Boiler size - conflicting engineer advice

[ashtons99]

I've had 2 different heating engineers round to advise on installing a combi boiler to replace existing 30 year old boiler. I'm wanting a Baxi Duo Tec. One has suggested the 33 will be more than sufficient for my needs, the other says the 40 will be better "just to be sure"

Now I know that there are loads of calcs and data sheets on the net but based on the fact that no doubt both will run the heating it presumably comes down to hot water demand and the ability of the house to supply water and gas at the necessary rate.

I have a 4 bedroom house with en suite and bathroom. 10 rads.

There are only 3 of us in the house and although we have 2 showers one is over bath and is rarely used, it's never happened that both are run at same time.

So the only issue I can see is if someone is in the shower and for example the downstairs hot tap is run.

I have read that having an oversized boiler is as bad in some respects as too smaller one, issues with economy and something called modulation??

So my question is, as a rough guide will the 33 comfortably meet our needs and provide a plentiful flow of water for a decent shower, or am I better with the 40 "just to be sure" or conversely will the operating differences between a 33 and 40 make negligible differences.

 

[Dan Robinson]

A 24kW will be fine - you'll never run two showers at once with a Combi anyway.

 

[Steelmasons]

A 24kW will be fine - you'll never run two showers at once with a Combi anyway.

Hey Agile sorry I mean Dan....I guess all those VAILLANT 31/37kw combis I fitted were oversized???
Under 10 litres a minute will work out just fine for the OP ,
God forbid when the cold snap is upon us with a cold water main temp below 5c..
Are you feeling ok Dan?..you don't seem your usual self??have you been out with Agile for a curry???
I'm sure Interslash combis exceed 24kw??...pretty much pointless wouldn't you say Dan?
One Agile is enough FFS...
Don't make the mistake of thinking whatever you say is sacrosanct....take a breather and chill..

 

[Snowman88]

Put a Duotec 33 in my 4 bed detached when I moved in, plenty big enough.

 

[Agile]

I doubt that Dan's internals will benefit from a curry for a while. Particularly as he seems to choose very hot dishes from the menu.

He needs to stay with the Japanese food, more healthy and somehow never seems at all filling. ( Good for the diet but not for that fully fed feeling. )

 

[Steelmasons]

Combi is the work of the devil..

 

[muggles]

Modulation will be your issue here, especially with a Duotec which is now an elderly design with fairly poor modulation

Boilers with a large modulation range have a lower minimum power output, and so are more efficient when running your heating. Something that is 30kW with 6:1 modulation will turn down to a minimum of 5kW, for example. If that boiler only had 3:1 modulation (IE a Duotec), it would have a minimum of 10kW. Larger modulation ranges reduce the amount of times a boiler will cycle on and off, thereby increasing efficiency as boilers use rather a lot of gas to fire up, so the more times they fire up, the more gas they will use

Your 4-bed house probably needs around 12-15kW when the heating first comes on, and this reduces as the system heats up. If the minimum your boiler can manage is 11kW, it's very soon going to start cycling and using more gas in the process. It may also heat up more slowly than a boiler with a lower minimum power could manage, and be less controllable

Combi boilers are a compromise and are generally sized to the hot water requirement, so measuring the incoming flow rate will give you a good idea of which output you might want, but remember that as the max output rises, so does the minimum, and so do your gas bills

 

[MJGas]

A 24kW will be fine - you'll never run two showers at once with a Combi anyway.

Hey Agile sorry I mean Dan....I guess all those VAILLANT 31/37kw combis I fitted were oversized???
Under 10 litres a minute will work out just fine for the OP ,
God forbid when the cold snap is upon us with a cold water main temp below 5c..
Are you feeling ok Dan?..you don't seem your usual self??have you been out with Agile for a curry???
I'm sure Interslash combis exceed 24kw??...pretty much pointless wouldn't you say Dan?
One Agile is enough FFS...
Don't make the mistake of thinking whatever you say is sacrosanct....take a breather and chill..

I think Dans posts was in jest.

 

[Steelmasons]

Hot water being the issue with a combi Muggles...turndown ratio being the least of the OP's worries..

 

[DP]

OP, I would be worried about 30 year old plumbing getting subjected to sealed system pressure
30kw odd boiler would be my choice

 

[Dan Robinson]

No it wasn't .

Absolutely naff all point installing a high powered combi. Look at the other thread that's popped up this evening with his 32kW Wooshitter.

 

[muggles]

Hot water being the issue with a combi Muggles...turndown ratio being the least of the OP's worries..

Hot water needs to be considered, yes, but my point was that there are more efficient combi boilers out there in real-world terms than a Duotec, which must by now be among the least efficient of all the combis

 

[Steelmasons]

A 24kW will be fine - you'll never run two showers at once with a Combi anyway.

Hey Agile sorry I mean Dan....I guess all those VAILLANT 31/37kw combis I fitted were oversized???
Under 10 litres a minute will work out just fine for the OP ,
God forbid when the cold snap is upon us with a cold water main temp below 5c..
Are you feeling ok Dan?..you don't seem your usual self??have you been out with Agile for a curry???
I'm sure Interslash combis exceed 24kw??...pretty much pointless wouldn't you say Dan?
One Agile is enough FFS...
Don't make the mistake of thinking whatever you say is sacrosanct....take a breather and chill..

I think Dans posts was in jest.

I'm not so sure after reading the possible "reverse return" issue.....hope he hasn't banged his head..

 

[Agile]

Mr Ashton, to see what size boiler you need, you really need to be more scientific.

That would involve measuring the flow rate from your hot taps or showers.

The 24 kW boiler will give about 9 litres per minute of hot water in the summer. A bit less when its very cold outside.

Whilst 24 kW is the "standard" and is typical in rented out flats, I always recommend a 28 kW or 32 kW because they can provide more flow in the winter.

So to be in control of your choice, you need to do some measurements.

A 40 kW may sound better but will still not run two proper showers. It is also likely to be less efficient on heating because its minimum power output will be higher and so cannot modulate down so well.

There are probably better boilers on the market too. Although the Duotec seems quite good value.

Tony

 

[Dan Robinson]

Can only speak from experience Steel .