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Heating system hysteresis and efficiency
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paulca:
I'm upgrading my custom control heating system.  Better put, I have "been" upgrading my custom heating control system and the core architecture needs redone.

While I'm at it, I want to address a problem.  A problem I thought I'd solved, but it resurfaces.

Cycling. 

So my "make it as simple as you can" approach resulted in a design which simply used a minimum expiry time on a heating demand of 5 minutes.  If anything turned the heating on, it would remain on for at least 5 minutes.  This works.  It results in a slightly over temp in zones by 0.5*C or more in some rooms, but it prevents on/off/on/off/on/off cycling every 2 minutes fighting over 0.1*C.

But it still results in 30-40 cycles per day this winter.  I feel I need to find other hysteresis methods.  At the same time I'm trying to get an answer from the boiler manufacturer as to reasonable cycle frequency over the boilers lifespan.

Problems and solutions:
1. Multiple zones request heating at different intervals.  These are processed individually.  So it's open to inefficient demand from individual zones in sequence when the demand could be handled more intelligently so multiple zones can be heated together instead of in turn.  Requires predicting if zones are close to triggering demand while the heating it already on for other zones.  To put it an other way, the living room might demand heating, its radiator comes on, the boiler comes on.  10 minutes later it stops demanding heating and it's radiator and boiler shut off.... but 2 minutes later the bedroom demands heating, as it's radiator was off it didn't get any.  This is a waste.

2. On at 16.9*C and off at 17.1*C margin cycling.  This is caused by the lack of temperature hysteresis control.  Effectively a zero dead-zone hysteresis.  If it's over temp, demand is removed, if it's under demand is applied.  Adding temperature hysteresis would provide a negative trigger (target -0.1*C) and a positive (continue heating until +0.5*C).  So a positive + negative hysteresis.   This is also required for the above to determine if a zone, while not triggering, could use heat as it's close to triggering.  Any zone under it's positive hysteresis limit could be heated as soon as any zone triggers it's lower negative hysteresis limit.

3. Lowering boiler output (flow temp) lowers cycling, but slows warming unused zones up for use.  A heating saving feature is to not heat (as much) zones not in use.  This requires I can heat them up quickly for use.  The solution to this, considering that lower flow temp will lead to less cycling, is dynamic flow temp control.  Just as the boiler itself will modulate back to maintain a flow temp, the flow temp can be modulated back as target temps are reached.  When a zone heated to 15*C comes into service, it can be rapidly heated with the boiler maxed at 85*C but the flow temp back well off as it approaches target of 19*C and then, maybe balance out with the boiler able to supply enough heat to stop the temperatures rising or falling. Another solution to this is predictive heating.  I already have an implementation of fore running schedules for a time in the future, deciding on target temperatures based on gradients.  So if I know in advance a zone will jump up in temp, I can pre-ramp it in advance and not require high flow temps.

Other things I could consider are load sensed, weather aware predictive heating.  Boiler manufacturers seem to do this and aim for long run times.  The challenging thing is how to implement it for multiple zones with varying load patterns.  I don't think my routine is stable enough to predict.

Sorry for rambling.  Having lack of coffee pot conversations in work isolation issues with lock down.



nctnico:
The solution is simple: make sure the system has a loop between hot and cold (return) at the fartest point from the boiler and let the boiler shutdown when the return water is hot. This means no (or very little) water flows into the radiators (which must have thermostatic control valves). Once the boiler sees the return water is getting cold again, it can switch on again. Since the heat will end up in your house, no heat is lost.
nali:

--- Quote from: paulca on January 29, 2021, 05:13:22 pm ---But it still results in 30-40 cycles per day this winter.  I feel I need to find other hysteresis methods.  At the same time I'm trying to get an answer from the boiler manufacturer as to reasonable cycle frequency over the boilers lifespan.

--- End quote ---

Take a look at your boiler's manual and see if it supports chrono-proportional thermostats. These basically PWM the boiler every few minutes as part of a PID control so it would easily handle 30-40 per day if compatible.

But that does seem a little high. I zoned my heating with motorised valves and occasionally suffered from hunting on/off. I cured that by throttling back the radiators, they were too high meaning a surge of heated air was shutting off the 'stat for a few minutes causing a cycle until the room heated up. Looking up how to balance radiators the starting point seemed to be to close off the lockshield valves and crack them open 1/2 turn which seems to work OK.

NiHaoMike:

--- Quote from: paulca on January 29, 2021, 05:13:22 pm ---1. Multiple zones request heating at different intervals.  These are processed individually.  So it's open to inefficient demand from individual zones in sequence when the demand could be handled more intelligently so multiple zones can be heated together instead of in turn.  Requires predicting if zones are close to triggering demand while the heating it already on for other zones.  To put it an other way, the living room might demand heating, its radiator comes on, the boiler comes on.  10 minutes later it stops demanding heating and it's radiator and boiler shut off.... but 2 minutes later the bedroom demands heating, as it's radiator was off it didn't get any.  This is a waste.

2. On at 16.9*C and off at 17.1*C margin cycling.  This is caused by the lack of temperature hysteresis control.  Effectively a zero dead-zone hysteresis.  If it's over temp, demand is removed, if it's under demand is applied.  Adding temperature hysteresis would provide a negative trigger (target -0.1*C) and a positive (continue heating until +0.5*C).  So a positive + negative hysteresis.   This is also required for the above to determine if a zone, while not triggering, could use heat as it's close to triggering.  Any zone under it's positive hysteresis limit could be heated as soon as any zone triggers it's lower negative hysteresis limit.

--- End quote ---
Both of those can be solved by making the hysteresis system wide. That is, all the thermostats raise their trigger points by the hysteresis value you set once the boiler turns on and lowers it when the boiler turns off.
james_s:
Most of our systems here are forced air, so the details are a bit different. Ideally the system should be sized so that it runs as close to continuously as possible on the coldest days and minimizes short cycling, although in practice that is difficult. Historically our heating systems tend to be grossly oversized which is nice in that you can warm up a cold house very quickly but they end up cycling frequently enough to really cut into the efficiency. Some of the fancier gas furnaces have multi-stage gas valves that can help with this.
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