30kW System Boiler Choice

[simond]

Chapeau

These things can be said with certainty because we in the trade are provided with insulation factors for different materials and seasonal weighted temperatures for a particular region and altitude. These figures are provided by CIBSE amongst others. They have had teams of academics creating the data, which has been proved in real life situations.

You have suggested reinventing the wheel, which doesn't help mankind.

Namsag has invented the perpetual motion machine, as apparently any property can be heated to a comfortable temperature by any boiler IF YOU JUST LEAVE IT ON LONG ENOUGH.....

And Alec has hijacked the post to make some erroneous and unsubstantiated claims about weather compensation. He isn't the Messiah, he's just a very naughty boy.

 

[ALEC1]

LOL

the point is that there are several ways of controlling heating systems, but only one is given airspace in the UK, that of simple on off control strategies.

there is no research to say that this is the optimum method to do so...its just done as far as I can tell because its always been done that way, for the past 50 years...

things do evolve and change...

and the benefits that I have seen inspire me to be more persistent, not less....

 

[mehran]

right... I going to back up Alec here a little bit.

in reality there will be two figures at work here.

1, the actual heat loss at any give outside temperature

and

2, the kw output need to equal and then over come the heat loss to heat the house to any given temp.

if you use controls that are always replacing the heat-loss to maintain a the internal temperature then you would need a smaller boiler then one that has to heat a stone cold house.

also you have to bare in mind that the fabric of the building will act like a heat store, so even if in the most bitter cold days the house will give you a buffer even if the boiler output is less than heatloss...(for a while)

 

[ALEC1]

coupled with the fact that at design temperature of -2c the boiler flow needs to be at about 70-80c...

at higher outside temperatures radiators radiators do not need to be so hot, or for that matter as set point is approached..

full load is rarely (if ever needed) except when you want to raise the temperature quickly...

 

[D_Hailsham]

Let's turn the problem around.

1. How much heat will it take to heat house A from -2C to 21C? Answer 30kW

2. How much heat will it take to heat house A from -2C to 9.5C? Answer 15kW

The heat loss per degree difference is exactly the same!

That's not turning the problem around, it's setting a different problem.

I think we need to go back to the beginning.

Boilers are sized for the external temperature being -2C. We assume that you may want to get the house to 21C. So on the coldest days (eg. When it is -2C outside) you boiler needs to add 23C of heat energy into the house.

If you have a 15kW boiler at the moment, and if your installer is correct about his sizing calculations needing 30kW, on a cold day your house will only reach 9.5C indoors.

An installer calculates that you need a 30kW boiler (and 30kW of rads) to raise the temperature 23C. So the heat loss is 30/23 = 1.305W/°C.

But you already have a 15kW boiler and 15kW of rads! So, if the installer is correct about needing a 30kW boiler, your 15kW boiler should only be capable of raising the temperature by 15/1.305 = 11.5C. This would give a room temperature, assuming -2C outside, of 9.5C.

1) The amount of heat (energy) required to raise a substance (the house) by 1C is related to the specific heat capacity of the substance. Here, increase in temperature is directly proportional to energy supplied.

2) The amount of heat (energy) required to maintain the house at that temperature is dependent on the heat loss. In a house that is basically how draughty it is and how well it's insulated. Heat loss, if only considering conductive heat loss, is proportional to the temperature difference between the inside and outside of the house. But heat loss due to draughts is going to be a bit more woolly to calculate.

No heat loss calculation method that I know of takes into account the specific heat capacity of the building. All it considers is the maximum anticipated heat loss.

What can be said with certainty is that a boiler capable of outputting 30kw will raise the temperature of a house quicker than a 15kw boiler. And the 30kw boiler will be 'on' less time when maintaining any temperature.

Are you advocating (a) deliberately over-sizing just the boiler, i.e installing a 30kW boiler with 15kW of rads, or (b) over-sizing the complete system (boiler and rads)?

If (a), the boiler will just modulate down to provide 15kW and most of the year will be running in on-off mode and short cycling.

If (b), this was the policy of "The Gas Board" when gas central heating started being installed and they had the monopoly. Their sales literature would say "Guaranteed Room Temperatures", with no conditions attached. The only way they could do this was by deliberately over-sizing the system. Gas was cheap and the Gas Board sold the gas!

Both options are just wasting the customer's money.

 

[chapeau]

This is all getting far too complicated and when things get complicated then mistakes are made. The physics is remarkably simple.........

If you have a perfectly insulated house, you could raise it to 100C using a 60watt light bulb.

You cannot do this because in the real world the house would be losing that heat just fast as you are putting it in.

Energy In - Energy Loss = Energy available to raise temperature of house

Giving specifics with 15kw or 30kw is not possible without knowing a lot more about how the house is built.

If you leave all the doors and windows open and there is a breeze blwing then the house will approximate outside temperature whether you have a 15kw or 30kw boiler.

Simples

 

[ALEC1]

complicated no, involved may be...no heating installer should be in business if they are struggling with whats being discussed here...

 

[simond]

where does that leave you?

 

[ALEC1]

I have no idea!

there is no way of being an expert in this industry its not set up for expertise...

its set up after a fashion to move product...

 

[DeltaT2]

This is all getting far too complicated and when things get complicated then mistakes are made. The physics is remarkably simple.........

If you have a perfectly insulated house, you could raise it to 100C using a 60watt light bulb.

You cannot do this because in the real world the house would be losing that heat just fast as you are putting it in.

Energy In - Energy Loss = Energy available to raise temperature of house

Giving specifics with 15kw or 30kw is not possible without knowing a lot more about how the house is built.

If you leave all the doors and windows open and there is a breeze blwing then the house will approximate outside temperature whether you have a 15kw or 30kw boiler.

Simples

100% agree mate.

As for ALEC1, any weather com. system will be adjusted to a ratio between the external & internal temperature. And as for having it controlled by WC only & a boiler lower than the heat-loss, you need a 'check-up from the neck-up' mate. I'd like to see you sell that Mantra in Aviemore??!!!.................

 

[ALEC1]

You are missing the point.. If you run a heating system 24/7 with two internal temperatures of say 20c and 14c the boiler never needs to go to full power...the maximum calculated heat in put is to raise the inside temp from -2c (both in side and outside) to 21c in usually two hours



If you remove the two hour time frame because the heating always available to maintain a temperature you can use a smaller out put boiler


In reality boilers need only be as big as the radiator output any way...

 

[DeltaT2]

C'mon mate, you can't say that, the heat-loss of any building has so many factors, not just the U-values, air infiltration etc etc.

The only systems that should be run 24/7 with a condensing boiler is UFH, where the mean water temperature is calculated based on the heat-loss & you're dealing with a radiant heat. Radiator convected heat is totally different. Then you have the building fabric heat-loss, where things like wind chill is a big consideration. Then you have the unseasonal weather changes, we can get 4 seasons in one day here in Scotland.

There will always be a big correction factor when a building is heated by convected emmiters.

So with radiators over-size boilers by 20-30%

 

[ALEC1]

stable state heating is pretty standard in middle europe...and it is run 24/7 (but temperature profiled) thats where the technology is developed..

I am not sure what you are actually doubting...

Why would you over size a boiler if the radiators have an output of 18kws (needed at -2c inside, -2c outside), all the rest of the time the boiler is oversized

 

[DeltaT2]

stable state heating is pretty standard in middle europe...and it is run 24/7 (but temperature profiled) thats where the technology is developed..

I am not sure what you are actually doubting...

Why would you over size a boiler if the radiators have an output of 18kws (needed at -2c inside, -2c outside), all the rest of the time the boiler is oversized

Is there such a thing as an oversized modulating pre-gas boiler, with the WC controlling the modulation???

Hey, Aviemore ain't mid europe & stable state heating is for highly insulated homes with very limited air infiltration.

It's a brave man that employs that Mantra in the Highlands of Scotland. There was one Dutch tw.t that lived in Ullipool, he thought he know it all & my mate in Inverness is still replacing his cock-ups.

C'mon mate, you're talking complete pi.h!!!

 

[ALEC1]

yes... of course there are...all of them that take an out door sensor control modulation by calculating the optimum flow temperature, and hence the kw output...its pretty simple stuff...

what boilers do you fit Δ t? and with which controls?