Short Cycling?

rcleimbach

I have a question about my boiler which I believe is short cycling.

Some background:
Buderus G115WS-5 with Riello burner was installed by a heating contractor in Oct 2022. They also installed a Burnham AL50SL indirect hot water heater. There are 5 heating zones in my house plus the zone for the indirect hot water heater. The zone valves are Honeywell and it has a Grundfos Alfa circulator. My house is located in Western Massachusetts and is about 2100 sq ft. This is a hot water baseboard heating system. It is a second home and I do not live there full time. During the winter months the thermostats are set to 55-60 degrees when we are not there. When there, we set to about 69-70 degrees.

Last winter a typical heating cycle was about 4-5 minutes on average. I questioned the service tech about whether these cycle times were long enough and he said it was OK. I was starting to think that they oversized the boiler for the size of my house. I also think the number of short zones could be contributing. This house was built in 1988 and I believe they over designed it with 5 heating zones.

Fast forward to this July 2023 when I had the first annual tune-up performed. This same service tech changed the burner nozzle from a .8 to a .85 and also removed I believe 2 baffles. Now I am seeing cycle times of around 2-3 minutes. I know these short cycle times are really bad for the system. Would these changes cause a reduction in the cycle times or could there be another issue?

Looking for advice on how to move forward with my heating contractor. I want to ask them to go back to a .8 nozzle and reinstall the 2 baffles to at least get back to 4-5 minute cycle times. But I am still concerned with 4-5 min cycles times still not being long enough. I spent a lot on money on this system and I am starting to have buyers remorse.

Can anything be done to increase the cycle times?

Thanks in advance for any advice.

KC_Jones

House built in 1988
Boiler installed with a net output of 119,000 btu
2100 sq ft house

That's 56 btu/sq ft. My 120+ year old house with original single pane windows and limited insulation is barely even 40 btu/sq ft. Your modern house should be, in my opinion, 20 per sq ft tops, probably less.

So yes, the boiler is massively oversized. You could have easily used the 115-3 and even that is probably too big.

It's installed now, and no way the contractor will fix their obvious screw up. Best you can do is find a reliable tech, who knows combustion and finds out how low Bosch will allow that unit to be down fired. It probably won't be much, but anything is better.

Ironman

As KC said, that boiler is way oversized. Your house probably needs less than half that and that’s on the coldest night of the year (design temperature). When it’s 35* outside, you only need half of that needed at design temp (25k btus?).

Add to this that the house is over zoned and the problem is compounded more when only one zone is calling. You should look at combining as many zones as possible. That only requires some minor control wiring.

The other option would be adding a sufficiently sized buffer tank.

The G115 should be returned to the correct nozzle that’s spec’s.

You cannot down fire a Buderus since the block is already up-sized for more efficiency; you can only up-fire by 10%.

The baffles should only be removed to increase flue gas temperature on a less than ideal chimney.

EBEBRATT-Ed

Buffer tank as @Ironman said

KC_Jones

Ironman said:

The G115 should be returned to the correct nozzle that’s spec’s.

You cannot down fire a Buderus since the block is already up-sized for more efficiency; you can only up-fire by 10%.

Thank you for the correction on that particular boiler.

rcleimbach

You guys have confirmed my suspicions. I need to have a hard conversation with my heating contractor, but I want to make sure I have all my ducks in a row. So I need proper information to support my claim that it was over sized.

I have learned the hard way that a heat loss calculation is the most definitive way to size a new boiler. However, it was mentioned above that a modern day house will require about 20 BTU/sq ft. Is this a "general rule of thumb" and would my heating contractor laugh at that from a sizing perspective?

I have measured the length of all the baseboard in my house and it comes to 94 feet. If I use a heat output of 600 BTU/hr per foot this equates to 56,400 BTU/hr. However, I realize that this is also somewhat simplified as it does not take into consideration windows, doors, type of insulation, etc. I am up for performing a complete heat loss calculation if anyone can point me to an online calculator. How would you recommend I broach this conversation with my heating contractor? I suspect they will become defensive as I am calling out their work.

Zones - it was mentioned that some of the zones can be combined together and that it only requires some minor control wiring. Can you help me understand this? I thought the only way to combine zones would be to do some re-piping. My second floor has 3 bedrooms. The master and its bathroom are on one zone and the other two bedrooms and a bathroom are on another zone. Are you saying that I can combine these into one "effective" zone by having one of the thermostats control two zone valves? So anytime the one thermostat calls for heat, it opens 2 zone valves? Then I would be able to eliminate the thermostat that is not in use anymore?

Buffer tank - I understand the premise of this, but seems to me to be a "band aid" to alleviate the short cycling. You are increasing the thermal mass anytime a zone calls for heat and then elongating the cycle time. It seems to me that this would increase my oil consumption and thus my costs over the lifetime of the system. I get that this is a trade off of premature wearing out of the boiler.

What is the ideal cycle time range for this Buderus model? I keep reading that in general a cycle time should be in the 8-15 min range.

Thanks again for everyone's help!

KC_Jones

The 20 I mentioned is sort is a reality check idea to see if the boiler you have is even in the balllpark. Some contractors don’t do heatloss and uses numbers like 40-50 per sq ft, but that’s outrageous.

In your case sizing to the radiation would have been significantly better even though it’s not right.

Based on your radiation, your system is capable of a max output of 57,400, so any boiler output above that isn’t able to be used at all. The limit for output is always the radiation.

bburd

I would not combine zones to reduce short cycling. While it can increase wear on electromechanical components and slightly reduce fuel efficiency, making the change would cost money, reduce the close temperature control and potential fuel savings of multiple zones and possibly lead to increased fuel use due to overheating in some of the combined areas.

Regarding the bedrooms in particular, doesn't it make sense to be able to turn down the heat in those you are not using? If not now, then possibly when the kids (if you have them) move out?

Sometimes a buffer tank is recommended to reduce short cycling, but that would cost so much to install that I think the net effect would be an increase in total heating costs.

IMO some people here worry too much about this. No system is perfect, and the cure may be worse than the disease.

EBEBRATT-Ed

As @hot_rod always says "the baseboard drives the system" and the boiler responds to the baseboard. You boiler is way oversized if it cycles that often.

Yes, you can make several zones into one large zone which will likely help but comfort may suffer if the zones do not have the same heat loss exposure. But it is small expense to try that first. Just reconnect the thermostat and zone valve wiring.

But you need a buffer tank to make it work right.

A buffer tank is not usually a band aid. Used properly it helps when you have a lot of small zones even with a properly sized boiler.

In your case the buffer tank is a band aid because the install was not sized correctly.

Sometimes band aids are the only fix short of a new boiler.

I would consider a boiler that is not burning for at least 15 min a short cycle. Starts and stops kill equipment

fentonc

I have a 3-zone system and a boiler that's way oversized for the total radiation installed. After the initial burn which is 2-4 minutes, like 95+% of the burns my boiler initiates last for ~76 seconds while it heats the water 20F and the aquastat cuts it off again.

@rcleimbach - adding zones requires re-piping, but combining zones just involves a couple of wire nuts and some low voltage wiring. Having gone down a similar rabbit hole, the wiring is pretty simple to understand with honeywell zone valves. Each zone valve has 4 wires - two of them provide 24V power to the motor that opens the zone valve, and two of them are an end-switch that makes contact when the zone is open. Your 24V transformer has 2 contacts - R and C. Typically the C contact is wired to one side of all of the zone valve motors, and the R contact is wired to all of your thermostats, with the the 'W' wire from each thermostat coming back to the matching zone valve. When the thermostat closes it's contact, the zone valve motor gets 24V power and opens. All of the zone valve end-switches are wired in parallel to the two "T-T" contacts on the boiler, so that if any of the zone valves open, it connects the two boiler contacts together through the end switch and 'calls for heat' at the boiler (which will turn on the circulator and start maintaining the temperature of the water).

You have two 'easy' options that don't involve re-piping or buying an expensive controller:

• To combine zones, you can disconnect the desired thermostat 'W' wires going to the zone valves, and connect them instead to the 'W' wire coming from the thermostat you would like to control them. When the remaining thermostat calls for heat, all of the zone valves connected to it will open and the zones will all act in sync.
  
• For zones where you might not care as much about temperature swings (especially pretty small zones), you can just disconnect the end switch wires coming from the zone valve. The thermostat will still open and close the zone valve, but the boiler will ignore it until another zone valve with its end switch still connected calls for heat. This is kind of a poor man's 'zone synchronization', as it means that zone will just wait until another zone calls for heat, and then the boiler will see both zones in terms of heating load.
  

Hot_water_fan

A heat loss isn’t necessary here: you need a much smaller output than any oil boiler can provide (about 64kbtu or .6gpm). So buffer tank. 

hot_rod

Gosh the big selling feature of hydronics is zone-ability for comfort and efficiency. I would combine zones as a last option. Since you cannot resize the boiler, the buffer tank is the best fix.

Is it worth the $$ to do so? That is a tougher question to answer. How much will the cycling effect the lifespan, or health of the boiler? It's certainly not a good operating condition, the boiler and flue piping may be paying the price.

For full time home occupants, just the sound of the boiler yo-yoing all day is worth the $$ to correct.