Boiler cycle time with outdoor reset

lanem2494

Hi all,

Happy New Year.

I have a three year old Viessmann Vitodens 100-W (model B1HA 125) boiler that I recently hooked up to an outdoor temperature sensor to enable the outdoor reset functionality of the system.
I have a 1,900 sq foot colonial with fin tube baseboard and two Nest thermostats (one on each floor). The house was built in 1985 and I would say it has above average insulation - all new replacement windows five years ago in addition to an attic with plenty of blown in cellulose.
I am located in MA northwest of Boston.

I'm still learning some of the ins and outs of outdoor reset, but from what I've read up on it seems like the lower supply temp and the longer cycle time I can get away with, while still having the house comfortable, the better. I currently have the set point on the boiler at 65 degrees F and the curve at 1.6, as well as each of the two thermostats set for 65 degrees F.

Since installing the outdoor sensor a few days ago, I'd say that each cycle time for each zone is lasting around 1 hour give or take before the inside thermostats are satisfied and the boiler cycles off. I haven't noticed much of an overshoot on either thermostat but the downstairs thermostat has occasionally hit 66 degrees F. It's probably around another hour before the thermostats call for heat again and the cycle restarts. The outdoor temps during this time have ranged from the mid 40s to the upper 20s.

I'm curious to know if there is a general or target amount of time that each boiler cycle should last given my situation, or if anyone in a similar situation can share their experience.

I am thinking of gradually lowering the curve and monitoring the performance until I find the lowest but most comfortable setting - thoughts?

Any and all feedback is appreciated.

Big Ed_4

The ideal set up is to heat the home with the burner running constant on the coldest day of the year ...

Jamie Hall

You can probably lower the curve... some... but be aware that there are conditions other than outdoor temperature which affect your heat loss -- particularly solar load or wind. Thus an "ideal' curve such as @Big Ed_4 suggests is possible, it also will leave you cold on a windy or dark grey winter day, and we do get those.

It's a compromise...

EdTheHeaterMan

The ideal operation for any modulating boiler is to adjust the firing rate based on demand so the burner never shuts off. (unless the demand is lower than the minimum turn down rate of the burner).

Thermostat cycling off may be a result of one zone reaching desired temperature before the other zone. When that happens, the return water temperature to the boiler will increase as a result of the lower demand on the system. This information will be used by the on board computer to further reduce the firing rate allowing the burner to continue to operate more efficiently. Using the thermostats to determine if the burner is operating may not be the most accurate way to do it. Unless the NEST app shows that both thermostats are satisfied at the same time.

By reducing the reset curve a little at a time, you may get a slightly better efficiency but the amount is going to be less than 5% at this point. You are already getting most of the benefits of the boiler design. It will not hurt to experiment with lower reset curve adjustments as long as you don't sacrifice comfort.

NEST thermostats are designed to be used as automatic SET BACK thermostats. ODR and SET BACK are not a combination that works very well. ODR is great at maintaining a given temperature in a building. ODR will severely reduce the system's ability to recover from a lower SET BACK temperature

lanem2494

Big Ed_4 said:

The ideal set up is to heat the home with the burner running constant on the coldest day of the year ...

Understood, thank you for the reply. This is known as the “design day” correct?

lanem2494

EdTheHeaterMan said:

The ideal operation for any modulating boiler is to adjust the firing rate based on demand so the burner never shuts off. (unless the demand is lower than the minimum turn down rate of the burner).  

So should this be the goal on any given day and outside temperature (and not just the coldest day as Big Ed_4 suggested)?

NEST thermostats are designed to be used as automatic SET BACK thermostats. ODR and SET BACK are not a combination that works very well. ODR is great at maintaining a given temperature in a building. ODR will severely reduce the system's ability to recover from a lower SET BACK temperature

Yes, thanks for sharing. I read this often during my research recently. To be honest I never used the automatic or scheduled set back features of the Nest in the past, but I may experiment with a slight set back (1 or 2 degrees) going forward at night and/or when I’m away and see how it works with the ODR. Regardless the the Nests will be basically decorations at this point 

EdTheHeaterMan

Yes, on commercial systems Outdoor reset is used to maintain the same temperature in the room as the outdoor temperature lowered. Some commercial systems would have modulating burners on several boilers that would stage in and out as the demand changed. So something was always operating until you reached WWSD. In your case you would always have some gas flame until you reach the minimum turn down BTU input.

I remember a story about a retired boiler man that was living in a retirement building and he would tell the other "inmates” that it was cold outside or it was warm outside based on the water temperature in the heating unit. The old man still knew his fundamentals!

If you are going to try using setback, then you will want to leave some or the ODR savings on the table. If the reset curve is too close to "constant run" then there will be no capacity for raising the temperature for recovery.

hot_rod

I suspect the Viessmann has a boost function. So if it struggles to keep up on ODR supply temperature will automatically boost over the ODR setting. It’s based on time and may be adjustable.

Hot_water_fan

I'm curious to know if there is a general or target amount of time that each boiler cycle should last given my situation, or if anyone in a similar situation can share their experience.

It could easily run for months straight - but this particular Viessmann doesn't have a great turndown ratio and it's well oversized, so you won't get that in practice.