Biasi Riva HE - Flow temp issue?

[fender]

Hi guy's,
I'll try to explain this as best possible.
Installed in 2009 and never an issue with it other than the aav had to be replaced about a year ago.
Anyway, the issue is with the flow temp to the rads.
In the colder weather we've always used a flow of around 70c, as we know that the rad in the living room especially is undersized, or the living room should have had one more rad.

Fired up the heating the other evening, and the flow temp is not behaving as it used to. It used to pretty much have a consistent flow of around 70.
Now it runs up to a 70 flow, then just drops to 50, goes back up to 70 and the cycle repeats. So, an average flow temp of 60.
It's as if the gas valve is not modulating, or a problem with the pcb?

Thanks in advance.

 

[Agile]

There can be all sorts of reasons.

But all would be identified by a conscientious service.

Presumably you don't have it properly serviced each year?

Can you quote the exact boiler model from the data label?

Tony

 

[fender]

Hi Tony,

Thanks for replying.
It was actually you who helped me identify the leaking aav last time...

Yes I do have it serviced annually, but it's not due for another 8 months.
The model is M110 24 SM/C

Thanks again.

 

[Agile]

When it reaches the set flow temperature then does the burner go off?

If so the heat output is set too high!

And the anticycle time may be too long.

But on those PCBs its a pain to set either.

But if you follow the instructions EXACTLY then you can probably set it lower and avoid most of your problems.

Work out the heat output of your rads and aim to set it to that.

Tony

 

[fender]

Cheers,

Yes, the burner does go off. It's like it cycles on/off rather than modulate which I assume it's meant to do?
I kind of assume that it senses the flow and return, and according to the difference it calculates the load and modulates accordingly. This is kind of the basis on which inverter a/c works on these days. Rather than just go on/off with a dead band, it senses the flow and return air temps, room temp and simply ramps up / down according to the load, trying to match the load at the time rather than cycling.

Anyhow, I'll look into the control strategy a bit further if I can and get back to you.

Cheers again..

 

[Agile]

Its simpler than that!

As the flow approaches the set temperature it modulates back.

But the minimum heat output is probably about 8 kW and unless you are in a mansion or its -3 C outside the actual heat loss will be about 2-3 kW at this time of year so the boiler has to just turn off and stay off for the anticycle delay.

What style of house is it and do you know the heat loss?

Tony

 

[fender]

Ah,

Yeh, that makes sense - a kind of simplified version of what our equipment does.
It's terraced (well insulated) - not calculated the heat loss as yet, but I could do that shortly.
As you say, the heat loss is actually going to be kind of low with the current outside ambients.

Thanks again Tony.
I'll let you know what I find.

 

[Agile]

You should set the heat output to about 10 kW and that should slightly improve the situation but there is no possible cure with a boiler with a 3:1 turndown ratio.

Some of the latest makers have recognised these problems and are selling boilers with up to 11:1 which would suit you fine if only you did not already have a modern boiler!

Tony

 

[fender]

Well,

This is six / seven years old now and done quite well tbh.
If anything significant fails on it, I think I'll just get it replaced rather than a repair.

In relation to the above, what would you recommend these days?

 

[Agile]

Its always a compromise!

Heating is about 85% on the gas cost. But the inefficiency caused by on/off cycling is not that bad and proper setup can minimise that to a good degree.

The Intergas is usually convenient to install and reliable but the turndown ratio is not very good.

The most efficient system for you would be a small heat only boiler and a cylinder, ideally unvented if your mains dynamic flow rate is good.

A combi does not work well in a low heat loss property.

Tony

 

[fender]

Cheers Tony,

I'll have to ponder this one a bit.
And....would you believe the blasted thing has started to behave now......typical.
I appreciate all your help and advice....good man.