Atag , viessmann 200 or worcester bosh

[mehran]

wrong(agile)... it makes not a jot of difference if you over size the boiler as long as you don't increase the low end modulation. you can always range a boiler down from maximum...but not up...

the OP is right in wanting a boiler that has a good modulation range. a boiler will spend most of its life in the low to mid range and not flat out.

a boiler that can go from 6-32kw is better then one that goes 8-26 even though it its size to match the house heat loss at -1/-3 or what ever low out side temp...

 

[Gasman77]

He will never admit it

 

[Agile]

I am impressed that the OP realises the importance of a boiler which can run at the lower power the house will need most of the time.

He has not said what power he has included in his calculation for HW. It should have been 2 kW.

These heat loss calcs are only approximate and many of the earlier programs came up with rather higher results.

The last figure he has stated is 25 kW but the HW content has not been mentioned.

That allows a boiler rated at 24, 25, 26 kW etc. I would not advise a boiler rated higher unless it modulated down further than the usual 3:1.

I totally agree with any others that its the minimum power output which is important as long as the boiler is capable of reaching the maximum required.

Unfortunately the manufacturers have not done much to deal with this problem. Its most significant when a high power combi is fitted to a small flat where the DHW power may be over 30 kW and the maximum CH demand is less than 10 kW so the boiler will always be cycling for heating.

Tony

 

[Gasman77]

knew it

 

[firekill]

thanks for all the replies , the HW demand is factored in to the calc at 2.5KW , i got it down to 25KW including the HW demand , by using a outside temp of -1 instead of -3 . believe me i was very anal when doing the calc , i measured everything twice & included all the wall & floor info. i am gonna put a wood burning stove in the lounge so i don,t want to oversize to much. question is , do i use the -1 or -3 calc . the 27KW figure was derived from -3 calc & included the hw demand ( 2.5kw)

 

[Biggles..]

Personally I'd use minus 3, however I would set the design to be hot water priority so you could ignore the 2.5kw for hotwater and make sure the system had weather compensation fitted.

I'll wait for the boiler repair man to correct us installers/designers and give his professional opinion.

 

[D_Hailsham]

I did the whole house method first & it came to 18.15 kw heat loss.

Does that include the 2kW for hot water?
Which region did you use?

the stars prog came out at 21kw heatloss.

Is that for 21C room and -1C outside?

I set the mwt to 65 as advised and stars prog said i needed 27kw with the hot water with the bigger rad sizes on 65 mwt.

The heat output required does not change with water temperature, but the radiator output does. This is because a rad output depends on Flow, Return and Room Temperatures.

The Stars program is very confusing on this point and I get the feeling that they do not understand this themselves.

I will try to explain, using an example I quickly cooked up using Stars. It's a single storey one-room building 6m x 7m, 2.4m room height, fully insulated. The pic shows the heat loss summary and uses a MWT of 65C.

Stars Example

• D_Hailsham
• 20 Mar 2010
• 1

How Stars calculates Rad sizes

The "Heat Loss" is 3730 Watts. This is the actual loss, before any allowances.

The "Adjusted" figure takes account of the change in rad output with temperature and the effect of the type of connection. The MWT Factor is 0.847. This means that the radiator will only produce 84.7% of its specified output when you use a mean temp of 65C. So you have to put in a larger radiator, i.e 3730 ÷ 0.847 = 4404W. The connection show is TBSE (middle buton). If BBOE was selected (left button) the Adjusted figure would increase to 4492 Watts. This is because rads connected this way produce about 2% less heat.

The "Required" number is "Adjusted" plus the "Fast Heat-up %". In my example this adds up to 4844 Watts. This is the size of radiator you buy; it is not the size of boiler you require.

You obtain the boiler size by increasing the heat loss (3730) by the "Fast Heat-up %" (10%), which gives 4103 Watts. To this you add the Hot Water Allowance (Sedbuk = 2kW, Stars = 2.5kW) This will give a boiler size of 6.1 or 6.6kW.

One thing I wasn't sure about was the ceiling temp in the program for the bedrooms. It automatically defaulted to -3, so I assume that's the worst case loft temp.

If your loft insulation is laid on the floor of the loft, the loft itself is not insulated, so the temperature inside the loft can fall to the outside temperature. That's why tanks and pipes in the loft have to be insulated.

 

[Agile]

I'll wait for the boiler repair man to correct us installers/designers and give his professional opinion.

Right on cue there then!

I had not noticed that he had mentioned the whole house calculation came up with 18.15 kW.

The questions then arises was that including the 2 kW HW allowance which as D Hailsham has pointed out included the 2 kW unless you take steps to remove it.

I would still agree that as boilers usually follow a 18kW and 24 kW rating step then a 24 kW would be the size to choose.

There are some descrepancies in boiler rating. Most manufacturers used to primarily quote the non condensing power output but they are now tending towards quoting a higher figure assuming some/fully condensing. They ought to all use the same method.

Tony

 

[firekill]

i have added up all the heatloss,s for the rooms added the 10% & 2.5kw for HW and it comes to 25.5 kw . this is with outside temp at -3 & all rooms done at either 18 or 21 degrees , bedrooms at 18 , lounge etc at 21 .So with those figure i could have a 26kw system boiler . the only reason i was quoting 27kw was because the stars prog was stating that figure on the required boiler page . i just want to get it right so we can at least say our system is as efficiant as possible. one more thing though i am now thinking of adding some cast iron rads in the hallway and landing areas .i will size them for the MWT of 65 . will this throw a spanner in the works . thanks for the info so far. ps i am sure the whole house method was 18kw without the HW added

 

[Biggles..]

Remember for greatest efficiency out of a cast rad,you should pipe them tboe.

 

[D_Hailsham]

There are some discrepancies in boiler rating. Most manufacturers used to primarily quote the non condensing power output but they are now tending towards quoting a higher figure assuming some/fully condensing. They ought to all use the same method.

I agree. There should be a British (European) Standard, just as there is for radiators.

One niggle I have is that manufacturers do not always quote the temperature differential used, so flow rates are impossible to calculate.

Continental manufacturers often quote outputs according to the German DIN standard. This can give produce efficiency figures of over 100%; but that's due to the way efficiency is defined.

 

[Agile]

This radiator classification using TBSE seems totally wrong to me!

With a squarish rad then thats fine however a standard cheap pressed steel rad 500 high and 3000 long is going to have virtually all the heat left at the connection end.

I am still suspicious this is not actually happening. With that connection if the aspect ratio exceeded 2:1 then a distribution pipe inside to collect the return from the far end would be needed to achieve the maximum output.

Tony

 

[Biggles..]

Tbse used to be piped flow at the top for even heating of the rad,thus avoiding your concern.

Manuf.. Are always going to use different standards to
each other so they can make their product look better than it is without a direct comparison.it all comes down to
marketing ploys.

If there's on set standard,some presumeably good manufacturers will be seen for what they really are

 

[D_Hailsham]

I have added up all the heatlosses for the rooms added the 10% & 2.5kw for HW and it comes to 25.5 kw.

That means the actual heat loss is 21kW.

I am sure the whole house method was 18kw without the HW added

The difference is probably due to the fact that your house has a mixture of solid and filled cavity walls. The whole house calculator cannot cater for this mix, it has to be one or the other.

As you are installing a system boiler, LCGS's recommendation to design for Hot Water priority and install weather compensation is a good one. You would then only need a 24kW boiler.

With hot water priority, whenever hot water needs reheating a motorised valve switches the flow so that it only goes through the cylinder coil and the boiler output is automatically set to maximum. There is no central heating for a short time, while the cylinder reheats.

Weather compensation automatically adjusts the boiler output according to the outside temperature.

Have a look at the Broag Remeha Avanta. These boilers can modulate down to 6kW, which means they will only revert to on/off mode when the outside temperature is above 15C. They can also be used with weather compensation and HW priority.

 

[D_Hailsham]

This radiator classification using TBSE seems totally wrong to me!

I agree, it does seem illogical. But if you look at the way the Stelrad Stars adjusts outputs for the three configurations you will see that TBSE and TBOE produce the same output and BBOE is about 2% lower. I would have thought that, as a radiator manufacturer, Stelrad would get that bit right!

With a squarish rad then thats fine however a standard cheap pressed steel rad 500 high and 3000 long is going to have virtually all the heat left at the connection end.

This assumes that the water at the opposite end to the connections is virtually stationary and the only flow is for a few sections at the connection end. Don't forget that the flow rate is relatively slow - a 600 x 3000 K1 rad produces 3kW and at 11C differential the flow rate is 4 litres per minute - and heat rises, so the incoming hot water will tend to spread along the top connecting tube before it cools and drops down to the bottom connecting tube.