Myson boiler short-cycling / nest thermostat Y-plan system

[Jon Parker]

I have revisited the radiators and found there does seem to be a small drop in performance, and the radiators at the end of the circuit are suffering particular poor performance.

Jon

[tabbypurr]

Lack of flow then. I'd look at the pump.

NT

[Bob Minchin]

You said the pump was at max already so I just wonder - assuming your system has a bypass valve or it may have one rad without a TRV giving a similar function, is passing too much through and not leaving enough flow for the rest of the system.

I tend to close the bypass valve to start with and get everything else working. then you should get the effect that the pump overrun time extends to a long time as the boiler/pump is failing to get rid of the excess heat. Open the bypass valve a bit at a time until the overrun time is reduced a few minutes.

Slightly perversely, balancing the rads means constricting the flow a bit at a time though the hottest radiators to increase the flow through the cooler ones. If you have an obvious hotter one then it is easy otherwise can be a bit hit and miss until you start getting some improvements.

[Jon Parker]

Towel rail in teh upstairs bathroom has 2 LSVs on, these are both left full y open to act as a bypass. There is also a bypass next to the pump by way o f a gate valve. This is in hte almost closed position, but I have experimen ted with different positions without any impact.

I've had my plumber / gas engineer mate round tonight. He's put a new pump in which is almost silent, which is a bonus. Unfortunately this had no effe ct on the short cycling problem.

So, we then wired the pump to a mains plug temporarily to force it to run, irrespective of what the boiler was doing. The result was that the pump ran continuously - obviously - and crucially so did the boiler.

We then closed off all radiators and opened them one at a time to fully ven t everything. There was a bit of gurgling now and then and then all of a su dden there was silence. Each radiator (including the conservatory at the en d of the circuit) got piping hot almost instantly.

We then opened up every radiator fully and again, all piping hot, very quic kly.

I can only conclude that the pump overrun function of the boiler is now wor king properly, and perhaps my attention will turn to the PCB inside the boi ler.

However, a contradictory observation was that when I measured the voltage a t the boiler ON terminal (i.e. the one that fires the boiler up when it rec eives power) when the boiler turned itself off during a short-cycle the vol atge dropped to zero. I assumed that the boiler was cutting out on it's saf ety thermostat and therefore the voltage would have remained at 240v.

With the now silent pump and satisfactory radiator performance if the pump is forced to run on I think I am going to plump for rigging up a run-on rel ay to the pump, which will have the effect of keeping it powered for 2-3 mi nutes (I will experiment with times to achieve continuous operation) after the boiler turns off.

This means that the boiler can continue to short-cycle if it wants to but t he water will still be circulated around the system, and the constant on-of f-on-off of my old pump will no longer keep me awake. This is important to me as I work shifts and sometimes sleep during the daytime.

Thoughts? Jon

[Jon Parker]

See my reply in another sub-thread about my new pump and forcing it to run on by wiring it to a mains plug (as a test!).

Only things not investigated now are the valve (only 3 months old) and the cylinder stat (also 3 months old).

Jon

[tabbypurr]

lly open to act as a bypass. There is also a bypass next to the pump by way of a gate valve. This is in hte almost closed position, but I have experim ented with different positions without any impact.

p in which is almost silent, which is a bonus. Unfortunately this had no ef fect on the short cycling problem.

, irrespective of what the boiler was doing. The result was that the pump r an continuously - obviously - and crucially so did the boiler.

ent everything. There was a bit of gurgling now and then and then all of a sudden there was silence. Each radiator (including the conservatory at the end of the circuit) got piping hot almost instantly.

orking properly, and perhaps my attention will turn to the PCB inside the b oiler.

at the boiler ON terminal (i.e. the one that fires the boiler up when it r eceives power) when the boiler turned itself off during a short-cycle the v olatge dropped to zero. I assumed that the boiler was cutting out on it's s afety thermostat and therefore the voltage would have remained at 240v.

p is forced to run on I think I am going to plump for rigging up a run-on r elay to the pump, which will have the effect of keeping it powered for 2-3 minutes (I will experiment with times to achieve continuous operation) afte r the boiler turns off.

the water will still be circulated around the system, and the constant on- off-on-off of my old pump will no longer keep me awake. This is important t o me as I work shifts and sometimes sleep during the daytime.

A minute overrun is enough.

NT

[Roger Mills]

Yes! If the boiler is cutting out as a result of the external demand being turned off, it's not a boiler problem per-se.

What turns the external demand on and off? You've guessed it - the NEST. That's where your problem lies. [Don't be misled by the fact (if it does) that it displays a flame symbol throughout to indicate that the demand is on. My Honeywell wireless stat does that all the time it's in proportional mode - even when it's cycled the boiler off.]

I'd be surprised if the Nest isn't configurable for minimum cycle times even if you haven't found how to do it.

[Jon Parker]

Apologies for not trimming. Using my phone to read and post.

I agree it feels like the nest but how can it be? Even if I wack it up to 30 it still cycles! That can't be right! And the CH call for heat wire stays at 240v!

In my previous post I mistyped "now working correctly" instead of "not working correctly" when regfering to the boiler's pump overrun function.

All my reasoning is telling me now there's an issue with the pump overrun function which I intend to solve with an overrun relay.

Hopefully that will be an end to my woes!

Jon

[tabbypurr]

then plainly it's not the nest stat cycling.

NT

[Jon Parker]

This relay looks about right...

Jon

[Roger Mills]

I'm confused! In your previous post you wrote "when I measured the voltage at the boiler ON terminal (i.e. the one that fires the boiler up when it receives power) when the boiler turned itself off during a short-cycle the voltage dropped to zero."

Now you're saying that the "CH call for heat" wire stays at 240v when the boiler cycles.

I had assumed these two commodities to be one and the same. If they're

*not* please explain fully exactly what you mean by each term, and exactly where you measured the voltages.

[Roger Mills]

That looks as if it should allow you to control pump over-run. I use a less elaborate timer designed for controlling a bathroom fan to provide pump over-run in my system.

But I struggle to understand how over-running the pump for longer is going to solve your problem. Surely, if the boiler cools down faster after it stops firing, its own thermostat will re-fire it *sooner* - thereby *shortening* the cycle time?

How does the pump behave at the moment? When the boiler is short-cycling, does the pump stop when the burner stops? If there's an external demand, the pump should still run regardless of whether the boiler stat tells the burner to fire. If the pump stops, it suggests either that the external demand has been turned off (which would down to the Nest) or that there's something seriously wrong with the boiler's pump control logic.

[Jon Parker]

Problem solved.

Took all of the wiring apart and put it back together again.

Everything now works. Not sure what's different but it works. No more short-cycling.

Thanks for all of your contributions.

Jon