Using TPI controller with zone valves - advice from smart plumbers please :-)

Discussion in 'Plumbers' Talk' started by Poucha, Jan 24, 2013.

  1. Poucha

    Poucha Member

    I'm looking to upgrade the controls on my heating system and would like to use a Honeywell CMT927 controller.
    One of the main features I want is the TPI, which regulates the heating better when it gets near to the temp set point, rather than having the hot/cold swings we get currently with a mechanical thermostat.
    However, our system has zone valves which need to be opened/closed when the system swiches on/off. They take minutes to activate (and are noisy!) and I don't want them worn-out by continuous open/closing cycles.
    Ideally, I want a system that keeps the zone valves open during the ON period and just cycles the boiler/pump on/off to control the heating within that period.
    Obviously this may cause issues when another zone calls for heat (e.g. I don't want the heating on if the hot water comes on in the summer!).

    Is there a system/box/wiring method that can give me a more steady control of the heating?
  2. tom.plum

    tom.plum Screwfix Select

    hi poucha, first of all I'm not a smart plumber,I'm just an old fashioned 'big hammer and bent pin' old skool type, but I know what you're asking is not logical, the mv.s need to be opening and shutting, thats how they control the temp and keep the system ecomic and eco freindly, If you keep the water cycling the boiler will also keep cycling, so thats like parking your car on the drive and keeping the engine running, Its just not done,
    peter will be along shortly, he's a smart plumber he's got degree's in something I can't even spell, but Ithink he'll agree,
    Oh and I'll be interested in what Devs thinks too,
  3. sam spade

    sam spade Active Member

    You need to replace the two zone valves with a  diverter valve with auxiliary switch. (This looks like a midposition valve but you can only have Port A open or Port B, you can't have both open at the same time.) Install the valve so the heating circuit is connected to the normally open Port (normally Port B) and the hot water cylinder to the normally closed port (Port A).

    When CH is operating the valve will stay permanently open, so there is no problem with the valve cycling open/closedf. When Hot Water is called the valve switches over so only the cylinder is heated. OK you are without heating for a while, but this is no different from what happens with a combi boiler. The whole output of the boiler will be devoted to Hot Water, so heating up time will be quicker.

    Wire the CM927 and the Auxiliary switch of the diverter valve to the boiler switching terminals. The motor connections of the valve are controlled by the cylinder thermostat.

    The CM907 will still sent instructions on/off to the boiler when HW is requested, but they will be ignored as the HW auxiliary valve will provide a permanent circuit across the switching terminals of the boiler.
  4. heatyman

    heatyman Well-Known Member

    To achieve your objectives you need a full compensating/optimising controller with sensors and valving to suit. Your potential savings would be nowhere near the cost, and in domestic circumstances would not perform anywhere near as effectively as you would expect, as the area is too fragmented to be monitored.
    What sort of swings are you getting at the moment?
  5. petertheplumber

    petertheplumber New Member

    Hi poucha, I feel you are going the wrong way, change your 2 two port valves for Honeywell they are quite and quick operating since they don't have to keep rotating to be on or off. Don't use the boiler to decide whether heating is on or off that is not cost effective and will wear the boiler out. Which is what tom said, A wireless programmable stat linked to the heating system will save you more money than anything else, domestically speaking, it  doesn't switch on and off it switches up and down to stop the property getting too cold. Also try and arrange the hot water to come on and different times to the heating, it will make the heating work better.
  6. Onetap

    Onetap New Member

    What boiler is it?

    I just had a read about the TPI controller and wasn't impressed. It seems to control the flow temperaure (good) by switching the boiler on & off (not good).

    If you have a boiler with a modulating burner, it would be more sensible to control the flow temperaure (good) by varying the burner output (good).

    TPI sounds impressive, but is merely a variation of PID that has been around, in heating controls,  for decades.
  7. Poucha

    Poucha Member

    Thanks everyone. It looks like I can't achieve what I want without spending a lot of money. The temperature swings about 1 degree, which doesn't seem much but at this time of year it goes from being too cold to too hot all the time. The boiler is an oil fired Trianco (240kBTU) and it's a big system with 40mm pipes to and beyond the zone valves - hard work to do any simple upgrades! There 3 zones (inc Hot water) so a diverter wouldn't work either.
    Also, there's no pump overrun so often the last burner blast gets trapped in the boiler. I suppose if it had overrun the temperature swing would be worse as it would keep heating for longer after the termostat switches off.
    I was hoping I could find a way to make the system more sophisticated without completely renovating it...
  8. petertheplumber

    petertheplumber New Member

    It looks like a 70kw input boiler so I stand by what I said earlier, change zone valves to Honeywell they are much quicker and quieter, and a wireless room stat will save you money. The Energy Efficiency council say that, I have fitted 8 in peoples houses including my own. And no complaints.
  9. Poucha

    Poucha Member

    Thanks Peter - changing the valves may be the only way to do it but the cost will rule it out. Please can you explain though:
    I understand that the new stats smooth the temperature swings by switching the pump/boiler on/off in cycles when it gets near to the set temperature - surely this mans that the valve would also have to open/close many times (my valves switch the relay that switches the pump/boiler)? - Because there are multiple zones, the valves have to be off when no heating is called for, in case there is demand in another zone. Or is there a thermostat that has two outputs - one that is always on during a timed "on" period (to hold the valve open), and one that has an "intelligent" output (to pump/boiler)?
  10. What do I think? I think 'whoooosh' (passes flat palm waaaay over head)
  11. sam spade

    sam spade Active Member

    As it's an oil boiler, you need to set the CM927 to 3 cycles per hour.

    So the times when the CH zones are open can be different, but the temperature will be the same as there is only one thermostat? Or are you thinking of two CM9XX thermostats, one for each zone.
  12. sam spade

    sam spade Active Member

    You can't use the bog standard V4043 valve in this installation. The pipes are 40mm!
  13. Poucha

    Poucha Member

    Yes Sam, I'd like to put a thermostat in each zone (house, annex) so they would come on independently.
    And yes, they are big fat valves and actuators so replacement would not be easy or cheap.
    Why would an oil burner need 3 cycles per hour?

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