Vaillant ecoTECplus418 cycling on start up

[sniktun1]

Boiler = Vaillant ecoTECplus418
Flow temp setpoint: 65°C
Anti-cycle time= 20mins
heat output: 18kW

bit of background - have a small bungalow, boiler generally worked fine with just the radiators (7 off them). i've since done an extension and added UFH with a manifold which is where the problems now are.

If I run the radiators for an hour, let them get up to temperature (and the boiler, set to 65°C flow) - then turn on the UFH manifold the boiler runs fine and controls to temperature. whole house works perfectly

If I turn on my entire CH at the same time (radiators + UFH manifold together) the boiler can never get out of it's start-up cycling phase. So it fires for roughly 1 min, get the flow up/above 65°C then turns off for 10 mins. then keeps doing the same thing. I'm assuming the boiler is trying to get the whole system up to temperature and a delta-T of about 20°C between flow & return before firing the boiler properly? When doing just the radiators this works, but adding in the mixed return of the UFH it just never gets there. 3 hours solid it won't get up to temp.

Is there a way to reduce this start-up cycle time? it doesn't seem to be linked to the anit-cycle time which I think is just relevant after the boiler is operting properly and up to temperature.

 

[cross thread]

Sounds like a flow issue possibly how the underfloor is connected, what happens if underfloor and rads on but underfloor pump is off

 

[sniktun1]

Sounds like a flow issue possibly how the underfloor is connected, what happens if underfloor and rads on but underfloor pump is off

It runs fine. I thought a flow issue initially, but if the rads are up to temp and then I enable the UFH the entire system works fine.

i've attached a sketch of the system, I don't think its connected wrong otherwise it wouldn't work once up to temp. Its more like the boiler needs to be told theres an UFH system on it so it has a faster start cycle procedure. not sure though

The only way the boiler works is to run the radiators for an hour first, then bring on the UFH. but thats a pain - ideally would like to come on together.

 

[Johntheo5]

If you have both on together and then swich off the CH, does the boiler continue to run? And if so what are the Flow/return temps?

Also what are the flow/return temps with both on?

 

[Mister Banks]

It runs fine. I thought a flow issue initially, but if the rads are up to temp and then I enable the UFH the entire system works fine.

i've attached a sketch of the system, I don't think its connected wrong otherwise it wouldn't work once up to temp. Its more like the boiler needs to be told theres an UFH system on it so it has a faster start cycle procedure. not sure though

The only way the boiler works is to run the radiators for an hour first, then bring on the UFH. but thats a pain - ideally would like to come on together.

Is the system on a single pump? Does the UFH come on when the HWC calls for heat? Are you using Vaillant or 3rd party controls? How is the system connected electrically?

 

[sniktun1]

No so the system has a pump as i've shown above, then the manifold has a 4-port mixing valve with integral pump for the UFH.

It has a standalone digital thermostat with a timeclock which enables the boiler, runs the pump and controls the 3-port valve - boiler then runs to its integral controls on the radiator setting.

Flow & return temp with radiator= 65/50
Flow & return temp with radiator & UFH when its up to temp= 65/45
Flow & return temp if I start both radiators & UFH together= 40/30 - can never get up to temperature and out of its startup phase (so just enables the boiler for 1 minute, then turns off for 10 mins. Does this for hours)

 

[cross thread]

your hot water return must be the last tee in before the boiler.And lower your underfloor pump setting to minimum ,to see what happens.

 

[Johntheo5]

No so the system has a pump as i've shown above, then the manifold has a 4-port mixing valve with integral pump for the UFH.

It has a standalone digital thermostat with a timeclock which enables the boiler, runs the pump and controls the 3-port valve - boiler then runs to its integral controls on the radiator setting.

Flow & return temp with radiator= 65/50
Flow & return temp with radiator & UFH when its up to temp= 65/45
Flow & return temp if I start both radiators & UFH together= 40/30 - can neget up to temperature and out of its startup phase (so just enables the boiler for 1 minute, then turns off for 10 mins. Does this for hours)

Try increasing the anticycle time from 20 to 25 or evend 30Cminutes, since its a lookup table, by recall, 20C + 65C = anticycle time of 4.5 minutes. 25C + 65C = 6.5mins and 30C + 65C = 7.5mins, the increased recycle time will get the flowtemp doen to 30 to 35C before refiring and a better chance of not reaching target temp+5C and burner trip/recycle..

 

[Johntheo5]

If you can get the UFH to run on its own after having the CH on as well, what are the boilers flow/return temps then, what is the UFH manifold flow temperature set to and any idea of the UFH actual flow/return temps and flowrates as per the loop flowmeters?

 

[jackthom]

At full system startup when the boiler quickly brings the flow temp over 65C and is about to cut out what is the return temp?
I’m assuming it must be fairly high and if so what path is this hot water taking through the system?

If on the other hand the return temp is low there isn’t enough flow through the boiler.
Both of these possibilities are a bit baffling to be honest considering the description of the layout.

 

[Johntheo5]

The maths are simple enough, if the boiler fires up at 60% output. 10.8kw, for 1 minute with a flowtemp of 35c, then a flowrate of 5.16LPM should satisfy these conditions to avoid the target temp exceeding 65C?

 

[sniktun1]

Try increasing the anticycle time from 20 to 25 or evend 30Cminutes, since its a lookup table, by recall, 20C + 65C = anticycle time of 4.5 minutes. 25C + 65C = 6.5mins and 30C + 65C = 7.5mins, the increased recycle time will get the flowtemp doen to 30 to 35C before refiring and a better chance of not reaching target temp+5C and burner trip/recycle..

Thanks, but this didn't seem to make a difference. Again it's in it's start up mode, so anticycle time doesn't seem to make a difference. Once it's up to temperature it runs fine.

I'm starting to think balance might be an issue, maybe I have the "primary" pump running too fast/slow compared to the UFH manifold pump? i've commissioned the UFH manifold to provide the manufaturers 2 l/min flowrate on each circuit (2x circuits), but I was half hoping the UFH mixing valve would react almost like a plate heat exchanger and not have a massive hydraulic effect on the main pump.

Main pump in a little grundfos alpha 15-50/60. Running at max speed which I thought was best with the added UFH system.

i'm doing some more testing tonight - see if any other "modes" will get it to work

 

[jackthom]

It’s still not clear whether the boiler cycling is the result of the return feed being somehow at a high temperature, which seems unlikely unless there is a short circuit bypass we don’t know about, or for some reason the flow through the boiler is actually reduced when both UFH and Radiators are on together.
I would expect flow to be increased when the two heating circuits are both open, but assuming there is a flow problem this doesn’t seem to be the case.

 

[Johntheo5]

IF starting up say a 5kw UFH from cold that normally runs with flow/ return temps of 45C/37C and assuming a boiler flow temp of 65C, then the flow through the boiler should be 8.96LPM with a dT of 8C, once up to temperature then the flow through the boiler will only be 2.56LPM with a dT of 28C, the remainng 6.4LPM will be recirculated through the UFH manifold so can understand if the boiler was tripping on a high dT with UFH up to temp but difficult see it tripping on UFH start up except the TMV is stuck in one position?

 

[sniktun1]

It’s still not clear whether the boiler cycling is the result of the return feed being somehow at a high temperature, which seems unlikely unless there is a short circuit bypass we don’t know about, or for some reason the flow through the boiler is actually reduced when both UFH and Radiators are on together.
I would expect flow to be increased when the two heating circuits are both open, but assuming there is a flow problem this doesn’t seem to be the case.

No I'm looking at the flow and return temps. Return is always low and just taking ages to heat up.

I did some more testing yesterday, the boiler fired for roughly 1 min - the flow temp got up to 65°C with the return temp at 25°C. The boiler then turned off (with the little waiting time symbol appearing). The flow temperature dropped roughly 1°C every 1 second until reaching around 35°C (so around 20-30 seconds, indicating to me very good flowrate and heat transfer). The return temp slowly crept up to about 28°C. The boiler then stayed off in this waiting mode with the pump running for about 10 mins.

It does this cycling for hours, with the return temp just slowly creeping up, when the return gets to about 37°C the boiler finally gets up and going and stays running. Once the boiler actually stays running the system gets up to a flow temp of 65°C and return temp of 50°C.

This is doing my head in lol.