Boiler adjusting max useful heat output

Now I've had a fault with the boiler fixed with a new control board. I've spent a lot of time looking at the thing and timing the operation burner etc. when it was playing up.

When cold in the morning it takes about 16 min to reach flow temp initially.

Then, when up to flow temps yesterday, in -1C outside temps, it would fire burner for approximately 2min 30sec and then rest for virtually the same 2min 30 seconds and follow that cycle all the while, whilst getting the room up to temp.

That seems like a lot of burner cyclying, which I suppose is caused by over sizing, it is a 32kwh combi? Does that sound excessive, it seems it to me??

I looked at the gas meter and in the "burner on cycles" it was using 1.68m3 per hour gas rate.

The specs say minimum modulation of the gas valve is 1.64m3 so I guess there could be a small amount of adjustment if I get an engineer out but probably not enough to be worthwhile?

at 1.68m3 I calculate the Kwh at 18.7kwh at assumed 78% that is 14.6kwh if my logic is right.

My 9 rads would be no more that 9kwh. So I guess the over size and lack of further modulation causes the short cycles?

There is an adjustment potentiometer on the circuit board I could get an engineer to lower useful output but would this take it lower than the lowest modulation the gas valve is set for? It doesn't seem likely to me?

I suppose all this is leading to a more modern boiler with a higher modulation ratio so it is more aligned to heat demand on CH mode...

I have now had both the ignition and main PCBs replaced in the last 2 years and thinking the burner on / off cycles are playing a part...

Advice very much appreciated. Thanks

 

Or

(1) fit a separate 9kW ( modulating down to 6kW ) boiler for the central heating and use the existing boiler for "instant" hot water.

(2) fit a smaller heat only boiler than can modulate down and install a hot water cylinder

For central heating ? Yes it is excessive, but that is what you get with combi boilers.

 

Yes, sorry when I was asking about excessive, I meant the on / off burner cycle periods. Sorry my post wasn't that clear.

Yes, I like your lateral thinking on the combi but the practicalities of two boilers in the house would mean it has to be replaced.... I'd love to change it back to unvented and heat only boiler but because someone in the past ripped that all out and put the boiler in the airing cupboard the costs would be beyond out reach.

So combi we are stuck with.....

 

Some boilers ( no names ) do not like being cycled. Firing when hot stresses a boiler significantly more than firing when cold. Some boilers can cope well with the extra stress ( good design ) while others suffer excessive wear and tear when cycling ( low cost / poor design )

 

You don't say what the heating flow temperature is set to on the boiler.
Is there some wiggle room for increasing this so the system can sink a bit more heat and get the house up to temp a bit faster with fewer burn cycles?

 

Sorry. Thanks for the reply. With the cold weather it is now set at 75C. I haven't got a suitable measuring device to measure the flow / return differential. are there any reasonably priced?

Edit: Also the rads are all single panel, so that's why the output is so low... of the 9 rads, one is in conservatory so never used except on frost. another is a towel rail. There are only two large rads [in the house] and they are downstairs and they're single panel 1800x600.

Edit edit. With the current freezing weather, it takes about 2 hours (boiler from cold so includes initial heat up 16 mins) to climb 2.5C. I.e. from 15.5 to 18C in the front room (most exposed, large windows). Double panels would help but space is an issue as it is not a big house....

 

Are we allowed to know which boiler you have (make and exact model)?

 

Of course. It old and not well liked on here, I know. It has been maintained properly since my ownership of almost 6 years. Before that who knows. Biasi Garda M90F32S.

 

Is the temp for CH set to max? If so try lowering the temp. I’ve found this can help cure short cycle

Also it can be better to keep the temperature set at 18 more of the time. Once up to temp it won’t need to be on as much. Tried this in my own house and the temp doesn’t fluctuate and the boiler stays on over run longer.

 

The maximum output of your boiler can be adjusted, but it must be done by a Gas Safe Registered engineer

Your boiler can modulate between 12.7kW and 32kW. Unless you have a very large poorly insulated house, the maximum you will need is probably under 15kW, and that will be in the coldest weather. Most of the year you will require less than the 12.7kW minimum, which can only be achieved by the boiler short cycling, i.e turning on and off.

The Baxi Whole House Boiler Size Calculator will give you an idea of the maximum heat requirement of your house.

 

Thanks v much for the reply.

Yes, I've found out it can be adjusted from the manuals but surely it can't be adjusted below the min output of the modulation of the gas valve? The 12.7kw is listed along side the min gas rate at 1.64m3 so it seems it can only limit to the gas valve modulation.

No problem, yes I have just had the control board replaced under M&S emergency cover I wouldn't be doing adjustments myself, the engineer who services annually would, but I don't want to waste my money and his time if there is nothing he can do, which I'm starting to think seems the case.....

Does 2:30 on 2:30 off sound horrific for 2 hours whilst the house gets up to temp in cold weather?

The 12.7KW in the manual as minimum useful output is at a calorific value of 34.02MJ/M3 which according to the bills I receive, is quite a lot lower than the MJ content of UK natuaral gas which is 39.4 on my bill?

So the min modulation on CH of 1.64 M3 I get is 18.3kw then 78% efficiency per sebbuk is say 14.2kw (versus the 12.7kw in manual specs).

Anyway, those differences are probably not that material in the scheme of this discussions here.

I used the baxi heat loss calc link you gave, it says 14.9kw. But that must be over rated? As the rad output inside our house (excl conservatory) is no more than 8kw. And at -1C on sat morning it still heated the house from 15C starting temp. So I would think heat loss is more like 7 or 8kw at -2C.

Entirely open to correction if i'm not understanding correctly or not on the right page.....

Is it worth a Gas Safe engineer doing any adjustments in these circumstances?

Many thanks again for replying

 

It is currently on 4.5 out of 7, which is corresponding to about 75C flow on the temp gauge. If I go much lower in these outside temps, due to single panel rads it does struggle to heat up rads and the house in reasonable time.....

When the weather moves back north of 3C I could drop it back a bit..... Thanks

 

I would love to know what, from your vaaaaaaasssst range of experience in these matters, you consider relevant models to these claims.

Make up your mind.

It most certainly can not.

No. He'll probably have topped himself before getting that far into the service call anyway.

Dude you have a Biasi. Just be grateful the schitbox is working at all; and beware of certain posters here giving advice they are ill equipped to give.

 

Yes, well all the rads incl the conservatory is probably approaching 9kw. Inside the house regularly in use less than 8kw...

Yes, I thought you might say that about the boiler. It has been ok in almost 6 years. Except the two PCBs. I guess I should start saving!!!

Thanks for the reply.

 

Indeed. Get a couple of grand set aside. The Biasi is crap, but they could chug along fine for a good while. You just don't want to get caught short and not in a position to pick a good replacement option (including the installer) over a convenient one (easy but expensive finance).

As for your original question, I'd be more inclined to think you have a sluggish pump, which, for the age of the boiler, is not a surprise.