[nielkfj]

In the small condo building where I live we have a non-condensing hot water boiler for the building hydronic system. (I've posted a couple other threads about it in the last few weeks.) The boiler is natural gas fired.

How would I calculate the predicted temperature and dew point of the flue gas coming out of the boiler heat exchanger before it enters the draft hood?

How would I calculate the predicted temperature and dew point of the flue gas coming out of the the draft hood before it enters the start of the flue?

 

[user_12345a]

What are the concerns?

There are charts you can find online showing predicated efficiency at various excess oxygen and stack temperatures and you can work backwards.

To see what's really going on get a pro in to do combustion testing.

 

[nielkfj]

What are the concerns?

I am looking into dropping the boiler water supply temperature in order to improve the energy efficiency. However, I need to check if flue condensation will be a problem.

There are charts you can find online showing predicated efficiency at various excess oxygen and stack temperatures and you can work backwards.

Do you know where I can find these charts? What are they called?

To see what's really going on get a pro in to do combustion testing.

I'd like to do this by calculation first before bringing in a pro to make the changes to the supply temperature and taking measurements. If the calculations show that flue condensation is a concern at the lowered supply temperature then I'll scrap the idea and save the expense of bringing in the pro.

 

[user_12345a]

There's no way to calculate accurately because every boiler/furnace is a bit different.

It has to be measured.

I don't think the stack temperature should be below 350F or so. It's not just a question of condensation in the boiler, the exhaust needs to be hot enough to rise up the stack and not condense.

Call a pro out to do a combustion analysis which includes measuring oxygen, flue temp, monoxide and more.

Without testing, I wouldn't go below 150F supply water temp. The return water temp will depend on the design of the loop - rads installed, etc, efficiencies vary, etc.


Keep in mind, if the heating system is cycled by a thermostat and heat is controlled properly, the savings of using reducing the water temp from like 180 to 150 are marginal.

The burner has to cycle either way and the efficiency gain is just from improved heat transfer.

Now, if the boiler is supplying heat continuously and an outdoor reset is being used to regulate heat output, you can see huge energy savings.

The problem is, at the minimum safe return water temperature, it will still supply too much heat most of the time.

The solution is to get mixing valves installed so the boiler return temp can be kept at a safe level or have a secondary loop with a heat exchanger.

The boiler is kept at 160 to 180f or whatever while the outdoor reset adjusts the supply temperature.


I don't know exactly how it would be setup - a pro should be consulted and do any retrofit.

 

[nielkfj]

There's no way to calculate accurately because every boiler/furnace is a bit different.

It has to be measured.

Really I only need a ballpark figure of the expected flue gas temperature in order to decide if lowering the minimum boiler water temperature is something worth pursuing. If it is, I will ask a pro to come in and test the idea, take measurements etc. to determine if everything is operating OK.

I don't think the stack temperature should be below 350F or so. It's not just a question of condensation in the boiler, the exhaust needs to be hot enough to rise up the stack and not condense.

It's actually condensation in the flue that I am concerned about. The boiler can operate at 105F return water temperature without condensation on the heat exchanger being an issue according to the boiler documentation.

But I take your point. Is there a way to determine the minimum exhaust temperature needed to allow the exhaust to rise up the stack?

Call a pro out to do a combustion analysis which includes measuring oxygen, flue temp, monoxide and more.

Without testing, I wouldn't go below 150F supply water temp. The return water temp will depend on the design of the loop - rads installed, etc, efficiencies vary, etc.

The boiler uses an outdoor temperature reset controller (ORC). What I am considering is dropping the minimum supply water temperature to 125 F. Since ORC is used, the controller would only drop the supply water temperature to the minimum during warm outdoor temperatures and the return temperature should be about 115-120 F I figure, which is safely above the boiler return minimum of 105 F.

Keep in mind, if the heating system is cycled by a thermostat and heat is controlled properly, the savings of using reducing the water temp from like 180 to 150 are marginal.

The burner has to cycle either way and the efficiency gain is just from improved heat transfer.

I understand the efficiency gain is about 1% for every 3F drop in water temperature. Currently the boiler is operating at 155 F minimum supply. So dropping it to 125 F supply is a 10% efficiency gain.

Now, if the boiler is supplying heat continuously and an outdoor reset is being used to regulate heat output, you can see huge energy savings.

ORC is being used already as explained above.

The problem is, at the minimum safe return water temperature, it will still supply too much heat most of the time.

Yes, this is also an issue as some parts of the building are too warm when the outdoor temperature is warm.

The solution is to get mixing valves installed so the boiler return temp can be kept at a safe level or have a secondary loop with a heat exchanger.

The boiler is kept at 160 to 180f or whatever while the outdoor reset adjusts the supply temperature.


I don't know exactly how it would be setup - a pro should be consulted and do any retrofit.

This would be the ideal solution. I've looked into it a bit. We could probably operated even lower, like down to 115 F if we installed a mixing valve.

However, due to the installation cost, I want to determine how low the supply temp can be without needing a mixing valve. One HVAC pro advised me that the boiler minimum supply temp in the ORC controller can be set at 20 degrees above the boiler minimum return temperature, and hence we could use 125 F minimum supply temp.

 

[user_12345a]

Get yourself a good digital cooking thermometer that can safely read up to like 500F+.

Experiment with different water temperatures, allowing it to reach steady state.

You stick the thermometer into the draft hood if it's natural draft.

If it's induced draft you'll need a small hole in the flue pipe and you can stick a screw in it to seal it. The flue runs negative pressure so it doesn't need to be perfectly sealed, just don't leave open hole.

Most high efficiency furnaces/boilers put out exhaust between 100 and 115F, so with a 105F return water temp condensation is a possibility especially at the beginning of the boiler cycle.

I doubt you'll be able to go lower than 140 to 150F supply.

With the mixing valve you may be able to have the supply start at like 90F at 65F outside and ramp up to the max for the system in extreme cold.

I suspect the payback would be pretty quick especially if there's overheating and tenants or owners are opening windows to be comfortable.

Is there a thermostat and zone valve for each suite?

What I know about this is only on paper/in theory, there are pros on this board who could get far more specific than me and recommend specific controls.

 

[beenthere]

He's asking this on both boards. And getting answers on the other board. As I recall, this is a 12 unit condo. And were not talkin g about a little 3000 dollar boiler. Or a small flue, I believe its a 14" chimney flue. And a 2 stage boiler. About 656,000 BTU input on high fire, and running around 56 GPM. And he hasn't even checked into how much temp drop is in the water piping to the furtherest 2 units, to know if they would be able to heat at the 125 min water temp he wants to run.

Until that test is done. Lowest safe boiler and flue temp don't really matter much. If it turns out that the water temp has to be a min of 135, or 145.

 

[user_12345a]

Well yah parts of the boiler could be below dewpoint even if most isn't and the stack temp is high enough to not get condensation.

If the individual suites have their own stats, I can't see savings being very high being it's not a condensing boiler.

If they don't and outdoor reset alone is regulating heat output, can see huge savings.

I don't know how building are done in the states but up here a lot of older ones have no zoning and use only outdoor reset; they overheat much of the season due to needing to maintain a minimum return temp.

 

[nielkfj]

Get yourself a good digital cooking thermometer that can safely read up to like 500F+.

Experiment with different water temperatures, allowing it to reach steady state.

You stick the thermometer into the draft hood if it's natural draft.

If it's induced draft you'll need a small hole in the flue pipe and you can stick a screw in it to seal it. The flue runs negative pressure so it doesn't need to be perfectly sealed, just don't leave open hole.

I probably won't try this myself. But it sounds like it isn't difficult, which is good to know.

It's a natural draft flue. The flue is 14" diameter and about 30 feet straight up the center of the building.

Most high efficiency furnaces/boilers put out exhaust between 100 and 115F, so with a 105F return water temp condensation is a possibility especially at the beginning of the boiler cycle.

I'm pretty sure it's not a high efficiency boiler. It's a Raytherm model H3-624-N, which is a non-condensing boiler.

I doubt you'll be able to go lower than 140 to 150F supply.

Given that it is a non-condensing boiler, which has a higher flue temperature (afaik), does your opinion change?

With the mixing valve you may be able to have the supply start at like 90F at 65F outside and ramp up to the max for the system in extreme cold.

That would be great. If it were the only owner, cost would be less of a concern and I would do what is needed to achieve the efficiency gains. But since there are other owners, they may not be amenable to spending the money.

There is a financial conundrum also - do we spend the money for a mixing valve on the existing boiler, or do we spend more money for a high-efficiency boiler.

I suspect the payback would be pretty quick especially if there's overheating and tenants or owners are opening windows to be comfortable.

The other complication is that the building has a make-up air unit also. It pressurizes the hallways forcing air under the apartment doors, then out of the building via bathroom, range hood, dryer and wood fireplace vents. So the building is continuously losing a lot of heated air regardless of if the windows are open or not.

Is there a thermostat and zone valve for each suite?

Yes. There are also a couple of "cabinet unit heaters" for the two stairwells, which are not on a thermostat - temperature is regulated manually by adjusting the speed of an internal fan.

What I know about this is only on paper/in theory, there are pros on this board who could get far more specific than me and recommend specific controls.

A theoretical perspective is also good to have.

 

[user_12345a]

With a thermostat in each suite, you may only save a few percentage points dropping the water temp.

It's not worth installing a mixing valve imo, it doesn't save energy when the return water temp to the boiler has to be kept high enough to reduce condensation and the system is zoned with thermostats. Doesn't increase boiler efficiency, may slightly reduce distribution loss.

It's only worth it when the outdoor reset is used to govern heat output - no zoning, as is often the case in older canadian apartment buildings.

Listen to beenthere's point about measuring return water temp directly and minimum temp required to heat the furthest units.

 

[nielkfj]

He's asking this on both boards. And getting answers on the other board.

Ya, sometimes I won't get an answer if I post to just one board. So I posted on two boards. I wasn't expecting the same people on both.

As I recall, this is a 12 unit condo. And were not talkin g about a little 3000 dollar boiler. Or a small flue, I believe its a 14" chimney flue. And a 2 stage boiler. About 656,000 BTU input on high fire, and running around 56 GPM.

All correct, except the nameplate shows "Input Btu/Hr: 627,000". Also, I can't recall what I said about the pump flow rate previously, but the estimate I settled on is 57.5 GPM, based on the curve for the current pump.

And he hasn't even checked into how much temp drop is in the water piping to the furtherest 2 units, to know if they would be able to heat at the 125 min water temp he wants to run.

Until that test is done. Lowest safe boiler and flue temp don't really matter much. If it turns out that the water temp has to be a min of 135, or 145.

125 F would be the minimum setting for the outdoor reset controller, which would only be the target setpoint at higher outdoor temperatures.

Also, I'm assuming some temperature drop can be tolerated because there were building envelope improvements made 10 years ago - extra insulation and new windows and balcony doors.

But I see your point that if there is too much temperature drop to the furthest apartments lowing the water temperature might not be an option.

 

[user_12345a]

125F supply is way too low, the entire point is to never have the return water temperature too cold, otherwise there wouldn't be a minimum.

How the loop was designed and performs would dictate how low you can go - I really doubt it's safe to go below 140F especially being a 2-stage setup.

On low the risk of condensation formation is higher than on high.

Tweaking the existing system I doubt you will save more than 10%.

It would probably take a major retrofit, like using more efficient pumps and going to a condensing boiler and reset to see major savings.

 

[nielkfj]

125F supply is way too low, the entire point is to never have the return water temperature too cold, otherwise there wouldn't be a minimum.

The boiler minimum return temperature is 105 F.

There is an outdoor reset controller (ORC) (Tekmar 261) controlling the supply temperature.

The 125 F setpoint I am considering would be the minimum supply temperature setpoint in the ORC. So it would only use 125 F supply during warmer outdoor temperatures. Since the outdoor temperature is warmer, the differential from supply to return should be less than at colder temperatures, say 10 F, on average. Which means the return temperature to the boiler would be 115 F.

How the loop was designed and performs would dictate how low you can go - ...

There were building envelope improvements made about 10 years ago - extra insulation and new windows and balcony doors. So there might be some room to lower the temperature without affecting comfort.

... I really doubt it's safe to go below 140F especially being a 2-stage setup.

On low the risk of condensation formation is higher than on high.

Yup. Actually, what I was trying to find out with this thread is how to calculate a theoretical flue temperature, so that I can estimate how close it would be to the dew point at the lower supply temp and on low-flame.

Tweaking the existing system I doubt you will save more than 10%.

It would probably take a major retrofit, like using more efficient pumps and going to a condensing boiler and reset to see major savings.

It all sounds good to me, but it probably wouldn't fly with the other owners. I'd like to get the existing system operating as efficiently as possible without risking damage to the equipment.

At this point my plan is to contact Raypak to see if they can estimate the flue temperature at lower water temperatures. If the estimated flue temps looks promising I'll then discuss lowering the supply temperature with the technicians operating the equipment, including using a combustion analyser to verify and measuring supply temperatures to the furthest apartments.

 

[user_12345a]

Rather than estimate, take measurements or call a pro.

Pipe temperatures can be measured easily, combustion/stack u should call a pro for when the return water temp is low.

Every setup is unique - heck, the btu heating value of natural gas varies a little between areas. There's no way to accurately predict exactly what will happen.

Going in circles here.


Keep in mind, manufacturers specify ranges, and when they say minimum, they mean absolute minimum.

Optimal/safe can be well above the minimum.

Don't think having a low load and lower water temp drop in mild weather is protective. You're actually higher risk of condensation damage in mild weather due to short burner cycles, and running on low fire increases the risk.

If boilers are anything like furnaces, the first stage btu input is 60 to 70% of maximum, so there can be a lot of cycling in mild weather despite being 2-stage.

It's absolutely not worth risking damage to the boiler and venting just to try and save a little fuel.