loss corresponds to outdoor temperature. You need less heat gain to match the lower heat loss when it's warmer then design temp. By lowering the water target temperature, you lower heat gain, allowing for longer zone on times. This statistically allows more zones to call together, longer, increasing load on the boiler, allowing for longer run time, and off time. (reduces cycle time) It also increases the efficiency of the boiler itself, as this is a function of incoming medium temperature. If multiple stages, or modulation is available, this adds to this concept.

One critical lower limit of this though, is that water temp can't be below 140*F on non-condensing boilers, such as the OPs.

Cheers!

 

[user_12345a]

Without the reset outdoor temp won't have anything to do with it.

The natural draft boiler without damper has got to be super-inefficient with the burner cycling during a call for heat, the heat pouring up the stack between cycles.

 

[beenthere]

What does it have to do with outdoor temp? seems only indoor temp and the rads.

Cycling on and off like that can't be efficient, especially with a natural draft unit.

Colder the outdoor temp, the quicker the rad loses heat to the cold wall. And the water returning to the boiler is that much cooler, and the boiler has to run longer. The warmer the outdoor temp, the slower the rad loses heat to the warm wall. And the water returning to the boiler is that much warmer, so teh boiler doesn't need to run as long.