Oil Boiler Zone Valve Question

JD1234p

So I have a Honeywell basic battery digital thermostat, house is set to 68 all winter no schedules or anything. When its a bit warmer during the day ~35-40 degrees outside, when the thermostat calls for heat, the zone valve begins to open ~60sec. Once open, then the circulate pump turns on then eventually the boiler. Boiler runs for ~5-8 min turns off (i assume it hits the high limit temp) circulate pump is still going. Thermostat hits the temp its looking for and turns off the call, now wait the ~60 for zone valve to close then circulate pump stops. All is good.

Well at night when it gets colder and everyone is sleeping, when my living room zone is being called, same as above the boiler shuts off because it hits the high limit, the circulate pump is going (a bit longer than during the daytime since no one is in the room) the thermostat hits the temp its looking for and stops the call, now ~60sec wait for zone valve to close...circulate pump still going, the boiler hits i think the low limit and still thinks a heat call is calling and begins to fire up and runs for like 5 sec until zone valve closes. Depending on how cold or warm at night it doesn't always do this. How can I fix this, HL is 180 and LL is at 160 and the boiler is a tank-less coil for the hot water. Seems like if the zone valve closed quicker i wouldn't have this issue. Would lowering the LL to 140 or 150 give the boiler more time before it needed to fire back on giving the zone valve more time to close. Let me know.

Jamie Hall

Try playing with the cycles per hour or system type on the thermostat (it's in the settings menus -- check the instructions) and make sure that it's set for 2 cycles per hour or for hot water heat.

JD1234p

It doesn’t have those settings. Just hold temp and scheduling and time date. 

DJD775

How often does it happen? Happened on my old system once in a while and the conclusion I came to is that the water temp hit the high differential limit, thus firing the boiler, seconds before the contacts open on the zone valve to stop the call for heat. If this happens more frequently maybe there is a problem.

hot_rod

probably an oversized boiler? Ideally at design days the boiler would run non stop, adding just as much heat as the boiler was losing.

If you have spring return type zone valves, the end switch usually opens within a few seconds of the end of a heat call.

Heat motor type take a bit longer to close, but the pump should also stop within a few seconds also.

One trick we used back in the day was two stage thermostats, pump could run first using boiler residual heat, burner would fire if temperature continued to drop.

JD1234p

It fires back up because the amount of time the zone valve takes to close the boiler thinks a heat call is still active until the zone valve fully closed. 

Jamie Hall

JD1234p said:

It doesn’t have those settings. Just hold temp and scheduling and time date. 

I've yet to see a Honeywell which didn't somewhere in the set up menus -- but if it doesn't you may need a different thermostat. What model Honeywell is it?

Robert O'Brien

Why do you need low limit? Tankless coil?

EdTheHeaterMan

JD1234p said:

It doesn’t have those settings. Just hold temp and scheduling and time date. 

Post the thermostat model number.  The cycles per hour are sometimes set by a switch on the back or in the installers set up menu

Dave Carpentier

I got around that short-burn situation by splitting the boiler control from the thermostat-circulator control.
On mine, a 24v transformer feeds the tstats which control the zone valves and turn on the circulator.
The TT at the boiler was jumpered/strapped. The boiler always operates between the low and high settings of it's aquastat.
This worked great to prevent dropped calls from shutting off the boiler randomly just as it started a burn.
The drawback is, now it's an "always hot" boiler.

More recently, I improved the swing range of the boiler by using a Ranco .. but thats another story.

EdTheHeaterMan

Dave Carpentier said:

I got around that short-burn situation by splitting the boiler control from the thermostat-circulator control.
On mine, a 24v transformer feeds the tstats which control the zone valves and turn on the circulator.
The TT at the boiler was jumpered/strapped. The boiler always operates between the low and high settings of it's aquastat.
This worked great to prevent dropped calls from shutting off the boiler randomly just as it started a burn.
The drawback is, now it's an "always hot" boiler.

More recently, I improved the swing range of the boiler by using a Ranco .. but thats another story.

You have an "always hot" boiler... do you need to maintain boiler temperature for a tankless DHW coil? If not, then the savings or percieved efficiency of missing that occasional short cycle at the end of a call for heat is MORE than offset by maintaining a higher than needed boiler temperature.

Think about it. the boiler will cool to room temperature over time. the chimney draft will reduce as a result of the lower boiler temperature. The infiltration of air that enters the home to make up for what is lost up the chimney is reduced.

Add just 50 degrees to the boiler temperature from 70° to 120° (and 120° is a low temperature to maintain) and you will be burning more fuel to maintain that temperature for no other reason but to stop a 30 second burn at the end of a call for heat. If you can afford it in fuel cost to make you feel better, then more power to you!

JD1234p

The problem is…it only fires up for like 3-5 seconds then turns off once zone valve closes. Seems like a waste and can put stress on the boiler. 

EdTheHeaterMan

JD1234p said:

The problem is…it only fires up for like 3-5 seconds then turns off once zone valve closes. Seems like a waste and can put stress on the boiler. 

As I said, If you don't mind the added expense of maintaining some temperature higher than the boiler room, then go for it. As far as added stress... That boiler is already hot from being off by the high limit. what more stress is there? If another zone happened to call for heat just as the satisfied zone was satisfied, the stress of firing the burner would be the same... it would just last longer than 5 seconds or at least as long as the call for heat lasted on the second zone's calls for heat. But in your opinion and gut feeling, 5 seconds is wrong on so many levels, so let's spend more on fuel, so that you feel better about eliminating a short cycle. It's is YOU that has to live there... Make yourself happy. Burn the extra fuel. You will feel better.

JD1234p

I'm not a boiler guy so what do you mean as maintaining temperature higher than boiler room? Do i set the high limit higher? I was hoping to adjust the boiler so it wouldn't turn back on until a lower temp that way i have more time for the zone valve to close. Seems like once the boiler temp gets to ~160 when the circulate pump is still going it fires back up thinking its still calling for heat while the zone valve is closing. how can i get the boiler to fire back up maybe around ~150 degrees, giving me extra time for the zone valve to close and not have the short cycle.

mattmia2

The 2 ways to eliminate this would be to have an integrated BMS that controls the boiler, zone valves, and circulator based on the room temp and have it all integrated in one control or to have a post purge set up on the boiler. The simple controls you have don't allow for either of these.

Dave Carpentier

I think the excess boiler heat (due to "always hot") that's lost into the room is still inside the structure. In my particular case, it drifts up into the drop ceiling space and keeps the floor above a little warm.
Excess heat lost up the pipe would certainly be a thing though.
I would imagine that as the temperatures approach design temp, the difference between always-hot and run-on-demand starts to diminish... perhaps to the point of advantage going to always-hot ?
Granted, we dont spend much time at design temps (thankfully).