30kW System Boiler Choice

[D_Hailsham]

Now did you just make that all up or is it a collective of googling.

No to both parts.

When heating is being designed one of the factors that is not included is when was the heating last on.

Precisely. Heating is designed on the basis of raising the temperature from A to B, say by C degrees. It doesn't matter what the value is of A, the heating system will only raise the temperature by C degrees.

Maintaining the temperature at B is a completely different matter. It may take 15kW to heat a cold house from -2C to 21C, but it doesn't take 15kW to maintain the temperature at 21C, even if the outside temperature drops below -2C.

 

[chapeau]

Maintaining the temperature at B is a completely different matter. It may take 15kW to heat a cold house from -2C to 21C, but it doesn't take 15kW to maintain the temperature at 21C, even if the outside temperature drops below -2C.

You simply don't know enough about the house to be making definitive statements like that.

What if the outside temperature drops to -15C? You could keep pumping 15kw in and the internal temperature could end up at 20C quite easily, even less, it is impossible to say what temperature the inside of the house will stabilise at.

You are relying on your books of tables without understanding what they mean.

 

[holty]

I wpuld take advice of your installer, since you have expressed such faith in him. Or maybe not.....

I may not be as clever as your installer, but would ask you to consider the following;

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.

So i think you have been misinformed by a fool. Choose another installer, most of us are cleverer than your training chap, who I assume teaches 'career change chancers' as he wouldn't last long in front of a pro.

this was on the first page. after 7 pages. i think the op must be ****ing themselves laughing.

 

[DeltaT2]

Maintaining the temperature at B is a completely different matter. It may take 15kW to heat a cold house from -2C to 21C, but it doesn't take 15kW to maintain the temperature at 21C, even if the outside temperature drops below -2C.

You simply don't know enough about the house to be making definitive statements like that.

What if the outside temperature drops to -15C? You could keep pumping 15kw in and the internal temperature could end up at 20C quite easily, even less, it is impossible to say what temperature the inside of the house will stabilise at.

You are relying on your books of tables without understanding what they mean.

Correct, and heat-loss calculations are always based on a certain external minimum outside temperature...........-2, -1, -5, -3°C. So there will always be a large correction factor required in a country like Scotalnd with large weather variations - from brass monkies cold to f.cking boltic freezing!!!

Most homes are blasted with heat twice a day, cause most people are out at work, well, apart from the coffin dodgers & the scroungers. So, there's no point in heating any home at a certain level all day & using WC, which is a complete waste of money.

 

[Agile]

If this wasn't the case the boiler would have to run continuously just to maintain an internal temperature of 21C when it was -2C outside. But no boiler does this; it only comes on occasionally to produce enough heat to maintain the temperature.

Its not often that David says anything wrong.

However his statements above do not take acount of modern proportional room temp sensing.

Nor the user's setting of the boiler flow temp to give the desired room temp.

In a properly designed system the boiler will run continuously at a power output which maintains the required room temp!

Tony Glazier

 

[doitall]

In a properly designed system the boiler will run continuously at a power output which maintains the required room temp!

Tony Glazier

Get away Tony, you're living in cloud cuckoo land if you ever think thats gonna happen

Maybe in 50 years when Alec's sold us on OT and weather compensation.

 

[Agile]

Alex is fixated on WC based on external temp measurements.

Thats not bad but does not take account of internal temps where you actually live!

Most makers offer internal proportional boiler control on its own or as part of an inside/outside controller like the excellent Worcester unit.

Tony

 

[doitall]

If you want a boiler that never turns off I think you had better start listen to Alec, because you'll need some fancy controls to achieve it.

 

[Agile]

If you only have on/off stat on your boiler you can fit an indoor sensor ( just use an NTC ) and connect to the Intergas WC connector.

Then you will have proportional control and your boiler will run continously at low power and maximum efficiency!

Tony

 

[ALEC1]

Tony, did you know that combining a boiler with TPI controls is not recommended unless lots of parameters are changed in the controllers? Can you guess the reason?

 

[D_Hailsham]

What if the outside temperature drops to -15C?

A drop in the outside temperature of -15c doesn't necessarily mean that the inside temperature will drop by the same amount. The drop in internal temperature will depend on the construction of the house and the time elapsed.

Even when the outside temperature is well below freezing, the radiators were only just warm most of the day. They were probably very hot to begin with - while we were asleep. The boiler comes on occasionally, long enough to top up the lost heat.

It's not often that David says anything wrong.

I think others may disagree with you about that!

However his statements above do not take account of modern proportional room temp sensing. Nor the user's setting of the boiler flow temp to give the desired room temp.

That's all true, but it isn't relevant to the point I was making. I was comparing the heat required to raise the temperature from cold to that required to maintain the target temperature.

If it took the same amount of heat to maintain the target as it does to raise the temperature from cold to target, then the boiler would have to run at the same output all the time.

It's like driving a car! You need more power to accelerate from 50 to 70 that you need to maintain 70.

In a properly designed system the boiler will run continuously at a power output which maintains the required room temp!

You will need a boiler which modulates down to about 1kW for that to happen. Guess what! You can get one. It's made by Geminox. Others seem to be following - slowly. For example the latest Baxi boilers can modulate down to 3.5kW and Remeha are soon to launch a boiler with a 7:1 modulation ratio.

 

[ALEC1]

unsurprisingly the bills are no higher than for turning things on and off....and in almost all cases they are lower...depending on what was being replaced..

 

[D_Hailsham]

but your boiler must have been cycling surely, and what type of boiler is it?

It was a Apollo Fanfare (on-off type) and, yes, the boiler was cycling.

With compensation controls you get long continuous burns at the lowest level of modulation even for days...

But when you get below the lower modulation level you still have to go into on-off mode!

If you need 22kW at -1C for a 21C room temperature and you boiler only modulates down to 11kW (just an example ), you will enter on-off mode when the outside temperature is above 10C. And if you then throw weather compensation into the pot, so you only need a flow temperature of, e.g 55C at 6C outside, the boiler will go into on-off mode even earlier.

 

[ALEC1]

yes I agree and thats what I see....cycling at minimum out put...but thats why a lot of boilers fire at low rates and stay there....

 

[Agile]

Tony, did you know that combining a boiler with TPI controls is not recommended unless lots of parameters are changed in the controllers? Can you guess the reason?

Boilers are designed specifically with a particular application in mind.

Makers like Vaillant only make it possible to change some parameters when their own ( expensive ) controls are used!

Profit maximising engineering?