Cycling Ideal Standard

[Justino]

I am trying to prevent my Ideal Standard 280NF from cycling (rated down to 60,000 btu). I have lived in the house for the past 12 months, so it has perhaps done this before we moved in. Boiler is on the ground floor, the 3 way valve and HWC is on the 3rd floor.

Whether the boiler is supplying HW or heating or both (sealed system, with tank stat and room thermostat), once the boiler is up to temp (5-10 mins on hot water, longer if both) it begins to cycle (on the boiler stat), generally 30 seconds on, 1 minute off. I have no problem with hot water supply, or radiator temps, I just want to reduce the cycling as I assume it is inefficient and will add wear and strain to the boiler.

A new Grundfos Alpha 2L 15-60 has been fitted as the old pump rattled. However on all settings, auto, high or low speed has made no difference. I have pipe thermostats on the flow and return to the boiler. On hot water, with the boiler on the middle setting I am getting outlet temps of 68-70 degrees, with a return of 50-55 degrees with the pump on the highest speed. The tank stat is set to 60 degrees and is positioned approx. 1/3rd up from the bottom. I have flushed the system using x800 thinking the heat exchanger may have scale deposits, and it was cycling as the reduced volume of water was getting to temp too quickly. This hasn’t made any appreciable difference. I was considering a second treatment of x800, or perhaps DS3 for more lime scale targeted approach, but was unsure how to dose DS3 into a sealed system (mix it to a paste and add to vertical towel rail?). The boiler also has a new PCB as the old one gave up the ghost after a 2 week summer shut down whilst on holiday. A relay melted, presumably from turning on and off so many times when cycling!

If I turn the boiler temp up, this just delays the cycling, and it happens at a higher temp, with a corresponding increase in the return flow temp. I am aware the boiler is large at 60,000 btu, but as the return flow temps are 15 degrees lower, I didn’t think this was an issue, the system appears to be soaking up the heat output. The pump is a new one, on full speed, on 28mm pipe, I can’t imagine it is a case of not being able to get the hot water away quick enough, or a blockage. I have opened up the HWC balancing gate valve to aid flow, again no difference.

Any thoughts would be greatly appreciated. I know my boiler has a low and high stat, fan and pressure switch, and I am wondering if any faults in those items could explain the cycling. The cycling can happen at different water temps, dependent upon the boiler setting.

thanks

Justin

 

[Justino]

Slight correction, from a cold start it will run on hot water for 15 mins before it starts cycling. At that point flow is 65 degrees, return is 48.

 

[jackthom]

Correct me if I'm wrong but it looks like this boiler is non-modulating and it's output won't go below ~17kW. So it is probably producing more heat than the system can absorb once the circulating water has been heated to the operating temp.

On the hot water side a storage tank with an extra long coil can help transfer some of this heat more quickly, assuming one of these isn't already fitted. I doubt it would make a huge difference though without fitting a new boiler as well.

Modern modulating boilers will reduce their output as the water temp approaches the set target and will often go as low as 5kW to keep things ticking over until the demand from the thermostats has been satisfied.

 

[Agile]

Yes, obviously your boiler is producing too much heat and it may not be possible to reduce it within the maker's specs.

You can check the power input at the gas meter. See FAQ.

The flow rate is still too low! That kind of boiler expects a diff temp of about 11 C or less!

Yet another case of an over powered installation. Probably by a builder!

Tony

 

[Justino]

I can’t say for certain who put the boiler in, but I suspect it was the developer when the building was converted in the mid 1990’s.

I thought the system was transferring the heat given the flow and return temps, but following your logic, it sounds as if the pump, even on full speed isn’t getting the flow away quick enough before it turns off on the boiler high stat, the low return temp cools the boiler slightly, turning it back on, starting the cycling process? So by opening the HWC balancing valve fully will have helped the flow a little but not enough. I will need to see the effect on the heating once we start using that properly.

Presumably with a higher flow rate, the return temps will be closer to 11 degrees, and so the low stat of the boiler will keep it turned off for longer? That being the case could I open up the manual bypass valve next to the 3 port valve (presently shut) to get the return flow temps to an 11 degree differential? If this is achieved it should not turn back on until the flow return differential is greater than 11 degrees. However it’s a long bypass route with the boiler on the ground floor, 3 port valve and bypass valve on the 3rd floor, so some heat losses in the pipe run. I wonder whether this would be more economical on gas usage, diverting boiler output thought the bypass, than having it cycle?

 

[Agile]

The boiler turns solely according to the FLOW temperature!

Time the on and off cycles when everything is up to temp.

Then open the bypass a little and you can see what difference it has made.

But as you do that you will start to reduce the heat output to the rads.

Tony

 

[Justino]

Thanks, I will try that. What does the low stat within the boiler control?

Based on what appears a low return temp, I assumed my system was soaking up the heat this largish boiler was generating. If that is correct, is the cycling caused by the flow not being sufficient to move the hot water out of the boiler quick enough? That being the case, as it is a new pump, on full speed (setting 4, 45w on the Alpha display) on 28mm pipes, I can’t see how the flow could be any greater. This leads me back to my idea that if the heat exchanger had lime deposits, reducing the volume of water content to heat, then this could be a cause of the quickly reached high flow temps and the relatively lower return temps, but still cycling on and off on the boiler stat?

I have flushed (not power) with x800, leaving it in for a week to ensure it did its job, but would longer have been better?. I may add DS3 this weekend to doubly ensure no lingering lime deposits, and crank the boiler up to full? Any thoughts on that strategy?

 

[Agile]

Your problem is your boiler is old and does not modulate. Its power output is too high for your heat loss in this weather.

X800 does all its going to in four hours and should be removed within 24 hours!

X800 does little to real lime scale.

DS3 does dissolve lime scale but only works well at high temperatures like 80C. Its best to close all rads when using it so that it is concentrated on the boiler and cyl heating coil. You only usually need about 100g and its very insoluble and needs lots of hot water to dissolve it.

Its not likely that it is lime scaled though as it would be kettling if it was!

Tony

 

[Justino]

With the temp differential on the flow and return of at least 15 degrees, am I not getting sufficient heat loss? I appreciate it is not a modulating boiler, and its a high output, so not ideal, but with what i thought was lowish return temps, then it was absorbing the heat, so shouldn't cycle, or am I wrong?

thanks

Justin

 

[Agile]

Wrong!

Your boiler is designed for an 11 C diff. Yours is higher. Makes little difference in practice!

But your pump is undersized for the flow resistance of your system.

Tony

 

[Justino]

My logic was as the return temps were 15 degree lower than the flow, the boiler should keep firing, but it seems my pump even at full speed, can't get the heat away quick enough as it keeps firing.

If by adjusting the manual bypass to get closer to 11 degrees, the boiler will still turn off on the stat, but I suppose it should hold off for longer given the higher return temps?

So other than a smaller boiler, what options are there for more powerful pumps on 28 mm pipe than the Grundfos Alpha 2L 15-60 that can deal with pushing the water up 3 storeys?

 

[jackthom]

The only thing that will solve the cycling is an increased flow of colder water coming into the return.

As the radiators seem to be unable to remove enough heat from the circulating water for this I honestly can't see any solution.

Beyond a certain point it's possible that a better pump may only result in a faster 'flow' of even warmer water arriving back at the return.

 

[Justino]

Are you saying that a 15 degree differential is not enough when on the hot water only? I am coming to the conclusion that on larger boilers you need higher flow rates, as regardless of the systems ability to absorb the heat output, as the system gets close to its set point, then cycling will occur anyway?

Just checked the spec of my pump, it is 3.1 m3 per hour. cant see a 130mm pump higher than this.

 

[jackthom]

What you really need is simply a high enough flow rate to carry away the boiler's full power while it's heating up the system from cold.

Nowadays the magic of modulation then allows the boiler to reduce it's output to match the requirements of the radiators/storage tank.

That's what's missing in this case and I don't think it's fixable, especially so when only heating up the HW tank on it's own.

 

[Agile]

Your system is bound to cycle to some extent.

The flow rate is too low. But that will not be very important if its 15 C.

Its normal for that kind of boiler to cycle!

Tony