Has anyone had any success in preventing their boiler from short cycling?

dirtychinchilla

dirtychinchilla

dirtychinchilla

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Joined
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Woking
I'm trying to get the temperature up in our house. It's only 2.5 years old, so well-insulated and a modern boiler (Vaillant EcoFit Sustain).

It doesn't seem to be able to actually get the house to temperature, and it's quite annoying. We're preparing for our first child's arrival in the next week or so, so I need it to be warm.

I think what's happening is that the output of the boiler is too high, so it sends heat out, the radiators can't give it off in time, and it somehow ends with the boiler short cycling.

I have adjusted the settings to set the output to "Auto" which apparently matches output to system demand, but the temperature in the house has only very slowly been creeping up.

Any ideas?!
 
dlockers said:
Ah thought you meant just the boiler not the whole lot. A new build should hold heat really well?

Is it a combi or system boiler/with tank?

I have mine set to a 50 degree flow rate. The rads don't get anywhere near as warm and it takes longer to heat up, but it is basically sipping gas rather than pounding it and cycling.

@danlightbulb is the smartest boiler guy though IIRC.
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Hmm so I have upped it as the house is heating slowly, trying to do manual weather comp.

It’s a combi.
 
Hyburnate said:
That's the reason I ask, my mums got rooms with absolutely tiny single panel radiators, and the rooms just don't get warm.

I put new rads in our home and I overspecc'd to a degree so that the rooms get up to temp with a lower flow temp.
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Ah well I think the TRV's lack of control is more of an issue. I do think the radiator in our living room probably should have been two, and a lot of heat ends up going up the stairs. But the boiler stops pumping heat, so I'm putting it down to how the boiler is working being the issue...
 
danlightbulb said:
Boilers turn off if the return temp is too close to flow temp or if the stat tells it to.

If the rads are actually hot (are they very hot or just warm?) then they still should be dissipating heat into the room.

You say you upped the flow temp setting, has this helped or not?
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It's weird because I spent ages balancing them, even though they're ****. So the return temperature is relatively low - as low as you're likely to get it without having better kit.

I made a few changes so I don't know whether changing the flow temp helped. I will check tomorrow.

Hyburnate said:
Unless I'm completely missing something you have ambient temp and target temp.

Ignoring opentherm etc etc

If it's 16 and you want it to be 21, your boiler is sending heat out at the flow temp until the thermostat hits 21 and then it stops sending heat.

Where is your thermostat? Is it in the coldest room?
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Thermostat is in the same room where I'm monitoring the temperature.

jpaul said:
following other current thread on boilers - this boiler modulates 5:1 https://professional.vaillant.co.uk/specifiers/products/ecofit-sustain-25-30-and-35kw-48448.html
so minimum output maybe 5Kw ... and if the house only needs ~3 say, like other thread - it will switch on and off.
eg. on for 6 minutes off for 4
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That's what I set to Automatic, though i'm not sure that I trust it. The installer had set it to 18 kW!

danlightbulb said:
So according to Google there's lots of reasons a boiler could short cycle.

It might be worth getting someone in to service the system OP.
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Yeah I saw that. Quite annoying. I don't really trust plumbers tbh, maybe a heating engineer. It was recently serviced.
 
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Semple said:
Boiler stuff aside, your room doesn't need to be a furnace for your newborn. With 3 of you sleeping in the room it'll naturally be a bit warmer. Do you have any temperature sensors/thermometers in the room to actually gauge the temperature?
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I agree. It's the living room I'm trying to sort out really. We've got a couple of those egg things - one for the living room and one for the bedroom.
 
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dlockers said:
I think root cause of short cycle will be return flow temp is too high and it stops firing.

That's way too high for a new build. I have a 50 flow temp on my 1930s semi and it is never cold. Your rads won't be big enough to spit out 70 degrees before returning hot water to the boiler.
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You might be right. As I said, I do manual weather comp, but I don't think that's the route cause of the problem. I'll try a couple of things.
 
I don't know if I'm being dense here either, but I can't work out what to set the anti-cycling time to. Maybe because I'm not a heating engineer.

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rodders said:
What you currently set at, must be a few mins? allowing it to fire back up again. Up that and your pump will continue to pump the hot water around, rejecting the heat, but boiler won't fire and cycle.
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I set it to 15 mins and that may well have improve things.

Maybe it’s all three: reduce the flow temp, limit the capacity, and up the cycle time (or leave it at 15 mins).
 
Morning everyone,

Thanks for your replies. I got annoyed by the whole thing so tried to stop thinking about it over the weekend.

As @squerble says, the boiler can't module down enough. It was set to 18 kW heating output!!! The house is three bedrooms, around 65 m2, so I think the peak heat demand is somewhere around 3 kW, assuming a terrible heat loss of 40 kW/m2. The boiler can only go down to 7 kW, so basically it's dramatically oversized. Therefore, the cycling problem won't ever go away, annoyingly.

I seem have got it to a relatively decent place over the weekend. The outstanding issue is that one thermostat is behind the TV, so when the TVs off it works fine. When the TV's on it gets a dodgy reading, so the thermostat has to be adjusted for that. I think I need to convert it, somehow, to a wireless one. Unfortunately, it's all Honeywell and I have no intention of changing all of it to something smarter.
 
dlockers said:
If it's just a dumb controller the swap to something better is pretty cheap and quite easy.

Amazon even offer the Hive unit (not quite as good as the Tado X but British Gas backed) with installation for +120 quid.
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Is that cheap, though?!

It's much more likely to be something like this:

 
squerble said:
If you're not looking to go for something "smart" (i.e., internet connected) then this would be fine.

EDIT: The tado and Hive linked by @dlockers are nice but be aware they runs on something called TPI when used as an "on/off" mode thermostat, meaning they'll kick the heating in and out for 5 minutes here and there to maintain temperature. This will further add to the cycling and honestly would be best avoided. I got a pair to replace basic on/off programmables and hated the TPI mode so went back to the basic ones we had.
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No, it doesn't need to be smart. I'd like it to be, but I don't see the value in it right now.

I'll just get that one from Screwfix and be done with it. Thanks :)
 
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