Big boiler too big for summer time?

[theSouth]

I've asked this question in the past but didn't really get an answer (that I understood)

Say I have calculated the requirements for a house that needs a BIG boiler with x power. X is enough for total requirements of big house (rads and hwc).

Now, what happens in the summer months when there is no need to heat the rads? When the x power large boiler is only required each day to heat the water in a normal sized hwc (for arguments sake let's say it's a 140l hwc)

I've heard boilers can modulate down, BUT only by so much. So what if the boiler can't modulate down enough to only the needs of the hwc during the summer?

Won't the boiler switch on and off repeatedly?

Basically if you need a big boiler for cold winters, how does the same big boiler work in hot summers?

Cheers

 

[Dan Robinson]

By switching on and off.

Which can be an issue.

 

[theSouth]

Would a "fast recovery" hwc help?

Such as Gledhill.

 

[Dan Robinson]

You need to look at the coil rating.

"Fast Recovery" is often a misnomer. These things are relative. Old cylinders only had a rating of 4kW or so.

 

[theSouth]

Primary heat exchanger rating is 21kW

Boiler 40Cdi

So I guess that means boiler needs to modulate down roughly 50% (roughly)

 

[Dan Robinson]

No... the coil rating is when it is starting from cold. As it heats up its ability to absorb energy reduces. The 40CDi can only modulate down to about 9kW from memory, so when the cylinder is about 75% charged the boiler will begin to struggle and cycle.

I have a 15kW boler that boosts to 22kW in hot water mode, with a minimum output of just under 4kW.

My coil rating is 27kW. My boiler only just starts to modulate down just before it reaches set point.

 

[theSouth]

No... the coil rating is when it is starting from cold. As it heats up its ability to absorb energy reduces. The 40CDi can only modulate down to about 9kW from memory, so when the cylinder is about 75% charged the boiler will begin to struggle and cycle.

I have a 15kW boler that boosts to 22kW in hot water mode, with a minimum output of just under 4kW.

My coil rating is 27kW. My boiler only just starts to modulate down just before it reaches set point.

Cool, a well designed system. What's that like

I'll hopefully be connecting up tomorrow so fingers crossed it not 'too bad'

Cheers for help (again)

 

[Agile]

Say I have calculated the requirements for a house that needs a BIG boiler with x power. X is enough for total requirements of big house (rads and hwc).

Cheers

Who told you to add together the CH and HW maximums?

I just use the CH and just 2 kW for water heating!

Which calculator did you use to give a heating load of 38 kW?

Is the house really about 10 bedrooms all heated at the same time?

Tony

 

[ScottishGasMan]

I'm with agile on this one, unless theres a crazy hot water demand, its quite possible to take the heating load, the hot water load, and just take the highest one as the required boiler size.

cylinders with high output coils (20kw+) will generally be a highly insulated unvented type, so they are normally left on 24hr, there for they are heated then the boiler can move to heating load when required. 140l isnt massive, so a 20kw boiler may be plenty depending on the house heat loss, (20kw will cover a large number or domestic propertys comfortably)

 

[ScottishGasMan]

I have a cylinder thats 180l (23kw coil from memory) and from memory a house heat loss of maximum 14kw (be less once house is insulated better)

I run an 18kw boiler that will run flat out on heating the cylinder like dans till not long before its up to temp. and heats house with no issue after that

 

[ScottishGasMan]

To add one last statment to that. Lots of new builds I see, take the house heat loss only, 22kw cylinders with 10kw house heat loss, they use a 12kw boiler and works fine. The only difference is that the cylinder heat up from cold will take a bit longer which most people wont notice as the cylinder only ever tops up rather than heating from cold.

 

[Dan Robinson]

I agree too. My hot water boost is not a documented feature .

I got a special cylinder in order to make the most of the condensing boiler. My Heatloss ideas than 15kw. Cc members will recall my various "projects"

 

[Agile]

In practice, in a normal domestic environment the hot water should be heated for 30 min BEFORE the heating so the whole boiler output is available to the cylinder.

But it is the energy efficiency which tells us to add just 2 kW for water heating.

Tony

 

[Dan Robinson]

But it is the energy efficiency which tells us to add just 2 kW for water heating

Really?