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Thread: Vaillant EcoTec 838 and hourglass

  1. #51
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    Quote Originally Posted by jdp80 View Post
    Out of interest, my 637 takes ~30 mins to hit flow temp first thing, it's rated to 24kW currently, doesn't appear to be cycling. Is this a symptom of the rating being too low?

    O/T: bruce_miranda did you get anywhere logging the ebus messages related to flow temp ?
    My Vaillant ecotec plus 831 seems to take a similar amount of time. The max central heating output is 24kw, but the boiler part load is setting is currently at 11kW.

  2. #52
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    Quote Originally Posted by bruce_miranda View Post
    Is there a mistake there? 30 mins to hit flow temperature? Your boiler should hit max flow temperature in seconds, not minutes. It's the return temperature that then rises to meet flow once things start going.
    I log a lot of data from the boiler every min. Flow temperature is just one of them.
    Yeah, 30 mins.
    Capture.jpg
    (approx 50% of zones on at 0630 + DHW .. DHW cylinder reached temp at 0745)

    I seem to remember you investigating the commands that get sent in order to verify the boiler's current maximum flow temperature ? I was playing with ebusctl but couldn't get anything to actually change the flow temp on the boiler. (I was going to implement different DHW/CH flow temps on a system boiler programmatically)

  3. #53
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    Quote Originally Posted by bruce_miranda View Post
    Is there a mistake there? 30 mins to hit flow temperature? Your boiler should hit max flow temperature in seconds, not minutes. It's the return temperature that then rises to meet flow once things start going.
    I log a lot of data from the boiler every min. Flow temperature is just one of them.
    Not a mistake. If you have 10 cold radiators and a cold boiler and all the radiators turn on fully at the same time the boiler does it will take quite a while for the output flow temperature to reach the set point.

    Initially there will be no rise in return temperature as cold water will be returning from the radiators which have considerable thermal mass, and the temperature differential the boiler can produce between return and flow will be limited and depend on the pump flow rate and the power output of the boiler.

    Lets say for argument sake that it can generate a differential of 20 degrees at your maximum pump flow rate. If your set point is 65 degrees then the radiator surface temperature at the cold end of the radiator (and therefore return flow temperature) would have to reach 45 before the 65 set point could be achieved. It will take some time for every radiator in the house to reach 45 degrees. Once the return exceeds 45 the boiler will start to throttle back to maintain the same output flow temperature with a smaller differential.

    If your boiler can really achieve a 40-50 (?) degree differential between return and flow then either the boiler is massively oversized for the radiators, or more likely your pump speed is too slow so that the water is unable to transfer heat away from the boiler quickly enough. This means the flow rate of the water is a bottleneck in your system as a given flow rate will have a maximum amount of heat in kW that it can transfer.

    I have a 23kW boiler (although turned down to 20kW I think, judging by the adjustment screw...) and about 16kW worth of radiators and I'd say it takes about 20-25 minutes to reach a 65 degree flow temperature from a dead cold start with all radiators coming on at once. Normally I don't do a dead start though as optimal start staggers the start time of each radiator and one bedroom is on at night, but if the heating is manually off and then turned on in a cold house all the radiators will come on at once. (And often the hot water as well, which is about another 5kW load)


    Quote Originally Posted by paulockenden View Post
    I'm not convinced that's true.

    The boiler is only fired when a TRV asks for heat, and that only happens once the valve has opened.
    Correct - when a set point is increased on an HR92 (such as a room schedule turning it up) it doesn't send back an increased heat demand until after the motor has finished opening the valve. So the HR92's will open before the boiler ever gets fired, which makes sense.

    Unfortunately it does it the same way around when closing when the set point is reduced - the motor closes the valve first and then sends a reduced heat demand afterwards, whereas IMHO it would have been better if it reversed the action when closing the valve - reduce heat demand first and then close valve.
    The controller doesn't fire the boiler and tell the valve to open at the same time. The sequence is controller sends new setpoint to TRV, TRV opens as required, if TRV needs heat it THEN asks the controller. Controller aggregates demand across all zones then signals the BDT or O/T bridge.

    At least, I think that's how it works.
    Yes that's correct, although I would also add that there can be a delay of up to about 4 minutes between when set points are changed until the new set point is actually sent to the TRV's - something that is very noticeable if you are manually turning the heating on/off. On the other hand after the TRV motor has finished opening the delay in sending a heat demand to the controller and then being forwarded on to the BDR91/OpenTherm bridge is only a couple of seconds.

  4. #54
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    In which case I still have my boiler turned up way too much for my under powered pump. And / or my bypass is set too low and is leaking hot water back into the return too early. My issue is I have too big a boiler and too small a pump, that is a fact. But it's made worse by having Evohome and OT. The first valve that opens starts the boiler at full power but because there is a lag in the other TRVs opening and because of the other issues I mentioned, my flow temperature rises too quick, in a matter of few mins!

    The graph of what today looked like doesn't look as pretty as the previous graph. Time for a new pump or to wind my boiler down further.

    Capture.JPG

  5. #55
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    Quote Originally Posted by bruce_miranda View Post
    In which case I still have my boiler turned up way too much for my under powered pump. And / or my bypass is set too low and is leaking hot water back into the return too early. My issue is I have too big a boiler and too small a pump, that is a fact. But it's made worse by having Evohome and OT. The first valve that opens starts the boiler at full power but because there is a lag in the other TRVs opening and because of the other issues I mentioned, my flow temperature rises too quick, in a matter of few mins!

    The graph of what today looked like doesn't look as pretty as the previous graph. Time for a new pump or to wind my boiler down further.

    Capture.JPG
    What do you currently have your 438 set on d.0 as max output?

  6. #56
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    Quote Originally Posted by mtmcgavock View Post
    What do you currently have your 438 set on d.0 as max output?
    30

    and I have a 15-60 pump set to max speed.

  7. #57
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    Quote Originally Posted by bruce_miranda View Post
    30

    and I have a 15-60 pump set to max speed.
    I'd be tempted to lower that a bit more, but it depends on your situation (How many rads, heat loss etc). I found that due to the EvoHome I've lowered my boiler to around 12kws (Non modulating) , due to the fact that not all the radiators come on at the same time with optimum start, different set points in temperatures and optimum turn off.

  8. #58
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    Quote Originally Posted by mtmcgavock View Post
    I'd be tempted to lower that a bit more, but it depends on your situation (How many rads, heat loss etc). I found that due to the EvoHome I've lowered my boiler to around 12kws (Non modulating) , due to the fact that not all the radiators come on at the same time with optimum start, different set points in temperatures and optimum turn off.
    Yup I agree. As I said, my boiler is too big and set too high and pump is too small. All of that combined means I have an issue.

  9. #59
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    Turned my heat down to 18Kw. Lets see what tomorrow's graph look like when everything starts from cold.

  10. #60
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    The graph was no better today even after I turned the boiler down from 30Kw to 18Kw. I suspect the issue is that my rooms don't all call for heat together in the morning. My schedule is dynamically updated based on room usage patterns so the heating comes on an hour earlier in each room of the house as it is used. So with only the Master Bedroom calling for heating first thing in the morning the Flow has nowhere to go so within minutes the Flow has risen and because it's running around the ABV, the return follows shortly after.

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