Vaillant Ecotecplus Target flow and room thermostat question

RK_London · 11 Jan 2024

I have a Vaillant ecotec plus 637 boiler installed about 5 yrs ago as a S plan plus unvented installation.

Question I have is the room stat is 1.5C below the set level (20C) but the boiler flow temperature is around 42C when the target is set at 60C. The boiler is oversized and I set the power to 14KW which has worked over the years. I was expecting the boiler to continue to fire up until either target flow or room temperature stat levels reached. This doesn't seem to be happening. Is this normal?

Johntheo5 · 11 Jan 2024

If (you mean that) the room temperature is 1.5C below its set point temperature of 20C then you would expect the boiler to remain firing except this stat is a smart one with TPI control where it may cycle the boiler on/off when it gets to this difference (1.5C). It should then fire continuously to get to and maintain its target temp of 60C, if the heat demand is lower than the boiler's minimum output then the burnr will trip at Targettemp+5C, in your case 65C and will refire after the anticycle time has elapsed. Also, of course, if the heat demand is > 14kw then the targettemp will never be reached.
Have a read of this. https://www.diynot.com/diy/threads/...aching-target-temp.622844/page-2#post-5641865

RK_London · 11 Jan 2024

thanks - I have 13 radiators with a total output of 13KW if all used at max setting so 14KW setting should be ok

Johntheo5 · 11 Jan 2024

Is the boiler firing up for only a minute or two, then cycling and repeating or is it firing continuously and just hanging there at ~ 45C?.

RK_London · 11 Jan 2024

Just checked the last 3 cycles. Boiler fires up for about 3 minutes 30 seconds then goes into pump over run before repeating.

Did 3 test cycles,

1st test cycle :Flow temperature was at 60C then went into pump overrun ( set for 6 mins as anticycling time is 6 mins at target flow temp of 60C). End of pump overrun flow temp fell to 46C.

2nd test cycle: Flow temp went to from 46 to 53C then went to pump overrun ending with flow temp at 43C.

3rd test cycle: Flow temp went from 43 to 48C then went into pump over run

Johntheo5 · 11 Jan 2024

Theoretically, the burner shoudn't trip until the flowtemp reaches targettemp+5C, 65C in your case,
Vaillants fire at ~ 65% of max output and maintain this for 60 secs.
So your boiler will fire up at say 24kw (range rating has no effect on this), if 60C is the very highest to allow another 3/4C before modulating down then it will require a flowrate of 25LPM to keep the dT to 14C (60-46), I cant see the circ flow of that boiler being 25LPM without a LLH so would suggest a anticycletime of 10.5 minutes, set anticycle time 35minutes which should mean the flow temp at fireup should be down to 35ish, giving the boiler a far better chance IMO of getting away first time, if the flowtemp is reduced to 35C before refiring then a flowrate of 13.8LPM will suffice. I know the second & third tests don't make much sense but maybe give that a go anyhow.

Is there a ABV (external bypass) installed?.

RK_London · 11 Jan 2024

My boiler does fire up at 65% max output but didn't hold this for more than a second or 2 in the three tests I did.

Will set anti-cycling time to 35 mins.

An external ABV is fitted ( discussed with you on a separate thread). Vaillant engineer insisted it was needed on this boiler, which is way oversized for my needs, even though there is an internal by pass valve fitted.

Johntheo5 · 11 Jan 2024

Which thread was that?.
Vaillants have a very "large" internal bypass which is also set very low but can be increased, the problem with internal bypasses is that they take ages to dissipate any heat since they are very short, an external by pass if one had it as far away from the boiler as possible will dissipate far more heat and get the flowtemperature down much faster.

RK_London · 11 Jan 2024

Your ABV comments were on my thread ' Central Heating Port Valve Water Hammer' 13th December

Vaillant engineer said exactly the same as you about the internal bypass on this Boiler. He said because it's 37KW, can take ages to lose heat so external bypass is needed to get flow temperature down faster.

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Vaillant Ecotecplus Target flow and room thermostat question

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