Low loss header - Do I need it?

[akist]

I have been getting quotes for my central heating installation and one quote I have received includes the installation of a low loss header and extra pump.

I have read an old article stating the purpose of the low loss header is to allow large volumes of flow through the radiators when the boiler heat exchanger is too small to handle that flow and would present a bottleneck.

I am installing a heat only boiler, and without any hard data, my gut instinct tells me that the boiler manufacturer would not produce a 40kW boiler only to be a bottleneck trying to power 30kW worth of radiators.

Am I understanding this correctly? (I am also going to email Worcester to see what they say).

 

[muggles]

Maybe more importantly, why is a 40kW boiler being fitted if you only have 30kW of radiators?

 

[akist]

My actual figure is 32664 watts, but I was allowing for future expansion, into the basement and maybe a conservatory if I ever get the money

 

[MNW67]

WB sent me this a few months ago.

 

[Johntheo5]

What output boiler is this based on?, most boiler rated flowrates are based on a dT (flow/return) of 20C so a 40kw boiler above will flow 1720LPH (28.7LPM) at pressure loss of ~ 3.3M, not too bad as it leaves around 3M to circulate through the system,, problems will arise if you require to run the rads at a lower dT, in this case a LLH probably required if the rad output is is 40kw.
A heat only boiler is more forgiving as you can install a more powerfull pump than in the system boiler and reduce the LLH requirement.

 

[MNW67]

What output boiler is this based on?

A range of boilers using the same heat exchanger

 

[BlueLoo]

A LLH is for coupling multiple boilers or pump returns, no?

 

[MNW67]

I have read an old article stating the purpose of the low loss header is to allow large volumes of flow through the radiators when the boiler heat exchanger is too small to handle that flow and would present a bottleneck.

Mostly. The heat exchangers on modern boilers are a lot more restrictive than on earlier generations. So, it can take an awful lot of pump power to force a high flow rate through them. See graph above. This means that with a very large system, the central heating pump has used much of its power just pushing the water through the heat exchanger and there is sometimes not enough power left to circulate the water effectively through all the radiators. One way around this is to use a LLH in conjunction with an additional pump. On one side of the LLH one of the pumps is used just to send the water through the boiler. Then on the other side of the LLH, the other pump is used to send the water around the radiators etc. Or you can use one very powerful pump instead.

 

[monkeysballs]

i also very much doubt you need a 40kw boiler or a 30kw! have you actually had your heatloss report done as adding up your rads isnt the way to do it!

i have just under 17kw of radiators at delta 50 yet i only have a heatloss of between 3 and 3.5kWh at -3c

you need to find out how much you loose and that tells you how big your boiler needs to be unless combi then you size it based on how much hot water you need at any one time

 

[BlueLoo]

Some of the usernames on here are ace.

 

[MNW67]

A heat only boiler is more forgiving as you can install a more powerfull pump than in the system boiler and reduce the LLH requirement.

I think this might be the pump used on the combi and system versions of this boiler. Quite a beast?

 

[akist]

i also very much doubt you need a 40kw boiler or a 30kw! have you actually had your heatloss report done as adding up your rads isnt the way to do it!

I went to a few online heat loss calculators, for each room in the house, picked the radiators, summed it all up. I did however calculate at -5C external temps so I presume the 32664 watts is a worst case scenario. For example last December/January I remember we had a 5 day spell of -C temps.

Worcester said that the boiler will modulate down to keep to the required flow temp, which is not what I had expected. I had expected that the boiler would allow the flow temp to drop as the return temp rises, and modulate up as the return temp drops. But it seems it simply wants to maintain its pre-set flow temp, come what may.

 

[monkeysballs]

how much gas did you burn last month ?

-5 is possible if your in scotland even lower the further up you go and if your more exposed but -5 in surrey you should be more like -2

you calculate for the average low you will still have enough energy to heat even down to -10

how old is the house how well insulated is it? as honestly if your thinking your going to be using 32kwh you must live in a converted church with open windows or a tent!


it depends what controls you have, weather comp will vary the flow depending on the outside temp load comp works like you say and
standard on off controls will just burn at what ever flow temp you have set until the stat goes its too hot and then comes back on when its too cold

 

[MNW67]

i have just under 17kw of radiators at delta 50 yet i only have a heatloss of between 3 and 3.5kWh at -3c

I have been thinking about heat loss quite a bit recently. In the recent cold snap I measured, as best I could, that my real world heat loss was about 9KW at about -3C. And then I tried to work out how to convert that into a suitable size for a boiler. Am I right in thinking that 9KW (output) would just keep my house at a steady state? What if I also needed to heat the hot water cylinder. What if felt really cold and wanted to quickly boost the temperature. What if the house temperature had fallen overnight and I wanted it to heat up quickly in the morning. What size of boiler would give me headroom for all those eventualities. I would be really interested in any thoughts as I have been puzzling over this.

 

[akist]

I think it is like having a big or a small engine in a car. The small engine is more efficient. The large engine is quicker.