CPH for hydronic heating

ben_18

What CPH setting would you recommend for a Honeywell thermostat on a hydronic (hot water) heating system?
The radiators are the slant fin type.
Would you recommend a different CPH if the radiators were those big old cast-iron ones?
(I would assume that the slant fin could use a higher CPH)

ChrisJ

I would expect cast iron to use less CPH than your slant fin setup.

Personally, I'd try 3 CPH for a while and see what you think. If the system seems to short cycle drop it down to 2 CPH. If it runs well, try 4 CPH.

It's not going to hurt anything, but it will take several days for the thermostat to settle down after you make the change. If you go from 2 CPH to 3 CPH it's almost guaranteed to short cycle for a bit before settling down so don't make any quick judgments. Same with going from 3 to 4. Reducing cycles, such as from 4 to 3 will probably cause an overshoot for a while as well.

The two important things here are to wait a few days and then pay very close attention to what the system is doing.

Short cycling (system turns off before room hits set point and then turns back on shortly after) reduce cycles.

Overshooting (room over shoots set point a lot after thermostat stops calling for heat) increase cycles.

ben_18

@ChrisJ Interesting. Thanks. It's at a friends house so I can't monitor it.

ChrisJ said:

Short cycling (system turns off before room hits set point and then turns back on shortly after) reduce cycles.

Overshooting (room over shoots set point a lot after thermostat stops calling for heat) increase cycles.

This got me thinking, I have a steam one-pipe system in a 5 story 15 unit building that is overshooting the set point. It needs to go on for ~2 hours to raise it by 2 degrees and then overshoots. Its set at 1 CPH. Do you think 2 CPH might make it run better?

ChrisJ

ben said:

@ChrisJ Interesting. Thanks. It's at a friends house so I can't monitor it.

ChrisJ said:

Short cycling (system turns off before room hits set point and then turns back on shortly after) reduce cycles.

Overshooting (room over shoots set point a lot after thermostat stops calling for heat) increase cycles.

This got me thinking, I have a steam one-pipe system in a 5 story 15 unit building that is overshooting the set point. It needs to go on for ~2 hours to raise it by 2 degrees and then overshoots. Its set at 1 CPH. Do you think 2 CPH might make it run better?

That's a definite maybe.
Are your mains vented plenty fast and insulated?
Is the system balanced fairly well?

Where is the thermostat and how do other rooms behave compared to that location? How long it takes for heat to get from the closest radiator to the thermostat makes a difference as well. If that's delayed, it can cause overshooting as well.

I run my steam system at 2 CPH, and when it's below 10F I go to 3 CPH.

ben_18

Thanks. Just wondering why you went with the 2 CPH and not with 1 CPH for your Steam system?
Also, are you using a honeywell thermostat?

ChrisJ

ben said:

Thanks. Just wondering why you went with the 2 CPH and not with 1 CPH for your Steam system?
Also, are you using a honeywell thermostat?

I'm using a Honeywell VisionPro 8110 with the green display.
I went with 2 because 1 CPH causes way too big of a temperature swing for my taste. 2 seems to work well, but my mains are vented fast and I can get steam to all of the radiators in about a minute.

3 works well for me as long as it's cold enough out but by the time it's in the teens or low 20's out it starts turning the system off too soon.

ben_18

In this building the mains arent vented so then I guess going with 2 CPH wont make it run shorter.
I'm thinking that locating the sensor on the first floor might help turn the boiler off faster so that it won't overshoot. What do you think of that?

ChrisJ

ben said:

In this building the mains arent vented so then I guess going with 2 CPH wont make it run shorter.
I'm thinking that locating the sensor on the first floor might help turn the boiler off faster so that it won't overshoot. What do you think of that?

It's fine, as long as all of the other floors will still get enough heat.

Why aren't the mains vented?

ben_18

The boiler is way oversized and short cycles. I might be double the edr of the radiators.
So I just figured that venting the mains wouldnt help all that much.

ChrisJ

ben said:

The boiler is way oversized and short cycles. I might be double the edr of the radiators.
So I just figured that venting the mains wouldnt help all that much.

Quite the opposite. I'd say main vents, especially fast ones are more important with an oversized boiler.

Venting the mains will help get the steam to the radiators quicker which means getting a head start and getting rid of steam quicker.


Picture having a sump pump in a large pit and the pump isn't big enough to handle the in coming water. You have two options, wait until the water is near the top of the hole to turn the pump on. Or, turn the pump on while the water is near the bottom. Having fast main vents is turning your pump on near the bottom. The water is still going to keep rising, but it'll rise slower and give you more time before it's too high.

ben_18

Heres my logic. The boiler short cycles on pressure to 3 lbs every 5 minutes. If I put in main vents then it will short cycle more often.

ChrisJ

ben said:

Heres my logic. The boiler short cycles on pressure to 3 lbs every 5 minutes. If I put in main vents then it will short cycle more often.

Not following that logic.

ben_18

I'll have to think about it more.
My next question is how to set the pressuretrol on this oversized boiler that is short cycling every 5 minutes.
Should cut-out-- 3 lbs, cut-in--- 2 lbs
Or cut-out--3 lbs cut-in--0lbs
The second option will let all the air back into the radiators and might take longer to expel the air. The first option might not let so much air into the radiators but it will be cycling more often.
(This is keeping the system as is without proper main venting)

KC_Jones

The venting dictates the pressure. More venting less pressure. If you have a short cycling issue it's partially (or entirely) because you have no main venting.

Jamie Hall

ben said:

Heres my logic. The boiler short cycles on pressure to 3 lbs every 5 minutes. If I put in main vents then it will short cycle more often.

No. More venting -- a lot more venting -- will allow the boiler to get steam to all the radiators before it starts to cycle. Then, I'll grant you, it will start to cycle. You're stuck with that.

Second point: reduce that pressure. It won't affect the cycling rate much, but it will save fuel -- and all the vents and traps. They don't like 3 psi (some types of vents simply won't reopen until the system pressure drops below 1 psi). Try a 1.8 psi cutout with a .8 psi cutin for starters. In terms of fuel, all the fuel burned raising the pressure from around 1.5 psi to 3 psi is wasted, and who needs that?

ben_18

@Jamie Hall Thanks. A little background.
Its a 5 story 15 unit building. The boiler input is 1.2 million btu when the load only requires about 700,000.
After about 20 min the boiler turns on and off every 5 minutes on pressure.
I dont want the boiler turning on and off too often because that will cause wear and tear.
I only go down to the site once in a while so I would go and play around with the settings until i get it right.
Just wondering why do you suggest cut in at .8psi and not lower?

Jamie Hall

ben said:

@Jamie Hall Thanks. A little background.
Its a 5 story 15 unit building. The boiler input is 1.2 million btu when the load only requires about 700,000.
After about 20 min the boiler turns on and off every 5 minutes on pressure.
I dont want the boiler turning on and off too often because that will cause wear and tear.
I only go down to the site once in a while so I would go and play around with the settings until i get it right.
Just wondering why do you suggest cut in at .8psi and not lower?

Chances are very good indeed that with that much mismatch between the actual load and the boiler rating that you are going to get cycling, just as you describe. Think of it as a car with just two accelerator settings: wide open and idle. The only way you can stay within the speed limit is to turn the engine on, get up to speed, turn it off, coast, on and up to speed, and so on.

Fortunately, cycling as you describe is much harder on the nerves than it is on the boiler. It does reduce efficiency, but it really won't hurt the boiler much -- the various control contacts and so on are made to handle it. It doesn't even reduce efficiency all that much -- a couple of percent.

The reason I suggest a cut in at 0.8 psi and not lower is that pressuretrols are notorious for falling apart -- literally -- if one tries to set them lower. Hence a cut in at 0.8, which is near but not at the bottom of the range, and a differential of 1 -- giving a cutout of 1.8, which is higher than optimum but not totally unreasonable, and keeps the gadgetry working.

ben_18

thanks. How about on a pressuretrol? Would you set it lower?
Also, how much cycling would you aim for? 5 minutes on then 5minutes or 3 minute cycles. Or are you just concerned with the pressure?

Is it possible that this system needs to run at higher psi because there are no main vents?

kcopp

As @Jamie Hall said... Add more venting.... no main vents? then add some. Not possible to have too much venting.

Fred

In addition to what everyone is saying about putting vents on those mains, another thing to remember is that higher pressure in the system slows steam movement down., compounding your problem. Sounds counter intuitive but it is true. Lower the Pressure.