Optimising Vaillant Ecotec plus 637 always cycling

[Fireitup]

My new Vaillant Ecotec Plus 637 system is cycling constantly, despite reducing flow temperature and adjusting settings.

Details of the system
Vaillant Ecotec Plus 637 boiler
250L megaflow water tank
6-port manifold underfloor heating (4 ports as 1 main zone + 2 individual zones) coverage an area of 85sqm,
8 radiators + 2 towel rails

My installers recommendation was to leave settings as factory default, the UFH mixer on max but my installer doesn't pay my energy bills. I tried to get a couple of other engineers to look at this but had plenty of no-shows and decided to take it upon myself to try and sort this out.

Using advice given to previous users on here with similar experiences i've been changing setting but find boiler keeps cycling (egg timer flashing).
I've
Reduced flow temperature to 65°C
Water tank temperature below 62°C
Manifold mixer set to 45°C
D00: 20kWh
D01: 8 minutes
D02: 5 minutes

The boiler runs steadily for 5 minutes around 49°C (flow temp D40 at 49°C, return temp D41 at 44°C). The manifold feed temperature reaches a little higher than 45°C, while the manifold return is just over 30°C then all of a the boiler significantly increases flame, driving flow temperature to 70°C and triggering cycling behavior.

Would be grateful for advice on how best I optimize it for efficiency and stop the annoying cycling?

 

[Johntheo5]

Is this happening with UFH only on?, has the boiler got a LLH (low loss header)
The anticycle time d.02, if set to 5 minutes, is way too low IMO to achieve a successful fire up after recycle, would suggest setting it to 25 to 30 minutes to give a actual recycle time of 5.5/6.5 minutes at 65C target temperature. If you have flowmeters on the UFH manifolds can you add them up, by knowing the manifold flow/return temps then easy to calculate the UFH output to see if < the boiler minimum output.

 

[Fireitup]

Thanks for that. I do have similar issues with radiators.

I dont think there is LLH, UFH has its own pump if that makes a difference (sorry this is all new to me)

For main zone which uses 4 ports on UFH manifold all guages set to 2.0l/min. For zone 5 and 6 they are set at 1.5l/min.

Based on chart you have shared either 20 or 25 mins would be the sweet spot.

 

[Johntheo5]

Thanks for that.

I dont think there is LLH, UFH has its own pump if that makes a difference (sorry this is all new to me)

For main zone which uses 4 ports on UFH manifold all guages set to 2.0l/min. For zone 5 and 6 they are set at 1.5l/min.

If its only happening with UFH mode then add up the total loop flowrates, the UFH output is LPMX60XdT/860 kw (dT is the manifold flow/return difference). The minimum output of the 637 is ~ 6.5kw.

 

[muggles]

Fit Vaillant sensoCOMFORT controls. It'll be a huge improvement.

Do you actually need a 37kW boiler? Was a full room by room heat loss calculation carried out prior to specifying it, as regulations require?

 

[Fireitup]

I’ve worked this out to around 11X60X12/860= 9.2

 

[muggles]

Just re-read your original post. 85m² of UFH and 8 radiators? No wonder it's cycling its tits off. You probably need about 12-15kW, not 37.

 

[Harry Bloomfield]

Reduced flow temperature to 65°C
Water tank temperature below 62°C

Your flow temperature, needs to be around 10C above the temperature set on your cylinder, otherwise you may suffer a constant cycle of the boiler failing to satisfy the cylinder temperature.

Best option, is the Vaillant controls, which allow separate flow temperatures for heating versus hot water cylinder, as suggested by Muggles.

 

[Fireitup]

Fit Vaillant sensoCOMFORT controls. It'll be a huge improvement.

Do you actually need a 37kW boiler? Was a full room by room heat loss calculation carried out prior to specifying it, as regulations require?

Yes the builders subcontracted the Heat loss calculation to their engineer, I wanted the sensoCOMFORT initially but the engineer dissuaded me with disaster stories like a fool i bought his nonsense

 

[Fireitup]

Just re-read your original post. 85 of UFH and 8 radiators? No wonder it's cycling its tits off. You probably need about 12kW, not 37.

Apologies for the confusion, 85m² is area that the underfloor heating is covering, I think between 600 to 700m of UFH piping was laid.

 

[Johntheo5]

Check d.40/d.41 with rads only on.
Also suggest changing d.00 from (if on) auto to say 15/20kw, this may help with the cycling.

 

[Fireitup]

Check d.40/d.41 with rads only on.
Also suggest changing d.00 from (if on) auto to say 15/20kw, this may help with the cycling.

d.00 already set to 20kWh
Will check d.40/d.41 when they on next... what should the difference be?

 

[Johntheo5]

(I saw after posting that d.00 is set to 20kw)

d.40/d.41 dT ~ 10C to 15C on rads only but d.41 does not reflect this as depending on the flowrate the internal bypass will be partially open, this defaults to a fairly low 2.5M I think but can be adjusted to a max of something like 4.5M (will check Mis) but the pump mode must then be set to 100% flow or something like that.

What exactly is the problem, does the boiler fire up OK and then run for say more than 5 or 10 minutes and then cycle?, is so then there are generally only two reasons for this, one is that the heat demand is < the minimum boiler output, the burner will then trip when the flowtemperature rises by 5C, in your case, 65+5, 70C and go into recycle, the other reason is if the d.40/d.41 dT is > 30C, possible with the UFH only on,
"(I’ve worked this out to around 11X60X12/860= 9.2)", boiler dT = 12*((65-30)/(42-30)), 34.99C, the internal by pass might prevent this, boiler flowrate = 11*((42-30)/(65-30)), 3.77LPM. (manifold recycle 7.23LPM at 30C)
and/or is the boiler now getting away after cycling after changing d.02 to 25/30 minutes?.
Sensocomfort won't be any help here.

 

[Fireitup]

(I saw after posting that d.00 is set to 20kw)

d.40/d.41 dT ~ 10C to 15C on rads only but d.41 does not reflect this as depending on the flowrate the internal bypass will be partially open, this defaults to a fairly low 2.5M I think but can be adjusted to a max of something like 4.5M (will check Mis) but the pump mode must then be set to 100% flow or something like that.

What exactly is the problem, does the boiler fire up OK and then run for say more than 5 or 10 minutes and then cycle?, is so then there are generally only two reasons for this, one is that the heat demand is < the minimum boiler output, the burner will then trip when the flowtemperature rises by 5C, in your case, 65+5, 70C and go into recycle, the other reason is if the d.40/d.41 dT is > 30C, possible with the UFH only on,
"(I’ve worked this out to around 11X60X12/860= 9.2)", boiler dT = 12*((65-30)/(42-30)), 34.99C, the internal by pass might prevent this, boiler flowrate = 11*((42-30)/(65-30)), 3.77LPM. (manifold recycle 7.23LPM at 30C)
and/or is the boiler now getting away after cycling after changing d.02 to 25/30 minutes?.
Sensocomfort won't be any help here.

Your assumption of running for 5 to 10 minutes then cycling is correct. what i don't understand is why during that 5/10 minute period it hovers at 49°C then suddenly shoots up to 70°C but I've changed d.02 to 25 minutes and will monitor this over 24 hours.

By chance/luck radiators havent come on in few days as not been cold enough, will keep an eye on this. Do you suggest I set pump to 100% or wait to see what d.40/d.41 once radiators are on?

 

[Johntheo5]

Your assumption of running for 5 to 10 minutes then cycling is correct. what i don't understand is why during that 5/10 minute period it hovers at 49°C then suddenly shoots up to 70°C but I've changed d.02 to 25 minutes and will monitor this over 24 hours.

Is this happening with UFH only on?, if so then the controls might hold the firing if it sees the dT getting very high which it will on UFH only, next time, while its hovering like this check d.40/d.41 and also look at the s parameters, there may be a clue there.

By chance/luck radiators havent come on in few days as not been cold enough, will keep an eye on this. Do you suggest I set pump to 100% or wait to see what d.40/d.41 once radiators are on?

Suggest waiting to see d.40/d.41, don't see any reason why there should be a problem on rads only as long as rads/boiler demand > than ~ 6.5kw.

What is the pump setting just now?.

I think there is something in the (new) boilers (d) settings that implies that the boiler can be set to maintain a settable dT but its not clear to me,.