What`s the point?

[doitall]

Agile was impressed

 

[charliechaplinspants]

D_Hailsham

So if you want to deliver 15 litres/minute of hot water with a temperature rise of 35C your boiler will have to produce nearly 37kW.

How did you get the total of 37kw from that calculation and what is meant by a temperature rise of 35C? Thanks guys.

 

[DP]

35 degrees C is the yardstick that allows one to compare boilers outputs.

In winter water from cold tap can be around 5 degrees C (in Scotland anyway). Boiler if it is fitted with correct size of gas line, has stated through flow of water, will deliver 5 plus 35 degrees of hot water at the tap.

Undersize the gas line, you may not get 40 at the hot tap. Increase water flowing through the boiler, you will not get 40 degrees at the tap. Have poor cold water delivery, you will certainly have 40 at the tap and more (provided gas line is in order)

 

[charliechaplinspants]

Thanks DP, but how has D.Hailsham calculated a boiler size of 37KW is required for a flow rate of 15ltrs per minute? If he was here he would probably tell me

 

[DP]

Just accept it that he knows what he is talking about

 

[charliechaplinspants]

I know he knows what he`s talking about, I just wanted to know how to do the calculation, that`s all, do you know? Anyone know? Thanks guys.

 

[Agile]

Needed a 1500Ltr sectional storage tank and a 6bar pump.

3 x Gemini 500Ltr unvented calorifiers at 5bar, and 54mm copper pipe, 28mm to each shower.

I only recall seeing TWO unventeds at Bassett Road or was this somewhere else?

Tony

 

[charliechaplinspants]

An average older three bedroom semi in the UK usually works out at about 10-12 kW for heating and then you add on 2 kW for water heating.

This is what you posted on another thread Agile, yet in my particular case the advice seems to be that if your incoming main is good, (mines 14ltrs per min) you may as well put in a combi big enough to deliver this, so far it looks like I would need a 30kilowatt boiler or more mines an average three bedroom semi.

 

[doitall]

An average older three bedroom semi in the UK usually works out at about 10-12 kW for heating and then you add on 2 kW for water heating.

This is what you posted on another thread Agile, yet in my particular case the advice seems to be that if your incoming main is good, (mines 14ltrs per min) you may as well put in a combi big enough to deliver this, so far it looks like I would need a 30kilowatt boiler or more mines an average three bedroom semi.

Tony is talking about a cylinder in his last post and not a combi.

 

[charliechaplinspants]

Yeah, you`re right sorry, misunderstood because OP was originally talking about a Vokera combi. So what`s your take on it DIY, High flow rate, put in a large KW boiler for hot water? ( combi ). I`m talking about my situation here.

Just been on the Wiki section of the Forum and it states there that sizing a combi to DHW requirements is not the correct way to do it.

 

[doitall]

Yeah, you`re right sorry, misunderstood because OP was originally talking about a Vokera combi. So what`s your take on it DIY, High flow rate, put in a large KW boiler for hot water? ( combi ). I`m talking about my situation here.

Just been on the Wiki section of the Forum and it states there that sizing a combi to DHW requirements is not the correct way to do it.

Then Wiki is wrong with a capital W.

For an average size house I would be looking to go around the 30Kw for a combi, and it would be either the Broag or Atmos intercombi.

 

[charliechaplinspants]

Thanks DIA, everything seems to be falling into place except the fact that the boiler will be quite a bit oversized for the heating requirements, in order for the boiler to condensate properly the rads will have to be oversized. Would you agree? Or just fit rads to size of room, but if that`s the case, why not fit an old style combi ? ( I know you can`t ) must be a lot of pointless condensing boilers out there.

 

[doitall]

No, you size the rads to fit the rooms and required temperature, + 15% for extra cold days and silly winters.

The boiler will modulate right to to whatever the heating load is, bit like a pan simmering.

If you want to save on running costs look at weather compensation, either of the aforementioned boilers are ideal for it.

 

[Agile]

The boiler will modulate right to to whatever the heating load is, bit like a pan simmering.

Unfortunately that is not usually the case!

Taking the case of the three bed semi in the UK lets take a heating load of 11 kW.

Suppose a 30 kw combi is chosen to give about 12 li/min of hot water.

Most combis will only modulate down to about 33% of their maximum power or 10 kW in the case of this example.

However, the heat requirement of the house is a maximum of 11 kW and thats when its -1° C outside. Thats usually only for a very few days a year.

For the rest of the time when the load is only 3-5 kW the only way the boiler can deal with that is to cycle on/off at minimum power. Thats less efficient than when the boiler runs continuously.

Tony

 

[doitall]

The Broag Avanta 28c will modulate down to 5.8.

The Atmos intercombi 32kw will modulate down to 7kw.

My old Servowarm will modulate down to 2kw

With weather compensation it' isn't a problem anyway.