Boiler Overheating Very Hot!! - Central Heating System HELP!

[D_Hailsham]

Would I be correct in saying the btu/h watt output quoted by the manufacturer of any radiator is the maximum amount of heat output?

Not quite. The output of a radiator is that which it produces under conditions laid down in British Standard BS EN 442. This specifies the temperature of the water as it enters and leaves the rad and the temperature of the test room. All of these will affect the output of a radiator. The test conditions are: flow=75C, return = 65C, room = 20C. You could produce higher output by increasing the water temperature or reduce the output by lowering the water temperature.

With modern condensing boilers, the trend nowadays is to run heating systems at lower temperatures as it means the boilers are more efficient. This can be compensated for by oversizing radiators. However, as the system is only required to produce the maximum output of a few(?) days each year, many installers would not bother oversizing the rads for the lower water temperature.

If the height, width, length of a room, plus the amount of outside walls, the roofing insulation, the size of window, whether it had double glazing or cavity wall insulation, whether the room was a bedroom or lounge, the age of property etc etc would be taken into consideration of working out the correct heat loss programme?

All of these factors have to be taken into account when calculating the heat loss of a room.

If the heat loss worked out was 6,500 btu what would the amount of btu heat output required to heat the room be? 6,500-8,500 the minimum being the heat loss. What is the range between heat loss and heat output or heat input?
Does this make sense?

Not completely

The heat loss of a room is calculated assuming certain external and internal temperatures, usually -1°C and 21°C. If you want to allow for colder weather or a warmer house, you have to make the necessary adjustment to the heat loss; fortunately it's directly related to the difference between internal and external temperatures.

Having said that, it is standard practice to allow an extra 10-20% in the size of radiators and boilers to provide for faster warming up. This is useful if the house is unoccupied most of the day. This is in addition to any oversizing for low boiler temperatures.

 

[igiveitup]

Firstly, thank you very much for taking the time to reply and explain, it is very appreciated. My point with the height, width etc. Is a new build property with low ceilings and more energy saving windows, doors etc, would require less heat than say a Victorian Terrace with high ceilings, possibly original windows, doors and had been built with fireplaces, not central heating systems.
Would the calculations of heat loss prog be taken into account for different properties? Or is it just worked on an approximate guide.
If the heat loss had been estimated as 4,000btu/h
What size of btu/h watts would be required to heat that particular area? in order to work out the size of radiator required. Providing the heat loss had been estimated on the height, width, length, outside walls, single or double glazed windows, whether an upper floor or lower floor. What dictates the temperatures? is that a general rule of thumb, or is it todays guide in alliance with a combi boiler.

If a terraced house was the end of a row, would it be classed as having more outside walls? than a middle of row terrace? would the adjoining wall be classed as half an outside wall?

Is there a weblink or organisation that would provide accurate estimated calculations of heat required if one put in dimensions and sizes. Also, would I be correct in saying that a convector radiators output is slightly lower than a traditional radiator?

 

[igiveitup]

Would I be correct in saying the btu/h watt output quoted by the manufacturer of any radiator is the maximum amount of heat output?

Not quite. The output of a radiator is that which it produces under conditions laid down in British Standard BS EN 442. This specifies the temperature of the water as it enters and leaves the rad and the temperature of the test room. All of these will affect the output of a radiator. The test conditions are: flow=75C, return = 65C, room = 20C. You could produce higher output by increasing the water temperature or reduce the output by lowering the water temperature.

With modern condensing boilers, the trend nowadays is to run heating systems at lower temperatures as it means the boilers are more efficient. This can be compensated for by oversizing radiators. However, as the system is only required to produce the maximum output of a few(?) days each year, many installers would not bother oversizing the rads for the lower water temperature.

If the height, width, length of a room, plus the amount of outside walls, the roofing insulation, the size of window, whether it had double glazing or cavity wall insulation, whether the room was a bedroom or lounge, the age of property etc etc would be taken into consideration of working out the correct heat loss programme?

All of these factors have to be taken into account when calculating the heat loss of a room.

If the heat loss worked out was 6,500 btu what would the amount of btu heat output required to heat the room be? 6,500-8,500 the minimum being the heat loss. What is the range between heat loss and heat output or heat input?
Does this make sense?

Not completely

The heat loss of a room is calculated assuming certain external and internal temperatures, usually -1°C and 21°C. If you want to allow for colder weather or a warmer house, you have to make the necessary adjustment to the heat loss; fortunately it's directly related to the difference between internal and external temperatures.

Having said that, it is standard practice to allow an extra 10-20% in the size of radiators and boilers to provide for faster warming up. This is useful if the house is unoccupied most of the day. This is in addition to any oversizing for low boiler temperatures.

Are you saying that the btu/h output of any radiator is purely down to the size of the radiator and the temperature of the water that comes out of the boiler to heat it? In other words if the boiler temp is set high, the trvs are set on high, but the room area is still cold, is the problem simply undersized radiators OR do different product ranges and different brands provide different heat outputs? Does the size of radiator depend on how long it takes to heat up. Would the heat output depend on the amount of steel in the rad as the old rads appear to hold heat far longer than new compacts/convectors, Is that also why radiators are oversized because of the low steel content. Is the temperature guide of 20c a general guide worked out on a chart regardless of each individual property. eg: A large rad can alway's be turned down, a small rad will never provide more heat

 

[D_Hailsham]

Are you saying that the btu/h output of any radiator is purely down to the size of the radiator and the temperature of the water that comes out of the boiler to heat it?

Basically, yes. The heat given out is related to the difference between the radiator temperature and the room temperature. The rad temperature is the mean of the flow and return temperatures. This is only an approximation as Newton's Law of Cooling comes into play.

In other words if the boiler temp is set high, the trvs are set on high, but the room area is still cold, is the problem simply undersized radiators OR do different product ranges and different brands provide different heat outputs?

If the boiler is set to a high temperature and the room is still cold, this will be due to undersized radiators. But don't forget that the radiator will have been sized for an outside temp of -1° and a room temp of 21°C. If the outside temperature is -10°C, the radiators will not get the house up to 21°C.

Setting the TRV to Max will not help if the rads are undersized. The numbers on a TRV represent the temperature you want the room to maintain (1= 12°C, Max = 30°C - this varies with brand and model of TRV). If the room is below temperature, the valve will be fully open
until the temperature us about 2°C below the required temperature. The valve then starts closing until the required temperature is reached; the TRV, normally, will only close completely if there is a secondary source of heat, e.g the sun shining into the room.

Does the size of radiator depend on how long it takes to heat up.

Don't really understand what you are asking. The size of a radiator is determined by the heat loss from the room.

Would the heat output depend on the amount of steel in the rad as the old rads appear to hold heat far longer than new compacts/convectors.

That debatable. The lower output of modern radiators is mainly due to them being run at lower temperatures. Pre 1997 radiators were tested with a difference of 60°C between the room temperature and the mean radiator temperature. So the water entering the rad could be about 85°C and 75°C when it left.

Is that also why radiators are oversized because of the low steel content.

No

Is the temperature guide of 20c a general guide worked out on a chart regardless of each individual property.

Are you talking about the room temperature?

A large rad can always be turned down, a small rad will never provide more heat.

That's correct; though if you have TRVs, the larger rad will turn down automatically.

 

[igiveitup]

Are you saying that the btu/h output of any radiator is purely down to the size of the radiator and the temperature of the water that comes out of the boiler to heat it?

Basically, yes. The heat given out is related to the difference between the radiator temperature and the room temperature. The rad temperature is the mean of the flow and return temperatures. This is only an approximation as Newton's Law of Cooling comes into play.

In other words if the boiler temp is set high, the trvs are set on high, but the room area is still cold, is the problem simply undersized radiators OR do different product ranges and different brands provide different heat outputs?

If the boiler is set to a high temperature and the room is still cold, this will be due to undersized radiators. But don't forget that the radiator will have been sized for an outside temp of -1° and a room temp of 21°C. If the outside temperature is -10°C, the radiators will not get the house up to 21°C.

Setting the TRV to Max will not help if the rads are undersized. The numbers on a TRV represent the temperature you want the room to maintain (1= 12°C, Max = 30°C - this varies with brand and model of TRV). If the room is below temperature, the valve will be fully open
until the temperature us about 2°C below the required temperature. The valve then starts closing until the required temperature is reached; the TRV, normally, will only close completely if there is a secondary source of heat, e.g the sun shining into the room.

Does the size of radiator depend on how long it takes to heat up.

Don't really understand what you are asking. The size of a radiator is determined by the heat loss from the room.

Would the heat output depend on the amount of steel in the rad as the old rads appear to hold heat far longer than new compacts/convectors.

That debatable. The lower output of modern radiators is mainly due to them being run at lower temperatures. Pre 1997 radiators were tested with a difference of 60°C between the room temperature and the mean radiator temperature. So the water entering the rad could be about 85°C and 75°C when it left.

Is that also why radiators are oversized because of the low steel content.

No

Is the temperature guide of 20c a general guide worked out on a chart regardless of each individual property.

Are you talking about the room temperature?

A large rad can always be turned down, a small rad will never provide more heat.

That's correct; though if you have TRVs, the larger rad will turn down automatically.

Is the temperature guide of 20c a general guide worked out on a chart regardless of each individual property.

Are you talking about the room temperature?

Firstly thank you very very much for your clear explanations, and yes I'am talking about the room temperature, are the temperatures set differently for bedrooms, lounges, kitchens and bathrooms?
Also does the temp have to be set at 20c? could it not be set higher?

 

[D_Hailsham]

yes I'am talking about the room temperature, are the temperatures set differently for bedrooms, lounges, kitchens and bathrooms?
Also does the temp have to be set at 20c? could it not be set higher?

The temperature of 20C is the one specified in the British Standard used by manufacturers when testing radiators.

You can have a room at any temperature you want, but the radiator will have to be made smaller or larger, depending on whether you want the room to be above or below 20C.

If you look in radiator catalogues, e.g Stelrad Elite Catalogue, you will find a chart showing how you under/oversize rads for different temperatures.

The temperature you have in each room is entirely up to you. If, however, you always keep doors open all the time, there is not much point specifying a temperature of 18C in the bedroom and 21C in the living room as the heat from downstairs will rise up to the bedrooms, which will eventually reach 21C or higher.