Which size boiler for replacement

scfnet

Our Potterton Kingfisher II CF80 is on its last legs (rusted and slowly leaking heat exchanger) and we have decided to get it replaced. The cost of the repair was getting close to that of a new boiler and it was not guaranteed that it would be possible to repair due to the extent of the rusting.

Anyway we have had three quotes all very similar in price but for different sized boilers and we are not sure which way to go. Each fitter states that his option is suitable but I would like some more advice please.

The house is a detached 5 bed, 2 reception with a total floor area of approx 180 sqm. Walls are cavity insulated and the roof will be up to 200mm of insulation shortly. My estimates using online BTU calcs suggest somewhere around the 60000 BTU mark (including DHW).

The quotes are for

Worcester Bosch 30CDi System Boiler
Glowworm 30 Flexicom
Worcester Bosch 24Ri Greenstar

Are the 30kW boilers going to be overkill or will the 24kW one be struggling?

Any comments gratefully received.

Thanks

Alastair

Pincher

Small world, I replaced a Kingfisher II CF100 with a Flexicom 30SX in March 2010, for a 5 bed semi with two floors that has about 200m2.

It was a rust bucket at 30 years old, but not leaking, but my CORGI guy refused to service it any more. Still going strong after five years without service, but the £400 scrappage came along, so rest in peace CF100.

Output/input gives BTU 80,000/106,600=75.05% efficiency for the CF100. Assuming the same efficiency for your CF80, it was rated at 80,000 BTU, consuming gas at 23.4kW, and outputting 23.4 x 0.75 = 17.56kW: the actual heat you get for the central heating and water.

Maximum output of a 90% efficiency 30kW condensing boiler is 27kW, but you have to allow for when it is not condensing, so assume 75%, giving 22.5kW.

So you are replacing a 17.56kW output boiler with a 22.5kW (max 27kW) output boiler, a definite upgrade in heat generated.

The old setup had 100mm of insulation in the loft with solid wall,
and the CF100 was definitely struggling to keep the house warm.
We now have 200mm in the loft, but the solid wall is still the same.
The SX30 has no problem keeping the house warm. I also have two hot water cylinders (210 litres and 250 litres). These do introduce a heavy toll on the boiler, and the boiler cannot warm the house quickly if heating the cylinders as well. I therefore set the timer to heat the water at 04:00 to 06:00, when the central heating is off. I would say 30kW is about right for this house, with a little time juggling.

I have no experience of Worcester Bosch boilers.
You didn't say how many bathrooms. Consider a Rinnai gas water heater if you have three bathrooms. I put in an extra DHW cylinder for kitchen and en-suite bathroom, when it's cheaper and use up less space if I had gone for a Rinnai 16i. The existing cylinder supplies the other two bathrooms.

gas4you

Nice maths, but the 30Kw stated on a boiler is the output. The Heat input is greater than that and is what is measured when we gas rate an appliance.

Your 80,000btu boiler is the equivalent of a 24Kw boiler, so if you are warm enough now this will be all you need.

Chances are when your kingfisher was put in, it was over specced anyway.

You need to work out the heat output of all your rads, then add 3Kw for a standard HW cylinder and see what that comes to.

Pincher

I always assumed that they just round the numbers up to the nearest neat looking model number. So XYZ30 could mean 29.1kW or 31.3kW.

Also, we can't assume maximum efficiency from condensing mode anyway. So provided I give a clear working out, hopefully people know what I mean.

scfnet

Thanks for the replies.

Our current kingfisher copes OK but we are only heating half the house at the moment (and 1 of the 2 bathrooms) as there are only two of us at present! The hot water cylinder is approx 200L I think

I assume modern boilers are all modulating types so will adjust their output to suit demand, so for the extra few quid (depending on fitter / model choice) would I be better going for the larger boiler in case of needing the demand down the line.

However would this affect the efficiency of the condensing part (or the rest) as it is not running at or close to max output?

In the meantime I'll take a look at calculating the current load from the existing rads rather than assumed demand from the room size calcs as they manage to heat the rooms that we use OK.

Thanks again

Alastair

gas4you

If you went for the larger output boiler, then the installer would, or should, range rate it down to what your system actually needs.

Pincher

The concept of an "oversized" boiler debatable.

The Flexicom 18SX modulates from 9.3 to 18kW, and the 30SX modulates from 9.3 to 30kW. So provided the controller works correctly, the 30SX can behave like an 18SX.

This is from a Glow worm brochure.

"Intelligent room control explained.

As the temperature approaches the desired set point, the
Glow-worm Climastat or Climapro2 RF begins to lower the
flow temperature of water supplied to the central heating
system (radiators).
A lower flow temperature means that the boiler can
operate more efficiently in or closer to the condensing
mode. Energy costs are then reduced and a more stable
and comfortable room temperature is achieved."

You have to buy the extra controller (Climapro 2 RF is good) to get this capability. The traditional boiler flow temperature setting is just ignored. Because the controller is smart enough, oversizing is not an issue. The bonus is, you heat the house up from cold quicker with a 30kW.

On the other hand, I have a Potterton Promax 30kW in another property, which is very basic. You set the boiler flow temperature DIAL (analogue!), and the boiler modulates the flame just to keep that constant. Doesn't know what temperature the room is at, because the thermostat is just an on/off relay. So the only thing you can do is to set the boiler output as low as possible, and pray it will condense. In this case, oversizing can be an issue.

scfnet

@Pincher - I see what you are getting at, but I am going to be replacing the thermostatic valve heads with something similar to the Honeywell Hometronic system to only heat the rooms we want at the times we want so the traditional central thermostat or the climapro will not be of much use. If I wasn't doing this then I would consider that.

I have run various radiator sizing calculators and after adding on the hot water requirements it looks like the 24kW boilers would do, but after adding up the approx output of all the radiators I get to about 27kW so I am still not sure, I might plump for the 30kW and expect to waste some energy - it can't be anymore than I am at the moment.

Is there any preference on manufacturer? I have had a WB before (not condensing) with no problems and have heard that the Glow Worms are now made by Vailant so should be OK but have had bad reviews in the past.

Thanks for everyones comments so far.

Alastair

Pincher

From spec, I am quite keen on the new Glow worm Ultracom2 range,
which can work at lower flow temperatures for under floor heating applications, ideal for condensing mode. Unfortunately, I put in the Flexicom in March, and they bring out the Ultracom 2 in July. BUM.:o