Replacing Heat only boiler

[skipdaleplumber]

Hi all,
I need to replace an Ideal mexico 65. Does 65 mean 65000 BTU's?
If so does that simply mean I could swap it for an equivalent KW boiler, what ever size that would be, I guess about 20kw-ish.
Thanks

 

[transam]

3412 B.t.u's = 1 k.w, check data badge on boiler for output , but 65 , probably does mean 65.000 b.t.u.'s

 

[Agile]

You should calculate the whole house heat loss.

The existing boier may be totally oversized or the house may have been better insulated since it was fitted.

A typical three bed semi needs about 10-12 kW plus just 2 kW for hot water heating.

Tony

 

[MarkRainbow]

That's good advice from Agile, most boilers are oversized on the basis that people can't or can't be bothered to do the heat loss calculations just want to be sure it's big enough. It is actually quite time consuming to do an accurate calculation.

With older "on/off" boilers, oversizing had quite an effect on efficiency as the boiler would cycle, wasting energy in heating up the boiler itself each time. Most modern boilers however, modulate and adjust the amount of gas they burn to that required, by measuring the temperature of the water coming back to the boiler, so they burn at continuous rate rather than turning on and off all the time. I have however been told that boilers work most efficiently when operating at their maximum output... not sure how true that is though.

Also to bear in mind... Most heat loss calculation methods assume an outside temperature of -2 degC... and with the weather we've been having recently!...

 

[Agile]

Many current boilers are more efficient at less than maximum power as the HE is transfering less heat that the maximum output and so removes more heat from the flue gases. Those part load efficiencies are not often available.

But the downside is that most boilers only modulate down to about 33% of the full power. So if they are at all oversized they still end up modulating on/off for much of the year when its not -1°C outside but about 12° which only needs about 3-6 kW to make up the heat loss.

Maximum efficiency is obtained with the boiler running continuously running at a reduced power which is still within their modulation range.

Tony

 

[MarkRainbow]

Yes Tony.. And of course the room thermostat will make the boiler cycle also. I should read up on how the new TPI controllers work...

 

[Agile]

Worcester have introduced a proportional room stat which modulates the boiler back as the set point temperature is reached. Its only been around for a short while and they have not publicised it ( as usual ).

As far as I am aware they are the only manufacturer to have this without the feature being included in a weather comp unit.

Tony

 

[mehran]

As far as I am aware they are the only manufacturer to have this without the feature being included in a weather comp unit

a bit behind the times agile plenty of manufacturers can have modulating room stat controls...

 

[Agile]

Can you point me to some?

Ideally with links to the actual product on their web site?

Tony

 

[MarkRainbow]

I enquired about Worcester's a few months ago and they had just withdrawn them due to problems, despite them being released in Germany.

Honeywell claim to have TPI controller, as I think Danfoss do, but how a third party thermostat can modulate a boiler that has only an on/off call for heat, I can't imagine! Any clues?

 

[D_Hailsham]

Honeywell claim to have TPI controller, as I think Danfoss do, but how a third party thermostat can modulate a boiler that has only an on/off call for heat, I can't imagine! Any clues?

A TPI controller is not the same as a modulating controller.

The TPI controller just varies the proportion of time the boiler is on in proportion to the difference between actual and required temperature. If the actual is more than, say 2 degrees below required, the boiler will fire continuously. As the difference decreases the proportion of time the boiler is on during a, usually ten minute, cycle will decrease.

Modulating controllers act directly on the boilers gas valve, fan etc to vary the output to meet the requirement.