Gas efficiency - 33kW v 40kW combi boiler

[BHI]

Looking for a bit of an idiots guide to boiler sizing and gas efficiency please...

There are two combi boilers: a 33kW and a 40kW version.

As you would expect, 40kW version has a higher flow-rate than the 33kW - and therefore the 40kW version would be more desirable for the bathroom (bath and separate shower cubicle) and en-suite shower with body-jets. (All thermostatic valves).

If both boilers were connected to the same CH system (separately, of course!) would they use the same amount of gas each to heat the radiators up to the same temperature? ie. the 40kW version would use more gas per minute, but for less time - so the same actual volume of gas to achieve the same temperature rise? Or is my thinking completely out?

Would both operate equally in condensing (more efficient?) mode?

And what's modulation all about?!

Also on the DHW side of life, again would the 40kW be equally efficient in heating the required DHW demand (yet hopefully have spare capacity in reserve for other outlets)?

There's only £8.40 difference in the price of the two otherwise identical models!... So it now comes down to gas consumption, and a reality check of how much difference this would make (ie. DHW demand for a total of 1-2hrs a day?)

Many thanks in advance again to all you boiler gurus and fonts of knowledge!

 

[Mike Jackson]

The heating output from combis is generally less than the hot water output so you need to check what this is.

Are these two boilers just different models by the same manufacturer?

 

[BHI]

Many thanks for your reply Mike.

Yes - both are Baxi Duo-Tec 2 GAs.

So the CH requirement will be the same from both - it's just whether the 40kW will use more gas than the 33kW to provide this? Or will it just heat them up that much quicker (bonus!)? What about cycling on-off as a result though?

From reading this forum, it seems that combi boilers tend to be sized according to DHW demand?

But we don't want to end up with extortionate gas bills by opting for the 40kW instead of the 33kW if it's less efficient in whatever way... Otherwise, I'm not sure what the drawback of paying the extra £8.40 is?!

Thanks once again.

 

[Mike Jackson]

The output to heating is 26 or 33 Kw maximum which modulates down to 4.7 or 5.7. This won't make a great deal of difference to the gas usage or the heat up times but the 40Kw will give a better hot water flow rate as long as you've got sufficient water coming into the property.

If you want to use two hot water outlets at once it's worth fitting flow restrictors to all outlets to apportion the flow. I've got a 40Kw combi and have got 10l/min restrictors to the barh and shower and 8 l/min to all others. This means that I can stand under the shower and not freeze when SWMBO opens the hot tap at the kitchen sink.

 

[AWheating]

whats the total heat demand from your central heating system? most morden combination boilers have a output range for central heating mode, this is the range the boiler can modulate between when running the central heating. So if your heating demand is say 10kw and both boilers have a range lower than this then they should be quite simular in cost to run.

For a more specific answer tho you could phone baxi and give them your heating and hot water requirements and they could advise you in more detail and help you make the correct choice.

 

[BHI]

Thanks guys:

Mike - interested that you've also got a 40kW combi... Have you noticed horrific gas usage from before you installed it?! Like the idea of flow restrictor valves. When you say it modulates down from 26kW or 33kW to 4.7 or 5.7 - are those last two numbers also kWs or a ratio?

AW - yes, our rad calcs (averaged from many different methods) suggest just over 10kW requirement. Did briefly speak to Baxi, but you're never quite sure the advice you're being offered... Yes, the 40kW uses more gas (as it's 40kW), but couldn't get an "efficiency rating" for quantity of gas required to heat a specific volume of water by a specific temperature. Will check the tech specs again...

So far, tempted to pay the £8.40 extra for the 40kW then!

 

[Mike Jackson]

Those last two numbers are Kw as well.

I moved to a 40Kw condensing from a 35Kw non condensing and didn't notice a great deal of difference other than better water temperatures. I actually waste a lot less water now because of the preheat on the modern boiler.

Don't forget that the larger boiler will potentially be more expensive to install due to the larger gas supply pipe that may be needed.

 

[BHI]

Thanks Mike. Out of interest, what make/model do you have? How many rads and baths/showers/outlets does it serve? Not sure the Baxi Duo-Tec 2 GA we're looking at has a pre-heat / internal stored water option though.

We've got a 22mm gas supply most of the way, which the boiler needs.

 

[mfgs]

A combi may struggle to deliver enough hot water for a shower with body jets

 

[doitmyself]

As you would expect, 40kW version has a higher flow-rate than the 33kW - and therefore the 40kW version would be more desirable for the bathroom (bath and separate shower cubicle) and en-suite shower with body-jets. (All thermostatic valves).

Before deciding which boiler you need, you have to find out two things:

1. The heating requirement of your house

2. The incoming cold water flow rate and dynamic pressure.

You can find the heating requirement by using the Whole House Boiler Size Calculator

Flow rate and pressure should be done with a flow meter and pressure gauge, but there is a simple DIY method, which just requires a litre-marked bucket and a watch with a second hand. You just time how long it takes to fill the bucket from the kitchen cold tap to, say, the 10 litre mark and then simple maths will convert this to litres per minute.

As for measuring the pressure, you need two taps connected directly to the mains, e.g kitchen and garden. Measure the flow rate from one tap with the second closed and then with it fully open. If there is very little difference, you have good pressure. But if there is a noticeable difference you should think twice about having a combi boiler.

As for the boilers you mentioned, you must realize that installing a higher output boiler does not automatically give a higher flow rate - this is a fixed quantity - all it means is that the water can be heated to a higher temperature.

Because you are heating the water instantaneously, not storing it in a cylinder, you only need to heat it to about 40-45, not 60-65C. This means that, for a bath or shower, the hot water alone can be used, without the need to mix it with cold water to lower the temperature.

If both boilers were connected to the same CH system (separately, of course!) would they use the same amount of gas each to heat the radiators up to the same temperature?

A boiler cannot supply more heat than the radiators can dissipate, so if you have 20kW of rads a 30kW and a 40kW boiler will both adjust (modulate) their output down to 20kW.

Would both operate equally in condensing (more efficient?) mode?

That would depend on the design temperatures of the system. A boiler only condenses if the temperature of the water returning to the boiler is below about 55C. Most old heating systems were designed for temperatures between 70C and 80C, so a condensing boiler will rarely condense!

What's modulation all about?!

It means automatically adjusting the boilers output, between a lower and upper limit, according to requirements. The requirement will depend on the weather - you don't need so much heat in the spring, when it is 10C outside, as you do in the winter, when it is 0C outside.

When selecting your boiler, the flow rate will determine to HW output and the boiler size calculator will tell you the maximum heating output. YOu need to make sure the boiler can modulate below this - the lower the better - to provide the correct output in warmer weather. Obviously there will come a time when the boiler cannot modulate any lower, so it will then revert to on/off mode. But this is less efficient and shpould be avoided if possible.