Zoning / Boiler efficiency question

tSeacha

Say for arguements sake that you have a domestic dwelling with a 20kW oil boiler. The house has 3 zones, one for DHW and 2 for space heating. Ignoring the DHW zone and to keep things simple, each heating zone can emit 10kW of heat energy to match the boiler output of 20kW.
There will probably only be one heating zone calling for heat at any one time so only 10kW of heating will be required with a boiler generating 20kW. Is there 10kW of heating being lost out through the flue of the burner and if so is there some diversity factor that you should use when sizing boilers for zoned dwellings?

robd

tSeacha wrote: »

Say for arguements sake that you have a domestic dwelling with a 20kW oil boiler. The house has 3 zones, one for DHW and 2 for space heating. Ignoring the DHW zone and to keep things simple, each heating zone can emit 10kW of heat energy to match the boiler output of 20kW.
There will probably only be one heating zone calling for heat at any one time so only 10kW of heating will be required with a boiler generating 20kW. Is there 10kW of heating being lost out through the flue of the burner and if so is there some diversity factor that you should use when sizing boilers for zoned dwellings?

20kw is the max output. Technically and roughly 10kw is for heating, 10kw for hot water. If you've 2 heating zones that might be 6kw for downstairs and 4kw for upstairs. All very rough.

It's not lost. Boiler will just need to fire for less time. Also, you wouldn't install a smaller system as the output requirements are based on average temperature outside for region. You need the extra power for those really cold nights.

tSeacha

Am I correct in saying that a 20kW boiler will have a corresponding 20kW burner with a fixed fuel/air ratio and thereby giving 20kW of heat. If we say that the upstairs zone is calling for heat only 4kW can be transmitted through the relevant pipework as the DHW zone and downstairs zone are closed.
I understand that the boiler will switch off when the return temp reaches a certain threshold but is that not a seperate issue? If it is then is my original question still relevant?

robd

tSeacha wrote: »

Am I correct in saying that a 20kW boiler will have a corresponding 20kW burner with a fixed fuel/air ratio and thereby giving 20kW of heat. If we say that the upstairs zone is calling for heat only 4kW can be transmitted through the relevant pipework as the DHW zone and downstairs zone are closed.
I understand that the boiler will switch off when the return temp reaches a certain threshold but is that not a seperate issue? If it is then is my original question still relevant?

kw is a measure of power. The water is heated to a fixed temperature. It takes a certain power or kw's to heat to that temperature.

The quoted kw of a boiler is the power per second. Having higher power or kw just means it gets heated to that temperature quicker. Boiler will switch off till it drops and then turn on again. The total power usage and hence gas/oil usage is the same though.

It's not a separate question, it's all related to one another in mathematical equations.

I hope this makes sense. Having an engineering background myself, it works for me but not sure how well I'm communicating it.

yoshytoshy

All gas boilers are slightly different to each other ,in how they operate. A good boiler modulates it's output to suit your needs ,i.e a 24KW boiler will output as low as 7KW if the heat rises quickly enough.

The boilers I mostly put in are worcester and they never go off on temperature ,they will slow down to suit the heat requirements and burn a minimal amount of gas to keep the temperature.

ntpm

robd wrote: »

kw is a measure of power. The water is heated to a fixed temperature. It takes a certain power or kw's to heat to that temperature.

The quoted kw of a boiler is the power per second. Having higher power or kw just means it gets heated to that temperature quicker. Boiler will switch off till it drops and then turn on again. The total power usage and hence gas/oil usage is the same though.

It's not a separate question, it's all related to one another in mathematical equations.

I hope this makes sense. Having an engineering background myself, it works for me but not sure how well I'm communicating it.

+1. Good explanation.

Domestic Oil Boilers operate on a fixed heating output. They do not modulate their output unlike Gas Boilers (as earlier threads stated).

Basic concept: They fire up and transfer the heat from the combustion fumes/ gases across the heat exhanger in to the water.

The problem arises is the boiler is oversized, the burner will produce too much heat and the heat exchanger and the boiler thermostat will reach temperature too quickly and the burner will fire on and off too frequently (short cycling). The short cyling may not produce enough continous heat to heat up the system effectively. This is inefficient use of energy and may reduce longevity of boiler due to ware and tear.

You can however de-rate oil boilers to better match the average heat output and they will still manage to heat up all the zones at same time is required... it may take a little longer though.

Not too sure if you are installing a complete heating system.
FYI.
Ensure that the boiler controls incorporate a "Boiler Interlock". This ensures the boiler only fires when there is a call from either the hotwater motorised valve or either of the heating zone valves.

tSeacha

Thanks to everyone for their input so far. The idea of de-rating a boiler is interesting and I imagine it involves reducing the oil nozzle size and adjusting oil pressures/air flow etc.
Would I be correct in saying that the boiler switches on and off based on the temperature difference between the water flow temp and the water return temp with the temp stat measuring the water return temperature. Therefore is there a point when the heating circuit cannot transfer the heat from from the boiler quickly enough before a boiler high temp stat will shut off the boiler?
Following on from this is there a limit to the amount of zones or alternatively a minimum zone kW rating that should be installed with an oil boiler circuit?
Finally what timeframe would you put on boiler short cycling?