Yet another Viessmann (ultra) short cycling post

slisli

Hi all,

I own a Vitodens 200W B2HA series, 49KW. This is for a fairly large property. I have a low loss header, connected to two water tanks for hot water (works OK), and a large UFH system. Each UFH zone is controlled by thermostat independently.

When there is low demand (1 or 2 rooms), I get into the issue: where the boiler fires up, flow temperatures remain reasonably constant (both at the boiler, and at the mixer for the UFH circuit), and the Boiler Temperature shots upwards towards 70C. Before the initialization cycle finishes and the boiler can modulate downwards, the burner shuts. This all lasts maybe 15 seconds. This keeps happening over and over. If 3 or more rooms demand, then I get past the "initial" stages and all is well (cycles can still be short, (at 10-15 mins), but not ridiculously short (at 15 secs)).

My guess of that the situation is that the boiler tries to be smart and thinks there's no load, so shuts itself down (as no temperatures of any sensor go higher than when there is DHW demand). I'm wondering if there are any settings I could play with in the boiler (ideal solution) to disable such feature or make it less sensitive, or some hacks to go past the initial stages. My own thoughts:

-When burner starts, also activate DHW for 1 minute to create extra load to go past the init (not sure if this is possible / what controllers would allow this).
-Use a small buffer (there's not much space left / seems annoying).
-Find another way to increase the load for the first phases of ignition.
-Configure the boiler (unclear how)

Thanks so much!

JMWHVAC

What do you mean, "Flow temperatures remain fairly constant..." You should have enough flow thru boiler that the temp surges everywhere as it does in boiler. Makes me wonder if there is enough flow thru boiler. However, I have seen this annoying problem with Triangle tube boilers, where the temp surges past its differential (under low load) before it is thru its initiation. In my case it was a lot more than 15sec tho. Triangle tube does not allow me to adjust the differential which is why I went to another manufacturer where I can set a higher differential to mitigate short cycling. Seems you are saying viessmann does not allow differential adjustment??

Alan (California Radiant) Forbes

Why do you have a mixing valve for the UFH? Are there also radiator zones?

I would try reducing your slope. 

pedmec

you don't have enough of a demand. With only one or two rooms calling its not going to shed the ignition start up burner. In most boilers your ignition speed is 40-60% of the full load before it will ramp down. you need a place to shed that load. a buffer tank or combine the rooms to have a bigger demand.

slisli

@JMWHVAC that's right, during the short period (15 secs) of burn, the burner goes from, say, 40C to 70C, whereas the Boiler Flow Temperature remains at 40C (it may move a little bit upwards, but 5C or so at most). I'm not surprised that boiler flow would lag behind, though. But if indeed heat isn't being transferred fast enough from Boiler to Boiler Flow, what should I look to do? Boiler Heating Pump settings (increase pump speed)?

@Alan (California Radiant) Forbes I do have a mixing valve as I'm controlling two circuits: UFH and DHW (and, indeed, a third circuit of towel radiators).

JMWHVAC

Everyone agrees low demand makes these issues but if flow thru boiler is lacking that will make it worse. If there is a higher speed I definitely would try it and see if there is any difference. I put a few viessmann mod-cons in already but I don't remember what all settings they give us.

PC7060

pedmec said:

you don't have enough of a demand. With only one or two rooms calling its not going to shed the ignition start up burner. In most boilers your ignition speed is 40-60% of the full load before it will ramp down. you need a place to shed that load. a buffer tank or combine the rooms to have a bigger demand.

This ^^^

Alan (California Radiant) Forbes

The pump for your UFH should be located after the mixing valve, correct? Is that pump powered through the boiler or an external zone control?

I've seen issues such as yours come about when the V. boiler is not in control of the pump.

slisli

The UFH pump is controlled by V KM-Bus (which is activated by Heitmeiser thermostat). I tried increasing the internal pump speed (it was set at 6/9), but the cycling persists.

EBEBRATT-Ed

Those boilers as well as all Mod Cons hold hardly any water. Controls cant get a handle on a system with no water volume to work with. I would add a buffer tank.

Water chillers work with only a 10-degree TD on the water. Water volume must be at least 3 gpm/ton for comfort cooling and 6 gpm/ton for process or the chiller will cycle itself to death.

Boilers are no different short cycling causes ware and tear and damage due to constant expansion and contraction.

The boiler and it's controls need some water volume to work with. Zones that are valved off don't count.

A buffer tank is the only fix.

Alan (California Radiant) Forbes

I like what @pedmec said:

or combine the rooms to have a bigger demand.

Alan (California Radiant) Forbes

You can call those rooms an uninsulated buffer tank. 

V. technical support (844) 649-5886 may have the answer you want. 

GGross

The default programming for that boiler completely ignores thermostat input and runs the boiler based on supply temperature at the low loss header, set up like that will reduce short cycling. Other than that you can combine zones, add a buffer tank in place of the low loss header etc.

Personally I would get into the menu and just make sure it was not dropping the burner due to low flow first though. Those boilers will not show an error code when they are locked out due to flow (unless you have added an external flow switch to the unit)

slisli

Thanks for the suggestion @GGross ! How would I figure out if it dropped the burner due to low flow? I can attach some pics when I'm home, but there is a pump for the UFH circuit. Flow for when there is low demand is 6-10 liters / min if I add the flow in the manifolds (2/3/4 circuits or so).

GGross

It would be in coding address 38 in level 2 coding. Not easy to see as you need to have it open while the flow would drop out. The flow in the heating circuits doesn't matter too much, the boiler just cares about what the flow is through the heat exchanger. The only reason I can see this happening is if there is a bunch of air in the system.

slisli

OK -- the main pump is set at 6/9 and it actually is fairly noisy, so it's possible there's air in it (pic of the pump setting attached). It is incredibly trivial to reproduce the issue so I'll film the whole shut down whilst monitoring 38/L2 later today. Would this theory be compatible with the fact that when there's more load through the heating circuits the boiler doesn't appear to cycle during startup?

Alan (California Radiant) Forbes

@GGross said:

make sure it was not dropping the burner due to low flow

He's got a LLH so the boiler sees plenty of flow, no?

It seems as though the boiler's firing rate is too high on startup, at least when maybe only one towel warmer or one small zone is calling for heat. I'd see if there's any way to reduce the initial firing rate.

And to be clear, your system does not need the mixing valve because you have UFH and DHW. Your boiler will recognize the difference between a call for heat and a call for DHW. It will produce elevated water temperature for DHW production and reduced water temperatures for UFH, so if you had only UFH, you wouldn't need a mixing valve. The presence of towel warmers forces a two-temperature heating system and the addition of a mixing valve. Your towel warmers are the party poopers.

slisli

Thanks, good to learn. Are mixing valves bad from an efficiency point of view? I'd assume so even if what they mix is the return flow (due to passive heat loss). Also, at least as I have it commissioned right now, when there's DHW demand from the tanks, my UFH keeps pumping (this isn't the default configuration as I understand). Would you recommend setting it so that DHW blocks UFH (even if I have the mixing valves)?

slisli

"I'd see if there's any way to reduce the initial firing rate." <- looked through the manual and couldn't see a way, but that would be my first bet on how to fix my problem. I contacted Veissmann and suggested code 83 but I can't seem to access that.

EBEBRATT-Ed

A blowtorch with a cup of water. Makes no sense to me. Operating like that will kill the HX. An HX that holds no water will not last if it doesn't have flow and a place to reject heat. jmho

slisli

The house is 700 SQM. But the weather is mild at the moment, and not all the time we'll need >20KW output. Tricky load, and was hoping modulation is there for precisely this reason.

Alan (California Radiant) Forbes

That like 10 BTU’s per square foot. Must be a pretty tight house. 

How many towel warmers are there?  Does the thermostat for the bathroom floors control the towel warmers?

slisli

Not many people in the house / some rooms are guest rooms and are off : ) Towel warmers are not being used at the moment, UFH for rooms goes also through the bathrooms.

I did a test tonight after venting the boiler pump and I got a pretty decent burn with only one room demanding heat past initialization. You can see decent modulation too. We'll see if this last or if I was lucky this one time.