Ideal Minimiser FF80 Problems/Replacement

[XrayDave]

A boiler is sized for a worst case situation, normally outside temperature of -1C. At this temperature the boiler should be delivering its maximum output, which means the flow temperature will be at the highest.

So if the outside tempetaure is lower than -1˚C, the boiler will not be able to maintain the internal temperature?

If you can keep your house warm in the current cold weather with the flow temperature at 60C, the old boiler is oversized.

Errr....
Not at the moment. The boiler is broken and we have been without heating & hot water since 9pm last Wednesday evening (30th Dec). 3x 2kW electric convector heaters and the immersion heater are keeping one room warm and providing us with hot water at the moment!

There is a horrendous arrangement to get the bathroom warm for 1st thing in the morning, but I won't go into any details!! Hopefully the boiler will be fixed on Wednesday!

Dave

 

[D_Hailsham]

A boiler is sized for a worst case situation, normally outside temperature of -1C. At this temperature the boiler should be delivering its maximum output, which means the flow temperature will be at the highest.

So if the outside temperature is lower than -1˚C, the boiler will not be able to maintain the internal temperature?

Most heat loss calculators use a temperature of -1 C. But if you read the first page of the Sedbuk calculator you will see that it says:

"design external temperatures [is] dependant on the location of the property (included in the location factor)."

So if the lowest temperature in your area is frequently below -1, this has been taken care of.

In any case boiler sizing often includes an allowance or 10-20%; Sedbuk use 10%. The main purpose of this is to reduce heating up times, particularly when the house is only heated in morning and evening. But this allowance also gives a cushion against lower temperatures than expected.

If you only need a 15kW boiler (13kW for heat) and you install a 24kW boiler (22 for heat), you would be protecting yourself down to a temperature of -15C. Personally I think that's a teeny bit OTT.

If you can keep your house warm in the current cold weather with the flow temperature at 60C, the old boiler is oversized.

Errr....
Not at the moment. The boiler is broken and we have been without heating & hot water since 9pm last Wednesday evening (30th Dec). 3x 2kW electric convector heaters and the immersion heater are keeping one room warm and providing us with hot water at the moment!

Woops! I meant before your boiler broke down.

 

[XrayDave]

That's OK DH, 'twas easily overlooked.

We have had several occasions over the past few years where the temp has been down to -5 to -8˚C, so I think that I'd prefer to go with a 24kW boiler then for "insurance" because my elderly mother lives with us and she feels the cold terribly.

Given that 18kW seems to be required and our house is rather exposed at the top of a hill (there are large spaces between adjacent properties), the 24kW option isn't too much by way of overkill?

Thanks once again for all of your advice.

Dave

 

[Agile]

There you go!

We have given you all this good advice and you want to over-rate the boiler by 33% this giving you a reduction in efficiency for the next ten years.

It will not happen but if ever the boiler was unable to provide enough heat all you need to do is take out that fan heater for the day!

Remember once the hot water is heated you have another 2 kW from the calculation. The hot water should be timed to start 30 min before the heating so its already hot when the heat-up comes.

Here its -0.9 outside but still 22.4° inside. Yes, I know its a bit too hot inside but I am surrounded by Zulus!

Tony

 

[XrayDave]

Sorry Tony if it appears that I'm going against advice, just don't want to screw up by finding that the boiler can't cope with extremes of temperature.

Made a BIG mistake last time getting the Ideal Minimiser - what a DOG that turned out to be! Trouble was, 11 years ago, there didn't seem to be that much choice for condensing boilers.

My best estimte of the radiator load is nearly 18kW and that's without taking any hot water load into account. So, would the boiler be over-specifed by 33%?

Previous boiler (the Minimiser) seems, from what you and DH have said, that it was over-specified, running at 1/2 thermostat setting with radiators in the 50-60˚C mark.

If the weather is cold, like it is now, would an 18kW boiler struggle to provide CH and HW at the same time whilst maintaining 68˚F (20˚C) plus internally?

My mother would like a temperature of 22.4˚C, that's for sure!

Just want to make sure that I get this right so that when I get an installer to quote for a boiler instllation, I know what I'm talking about and can ask the right questions.

Temperature of the house drops noticeable when it is raining and there is a wind blowing due to the exposed nature of the house.

Tanks once again.

Dave

 

[XrayDave]

Just looking at the EST and sedbuk calculations again.

EST Calculator reckons 15kW boiler is required, sedbuk says 18kW

Sedbuk calculation makes a 2kW allowance for hot water whereas the EST calculation does not?

These both assume a window area of 17% of wall area?

EDIT: Actual window area is 13% of total wall area.

Thinking about the house, I need to check out the window percentage. I have a 2-storey wooden bay window - bay is 12' x 4' projecting out from the main building line.

18kW boiler sufficient for my needs?

Dave

 

[Tibbot]

Just looking at the EST and sedbuk calculations again.

EST Calculator reckons 15kW boiler is required, sedbuk says 18kW

Sedbuk calculation makes a 2kW allowance for hot water whereas the EST calculation does not?

These both assume a window area of 17% of wall area?

Thinking about the house, I need to check out the window percentage. I have a 2-storey wooden bay window - bay is 12' x 4' projecting out from the main building line.

18kW boiler sufficient for my needs?

Dave

I know you're trying to be thorough, but before getting too concerned about temperatures in your house, I'd be more worried that Hell might freeze over before you make a decision.

 

[XrayDave]

I know you're trying to be thorough, but before getting too concerned about temperatures in your house, I'd be more worried that Hell might freeze over before you make a decision.

Such a helpful post! Thank you

Having spent a lot of money only 10 years ago on a new boiler, I find myself in the same position of having to replace it.

Now possibly you can afford to throw money away, but I want to make sure that this time the money gets spent wisely.

The only reason that I am asking so many questions is that other people know far more than me and are prepared to take the time and trouble to share their expertise and help me to understand the reasoning behind their advice.

I wouldn't be asking the questions if I'd simply assumed that all I would have to do is replace my 10 year-old 27kW boiler with a new 27kW boiler. By asking appropriate questions, it seems that an 18kW boiler would suffice and be far more appropriate to the situation.

As far as temperatures go, I am making sure that my mother, who is over 80 and is not in the best of health, will be kept warm enough.

I hope this clarifies my position.

Dave

 

[Agile]

Its now -1.3° outside here but inside its gone up to 22.7° !

I have just rolled up my sleeves! But thats partly in response to the ginger wine!

OK, perhaps I can appeal to your deduction powers another way?

If your rads can only dissatate 18 kW whats the point of having a boiler with a greater output power than 18 kW?

Why do you imagine you are constantly reheating your hot water?

Heats up before you wake up and just needs topping up/reheating during the day ! Or do you have a cleanliness fettish that you take 18 showersd every day?

If the hot water was ever preventing you house getting hot then just switch on the immersion heater !

Then as the last resort fill a hot water bottle and go to bed early !

Tony

 

[XrayDave]

Ok Tony,

You've convinced me. Thanks for all of your time & trouble. I am now much wiser.

An 18kW boiler should do the trick.

Dave

BTW, the window area works out to be 13% not the 17% that sedbuk & EST assume, so this helps to convince me also

D

 

[D_Hailsham]

Just looking at the EST and sedbuk calculations again.

EST Calculator reckons 15kW boiler is required, sedbuk says 18kW

If you use exactly the same data the results are the same. So how does the Sedbuk figure suddenly increase by 3kW and not the EST?

Sedbuk calculation makes a 2kW allowance for hot water whereas the EST calculation does not?

EST includes 2kW for HW on the very last page (Required Boiler Output).

These both assume a window area of 17% of wall area?

EDIT: Actual window area is 13% of total wall area.

Thinking about the house, I need to check out the window percentage. I have a 2-storey wooden bay window - bay is 12' x 4' projecting out from the main building line.

It does not matter if the area is smaller than 17%, it just means the heat loss will be slightly less.

You only need to take into account the area of the wall occupied by the bay window, not the actual area of the window.

18kW boiler sufficient for my needs?

I think so, and so does Agile, but it's your call in the end.

Don't forget that plugging a 24kW boiler onto 22kW rads will not give you 24kW of heat. As I have pointed out earlier your rads will only be giving off 85% of their nominal output at best, due to the different temperatures at which current boilers run compared to your old boiler. Put another way. If you want to install a 24kW boiler you will need 28kw of rads.

 

[Tibbot]

Having spent a lot of money only 10 years ago on a new boiler, I find myself in the same position of having to replace it.

Now possibly you can afford to throw money away, but I want to make sure that this time the money gets spent wisely.
Dave

Then be prepared to spend it again, possibly in even less time. Modern gas boilers are not exactly built to last (although you might be lucky). Manufacturers only tend to quote 10 years. Unfortunalety no amount of forum research or hanging on to your installer like a limpet during the job will change that.

 

[XrayDave]

Then be prepared to spend it again, possibly in even less time. Modern gas boilers are not exactly built to last (although you might be lucky). Manufacturers only tend to quote 10 years. Unfortunalety no amount of forum research or hanging on to your installer like a limpet during the job will change that.

That's quite a scary thought - that boilers only last 10 years or so.

The Ideal Minimiser - although being a pain for most of its life due to Aluminium Oxide build-up which prevents firing - has done OK then to last 11 years??

Although I do watch tradesmen whilst they work (and often ask questions which is how I learn the time-saving tricks) gas installations are a real no-go area. I'm OK at plumbing, but gas scares me AND it is illegal, so I leave the installer alone. Even when the BG engineer comes to fix/service the boiler, I leave them to it as if it goes wrong or needs attention, there's nothing I can do anyway.

Hopefully the new boiler choice will last a long time after the system has been flushed, Fernox added and yearly servicing.

Dave

 

[Agile]

You only need to take into account the area of the wall occupied by the bay window, not the actual area of the window.

I think this ginger wine must be stronger than the rated 13.5% !

Its evidently affected my reasoning powers because I cannot see how the area of wall occupied by a window can exceed the area of the window !

Tony

 

[XrayDave]

Sedbuk calculation makes a 2kW allowance for hot water whereas the EST calculation does not?

EST includes 2kW for HW on the very last page (Required Boiler Output).

Doh! So it does.

Dave