another thermostat question

gschallert

Reading all of these thermostat topics got me wondering about CPH and swing in my own system and the in-laws new modcon boiler. I have no idea what mine is set to and I'll have to try and find the manual to look up how to do that.

I have a new standard CI boiler with finned hw baseboard and more radiation than needed for my heating load. How many CPH is recommended in this case?

In-laws just had modcon installed, they have mostly finned hw baseboard and one or two CI rads across two zones. I don't know if they got new t-stats or kept their existing either. Does a modcon have a recommended CPH setting or because it modulates is it supposed to cycle more like forced air furnaces?

What would constitute short cycling in each setup?

SWEI

Mod/con boilers and conventional thermostats do not often make great partners. Once an ODR curve is properly adjusted, a conventional thermostat is best used as a high limit control. Set it a couple of degrees above the space temp for which the ODR was tuned and just leave it there. This will reduce overheating in times of high solar gain, when the woodstove is covering the load, or when a big meal is cooking in the kitchen.

If you want the thermostat to have any real control authority, the ODR curve must be shifted several degrees (think 8-10°F) upward, which comes at a cost both to efficiency and to comfort.

The best solution is proportional zone valves, the most common example of which would be TRV's. These work with the ODR instead of fighting it like conventional (on/off) zone valves and thermostats do.

gschallert

Turns out I didn't have the installation manual for my thermostat, just the user manual. I did manage to find an online version of it though and once I found the instructions I discovered my CPH was set to 5 so I changed it to 3 for now and will try to monitor cycles/burn times for heating on my standard CI boiler (no outdoor reset) - it will let me choose 2 or 1 if necessary so depending on what I observe I might see if 2 can keep the house comfortable. I keep the t-stat at 70 with no setbacks.

SWEI said:

Mod/con boilers and conventional thermostats do not often make great partners. Once an ODR curve is properly adjusted, a conventional thermostat is best used as a high limit control. Set it a couple of degrees above the space temp for which the ODR was tuned and just leave it there.

We'll be staying @in-laws at least one overnight over the holidays so I'll try to gather some information when I'm there. From what I've read of the boiler (Vitodens 100) there is no "adjusting" for the ODR of this boiler. It's just a dial that can be changed as necessary to use a different preset curve for water temp depending on outside temp. I think he's got it set to 4 currently which seems to be keeping the house comfortable but not hot and it is condensing he said. I'm excited to check it out in person and see how much gas they've used and how often it's firing etc. Being a modcon, should it's CPH be set to 3 as well? The instructions for mine said to use 3 for any "hot water system"

SWEI

Properly adjusting the ODR curve is not all that hard: Crank the thermostat way (like 85-90°F) up and leave it there. Turn the Vitodens dial down about two 'points' from where it is now. Spend the next week or three fine-tuning the dial until the house remains comfortable across a broad range of outdoor temps. Once that's done, lower your thermostat setpoint until it's 2-4°F higher than the current room temp.

gschallert

SWEI said:

Properly adjusting the ODR curve is not all that hard: Crank the thermostat way (like 85-90°F) up and leave it there. Turn the Vitodens dial down about two 'points' from where it is now. Spend the next week or three fine-tuning the dial until the house remains comfortable across a broad range of outdoor temps. Once that's done, lower your thermostat setpoint until it's 2-4°F higher than the current room temp.

Seeing as how I'm only going to be there one night I think I'll stick to making sure the t-stat CPH is set correctly for hot water and clock how many times the boiler cycles per hour for heat.

I sat in front of mine for an hour or two to monitor boiler cycles and ended up setting my thermostat for 2 CPH and so far it's keeping the house comfortable from single digits to mid 30's over the last week. I wonder why they specifically recommend 3 CPH for hot water systems or if it was just the happy medium between steam and forced air.