Boiler recommendation

[antgel]

Hi all,

I've just done the boiler calculation for my flat, and it came out at

12.25KW, which I think can be handled by any boiler AFAIK. I have two main requirements. The first is that it runs quietly, as it's going in a kitchen/lounge. The second is that it is small (it's a flat, so I would like to save space). I'm open to anything efficency-wise, as long as it's good value. I'm not sure that in my 900sqft flat, an HE boiler would make a massive amount of difference to my bills, but I'm willing to listen.

Any recommendations (I want a combi)? Is it the case that getting a Potterton, which is what I have now (albeit around 20 years old) will enable the installer to reuse the flue? It would be a right pain to get to the outside of the building.

Antony

[RedOnRed]

I believe you can't use the same flue and since April 1st a new boiler must be Sedbuk A or B.

[Andrew Gabriel]

Some boilers have flues which can be fitted without access to the outside of the wall (e.g. when you're on the 20th floor), although this may require some extra optional bits and pieces for the flue assembly.

[John Stumbles]

How did you do your calculations (and how big's your flat)? I've just done the calcs for an end-of-terrace 3 bed single glazing medium loft insulation house and that's 10-12kW (depending on whether I go by my own calcs or those on

[1]) so you've either got a very lossy and/or big flat or your calcs are out.

[OldBill]

Both boilers I've fitted recently did not need access to the outside. Seems to be standard, especially on the smaller models that would tend to be fitted in flats. Its unlikely in my experience that a existing hole will be in the right place for a new boiler. The Potterton is likely to be rear-flued so its possible the Ariston Micro-genus might fit. Boilers fitted since 1-Apr-2005 don't have to be Sedbuk A or B if all the requirements on location can't be met.

[antgel]

Hmm. I used

and my flat's about

900sqft, double glazed, middle floor. Are you able to try this calculator with your flat? I tried the that you refer to, but I don't know what dimension to type the units in!

Antony

[<me9]

My current boiler is in the garage and the flue goes sideways and out into the alley (3' wide) between the houses. The alley is too narrow for a condensing boiler horizontal flue. It would be possible to re-locate in the kitchen or utility room but only if existing wall units are removed. Would the new rules allow a replacement non-condensing boiler, or would I have to lose cupboard space in the kitchen/utility?

[antgel]

Furthermore, I just used the

(dimensions were metres!), and it gave me the same answer, pretty much. Length 8.6m Width 11.1m Height 2.3m

1 floor One external wall along the width => external wall area 25.6sqm Flat. Double glazed metal frames. Unfilled cavity walls. (Was unsure about this, but whatever option I choose doesn't appear to make a large difference.) Roof is flat uninsulated. Location is SE.

Did I miss something?

[John Stumbles]

Yes, that's better: it agrees exactly with my spreadsheet (which I based on the manual worksheet method given as part of the C&G6084 training materials pack).

Metres. It's a bit confusing in that it uses 'length' for what I'd call depth i.e. front-to-back measurement.

However the idhe calculator is following the same model.

I don't know what figures you put in for your flat but I tried it with 10m x

10m, one external wall on each side, 2.5m height (1 floor, mid-floor flat) unfilled cavity wall, double glazed plastic/wood frame in SE England/Wales and got:

The WINDOW heat loss is 1.01 kW The WALL heat loss is 1.62 kW The ROOF heat loss is 2.67 kW The required boiler output is 10.88 kW

However yours is a mid-floor flat so the model should ignore roof losses, but if you change the roof insulation setting it reports a different heatloss, so it looks as if their calculator is borked. My spreadsheet shows 8.95kW for the same values. I've emailed you a copy separately.

[antgel]

Thanks John. Note that you have to replace google with antony for my email to work. I'm prepared to believe your figures, and there is only one external wall to boot. My flat attributes are in my reply to my reply above. Bottom line is that it looks like any boiler will do.

For the sake of progress, let's assume that your 8.95 figure is correct. I was looking at an Ideal Isar HE as it's nice and compact. Firstly the 24, but I then realised that the 30 or 35 would give me a higher flow rate, which I would probably go for.

I have done some research on modulation but don't quite get it yet. What exactly happens if my need at a given time is lower than 8.8kW? Does it short-circuit? Does it waste energy?

I'm now thinking about a Glow-worm cxi. The 24 and 30 output as low as

5kW and the 38 outputs as low as 6.6kW - all lower than any of the Isars. And they appear to be cheaper.

Bearing in mind my situation, is there any reason to pick the Ideal over the Glow-worm? And bearing in mind what I asked about modulation, what's better - the 30 with it's min 5kW output, or 38 with it's 6.6kW min output and 15.5L/min flow rate?

Who knew that boilers were so complicated? I swear that my parents' plumber just came along and whacked in any old thing... :)

Antony

[Andrew Gabriel]

That's kind of irrelevant if you're on the middle floor. If it did somehow feed into the calculations, I suspect it has calculated for a top floor flat with uninsulated roof. (I might still assume the flat upstairs wasn't heated though, so your heating can still cope if the upstairs flat is empty.)

[John Stumbles]

The scoring rules are quite explicit that you have to ignore any existing furniture & fittings when assessing whether or not you can make an exception to fitting a condi.

[John]

AND they have stainless steel heat exchangers not aluminium

Go for the best hot water performance but remember it needs a bigger gas pipe from the meter.

A tip - make sure you do a "very" good flush of the old pipework and rads.

[John Stumbles]

OK I've sent it there, but I've just realised you said you want a combi. D'oh! In that case forget sizing for the heatloss of your flat: the size of the boiler will be 24kW or so for a decent input into the hot water.

If you need less heat into your rads the boiler switches on and off. This wastes energy compared to firing continuously but if the boiler can modulate down to a low output it's more efficient than if it has to give a short blast at full output and then switch off again.

Modulation is particularly important for the hot water so that if you need less than full output the boiler can match the demand rather than switching off and on: this makes for uncomfortable showering!

I'd go for the lower output since I guess you only have 1 bathroom and won't need the higher output to hot water of the 38. I'm not mad keen on the Ideals. I don't know about the Glow-worms. I'd also consider the new Bosch Worcester Greenstar i Junior, although these are new models so I'd be relying on B-W's reputation rather than an established track record for these new designs.

Prolly not much to choose between Ye Olde Heatynge-only Boylers.

[Alex (YMG)]

Not sure if this would work in your situation, but Glow-worm do a 'plume management kit' which you can even make yourself - it's just 50mm pushfit waste pipe, pushed onto the exhaust outlet of the flue. I cannot remember the exact numbers but I seem to remember this 'drain extension' could run for something like 10 metres - if your garage was single story you could take the pipe straight up the wall from the flue, and then vent it backwards (i.e. facing over the garage roof) to avoid the plume in the alley?

Alex

[antgel]

Ok. I wish Google Groups would let me trim the quoted text a bit when I reply. I had no idea that for a combi, one doesn't need to size for heat loss, although it makes sense when I think about it.

I do actually have two bathrooms, dishwasher and washing machine. Do you still think the 12.3L/min flow rate of the 30 is going to be enough?

Lastly, do I need to convert the dishwasher and washing machine to cold fill only with a combi?

Antony

[Andy Hall]

If you expect to use the two bathrooms concurrently, then 12lpm is going to be vary much on the low side in the winter. The flow rate is specified normally with a 35 degree rise in temperature. When the water is at 5 degrees coming from the mains, for normal shower temperature of about 40 degrees, the effect will be that the boiler flow rate will become the total rate as you won't be adding cold water to it.

12 lpm for a shower is fairly poor, although not as poor as from an electric shower. For two bathrooms, you really need a larger boiler

- capable of at least 50% more than 12lpm.

A modern dishwasher is designed to run with cold fill and to heat the water from cold anyway. There are several reasons for this. a) the detergents work on the basis of contact time and using hot water reduces that. b) Using hot water initially will set any proteins on the contents and make them harder to remove.

Unless you are using the hot cotton washes, most fabrics and detergents are designed for cool water of no more than 40 - 50 degrees. Therefore, you are taking in relatively little hot water anyway.

By the time a combi is delivering hot water in either case, unless you buy one with a small storage vessel built in, you won't have hot water coming through before the appliance is full

[<me9]

The current boiler (glowworm complheat) has a 2m horizontal flue out into the alley, about 2.5m up. The outlet faces the wall opposite. If you can add 10m, then that could take it up and over onto the roof. The wall through the flue protrudes is a 2 storey gable, near the centre.

[Ed Sirett]

The Non-condensing Vaillant boilers let you do this but you have to have a

127mm core drill so you can push the large 'grommet' through the wall from the inside.

[Doctor Evil]

12 l/m for a shower is good, not poor. Most people who have combis always compliment them on the shower performance, and 10-12 l/min is about average. Poor is 5 l/min. You have been told this in the past, so why do you deliberately impart misinformation?

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