Oversized boiler - is it a problem?

[WhydidIstart]

Hi,

I’ve searched on the forum but I haven’t been able to fully answer my question. I understand it’s not a good idea to oversize a boiler, perhaps because of excessive cycling and it not running in condensing mode for long?

However I have bought a Vaillant system boiler 630 = 30kW output as it was the make I wanted and cheap! (person bought wrong one, left it for ages, company went bust, etc). It’s brand new boxed with all fittings and flue, etc.

The radiators need about 15kw and add a bit extra for a Megaflow indirect hot water tank and 30kw is still somewhat excessive. In reality when the house is warm, water hot and half the TRVs are shut the boiler won’t have much work to do at all.

Firstly is there any regulation that states the boiler can’t be oversized? Secondly do you think it will make much difference considering the boiler will modulate down to about 10kW or so and act like a smaller boiler anyway? Finally if it does make a difference, is it worth me selling this one and buying a lower powered one or should I stick with what I have. Changing it will cost me money, so I’m happy to put up with a slight loss in efficiency every year not to have to spend £500 now. What do you think?

 

[simond]

Keep the one you've got and downrate it in software. See the bottom of my post for the reason.

It won't be as efficient as a 618 due to the modulation ratio as you have said.

And you will need to size the gas supply as if it was running 30Kw, not 15Kw.

If it has been around a while it will have the old ecoTEC seals on the heat exchanger and be subject to the recall that Vaillant think they have completed.

Was it really worth it? Probably not. What can you sell a second hand boiler for? Jack Sh*t.

 

[WhydidIstart]

Ok thanks for the info. Are the old seals that you mention something I can see by looking at it or should I contact Vaillant with the serial number?

 

[Agile]

I am sometimes called to seriously over rated boilers!

Usually a 30kW or £35kW in a three bed semi!

They really do give a problem because even on minimum fire the reach the set flow rate in a minute and then go off for the anticycling delay so cause the system to heat up much more slowly.

Although you may think your rads need 15 kW its probably more like 8 kW or less most of the time!

In any case its the whole house heat loss that should be used in sizing the boiler.

I disagree with Simon about the gas supply. I would take the view that if the boiler is permanently down rated that its adequate for the gas supply to be sized for that power input.

Tony

 

[WhydidIstart]

I think 10kW is the lowest power it can be fired at. The hot water will be on for most of the time, so am I right in assuming this boiler will be ok if set to fire on low power and the hot water generally on at the sane time so there is a heat dump for the water at start up?

 

[Agile]

The cylinder will only absorb about 10 kW if the water is cold AND the flow temperature is at the rated flow temperature at which the absorbtion is specified. Often thats still at 80° C.

Whatever you will not alter the fact the boiler is overated and will cycle most of the time.

Tony

 

[WhydidIstart]

ok, I appreciate your experience so what would you do if it was between keeping the non-ideal 30kW one or having to buy a whole new boiler?

 

[Agile]

ok, I appreciate your experience so what would you do if it was between keeping the non-ideal 30kW one or having to buy a whole new boiler?

I would fit the 30 kW for a customer who needed one and buy a suitable one for myself.

In your case you could advertise the 30 kW on ebay and see if you could buy something which will be suitable for you.

Tony

 

[simond]

I disagree with Simon about the gas supply. I would take the view that if the boiler is permanently down rated that its adequate for the gas supply to be sized for that power input.

YOU may well disagree.

But for domestic boilers, CORGI stipulate that they could be turned up again and therefore the supply must be sufficient for the rated maximum output of the appliance, no less. On an ecoTEC a meddling customer could turn up the boiler using only the control panel.

Unfortunately 'your view' does not carry any weight with CORGI. Are you going to go back on the ones you've fitted contrary to the UK gas regs?

 

[Agile]

Yes, I know that CORGI would expect the gas supply to be fully compliant with the maximum power of the boiler. I would agree with them in theory.

I dont fit many boilers and I would never fit an over sized boiler anyway so that senario would never apply to apply.

Tony

 

[DP]

If boiler is to heat a Megaflow, would advice to ensure gas line is correct size (it must be correct size regardless- Gas Safety Regs require that). I fitted a Vaillant and a Megaflow for a friend. Believe it or not, the Megafllow will easily sink the power Vaillant throws at it. In my friend's case, for many minutes the boiler was on full power to heat the Megaflow.

 

[simond]

We've started fitting ACV cylinders because of their tank-in-tank design.

A measly 210 litre cylinder can sink 52Kw. Because you can store water up to 85C it has most of the advantages of a heatstore with few of the disadvantages.

 

[petergh]

Simond,
I have a Glow worm hxi30 2 years old. Definely too big , my gas usage now 25% higher than before when I had a nettaheat 16/22. Consumption monitored over 4 years.
Can I downsize this Glowworm with software? Who can do this for me?

Keep the one you've got and downrate it in software. See the bottom of my post for the reason.

It won't be as efficient as a 618 due to the modulation ratio as you have said.

And you will need to size the gas supply as if it was running 30Kw, not 15Kw.

If it has been around a while it will have the old ecoTEC seals on the heat exchanger and be subject to the recall that Vaillant think they have completed.

Was it really worth it? Probably not. What can you sell a second hand boiler for? Jack Sh*t.

 

[Beerlover]

First work out your heat requirements for the house, eg Megaflow 210 has a 21Kw heat input.

Then follow this

Untitled

• Beerlover
• 3 Oct 2010
• 1

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Your control will look diffent to this but the principle is exactly the same

 

[D_Hailsham]

First work out your heat requirements for the house, eg Megaflow 210 has a 21Kw heat input.

Then follow this ...

I think you will find that only the 38hxi can be range rated.