Cold House? CHECK YOUR RADIATOR BTUs

flashnazia

First let me start on the basics.

Better explanations of technicals or corrections are appreciated.

For those that don't know,

BTU - British Therman Unit.

It a measurement of heat. 1 watt=3 BTU

Your radiators should produce enought BTUs (heat) to match the BTUs for the room.

The amount of BTU the room needs depends of factors like height, width, length, size of windows etc. (Bigger windows need more BTU etc.)

It is based on estimations so can vary widely.

Heating engineers use rough guesses like,

For bedroom and hallways, Height(feet)xwidth(ft)x length(ft)x4

For other rooms, Height(feet)xwidth(ft)x length(ft)x5

Or
50 Btu per square foot

Online BTU calculators,

http://cultfollower.net/radcalc2/BTUcalc.htm
http://www.radcalcs.com/ (this one is over generous!)

The radiator in the room should provide enough (if not more) BTU for the room.

You can find out the BTU of a radiator by looking at the manufacturer's leaflets.

In my opinion, it is better to get a higher BTU radiator to meet your room's needs (and adjust with a thermostatic radiator valve) then to have one that is too small.

I am saying this because,

All this time I have had the wrong rads in my home (I didn't know).

Every winter I have freezed despite central heating on full blast and compensating with fan heaters and gas fire.

I thought it was just one of those things because of the age of the house.

All that fuel wasted (because the boiler has to fire for longer) as the rads weren't emitting enought heat for the rooms.

moncman

Well yes and no.

The heat didn't just vanish because the radiators were a bit puney.

What happens is that the fluid (water) in the system will return to the boiler hotter than it would have done if more heat had been allowed to convect into your room.

The problem is that all the while you are pumping fluid round the system in a vain attempt to raise the room temp the pipes are losing heat to other cavities (under floor/in loft wherever). The longer the system is burning to try and raise room temp the more the inherant inefficiencies will be costing you money. If the heating is only on for a couple of hours to get the house warm then obviously that is a good thing but you can be rest assured the boiler and pump will be working at max chat and so you could be burning a heck of a lot of gas during that time.

flashnazia

moncman wrote: »

Well yes and no.

The heat didn't just vanish because the radiators were a bit puney.

What happens is that the fluid (water) in the system will return to the boiler hotter than it would have done if more heat had been allowed to convect into your room.

The whole point is that the radiator should give out as much heat as the room loses for the room to stay at temp.

If I have small rads it can't match the heat loss.

But it is still giving out heat.

So water can't return to the boiler still hot?

Cardew

The point moncman is making is that under size radiators do not dissipate as much heat as larger radiators, so the water returns to the boiler warmer than it would do with correct size radiators; therefore your boiler will not fire for as long.

Take an extreme example. All you have switched on in the house is a small heated towel rail(e.g. all radiators off) That can only dissipate a little heat so the boiler will only fire for very short periods as the water returning to the boiler will not have lost much heat.

So you having undersize radiators will not have cost you any more money to run than having correct size radiators. The only difference, as stated above, is that you will lose a little more heat from the pipes(although of course that heat goes into the house.)

Cozworth806

I read when I was looking for my system that it is better to get more radiator space than less as it is certainly more effiecient to have a boiler heating the correct amount rather than too few that will require longer heating times.

I certainly had this in my living room, where I should have got two smaller rads rather than one massive thing, as it would have been easier to balance the heat between the two.

flashnazia

Cardew wrote: »

The point moncman is making is that under size radiators do not dissipate as much heat as larger radiators, so the water returns to the boiler warmer than it would do with correct size radiators; therefore your boiler will not fire for as long.

Take an extreme example. All you have switched on in the house is a small heated towel rail(e.g. all radiators off) That can only dissipate a little heat so the boiler will only fire for very short periods as the water returning to the boiler will not have lost much heat.

So you having undersize radiators will not have cost you any more money to run than having correct size radiators. The only difference, as stated above, is that you will lose a little more heat from the pipes(although of course that heat goes into the house.)

Assuming you have one radiator of 5000 BTU in a room that needs 10000.

This means the room is losing 10000 btu but the 5000 rad is unable to replace the heat lost as it doesn't producr enough.

The radiator will also get colder much quicker because the room is cold and the heat tranfers.

If you have a thermostatic control in the room set to a certain temp it will never send the off signal to boiler as the temp will never be reached.

freddysmith

Interesting thread raised by the OP. Its given me some thoughts on checking my own home heating system.
I have quite a large void beneath my house and the heating pipes are not insulated or have deteriorated.

Cardew

flashnazia wrote: »

Assuming you have one radiator of 5000 BTU in a room that needs 10000.

This means the room is losing 10000 btu but the 5000 rad is unable to replace the heat lost as it doesn't producr enough.

The radiator will also get colder much quicker because the room is cold and the heat tranfers.

If you have a thermostatic control in the room set to a certain temp it will never send the off signal to boiler as the temp will never be reached.

As stated above:

The heat didn't just vanish because the radiators were a bit puney

To produce xxx BTUs of heat will use xxxkWh of energy. A small radiator will dissipate less heat than a bigger radiator.

You do not seem to appreciate that the boiler will switch off when the water temperature reaches the set level. As the water is returning warmer the boiler will fire for less time.

Using your logic (i.e. that a 10,000 BTU radiator costs less to run than a 5,000BTU radiator) if you put in a 20000 BTU radiator the system will cost even less to run, a 40,000 BTU radiator and cheaper still!!!

We are not disputing that you should have correct size radiators in your room, but having smaller radiators(under-size) does not waste fuel "because the boiler fires for longer" as you state. In fact the boiler will fire for less time and will use less fuel.

flashnazia

Cardew wrote: »

As stated above:


You do not seem to appreciate that the boiler will switch off when the water temperature reaches the set level. As the water is returning warmer the boiler will fire for less time.

No need to get personal.

I'm talking about my system that only switches off when the thermostat in hall reaches a certain temp.
So in my situation the boiler will fire for longer as the rads will never allow the room to reach temp.

I've asked a heating person and he agrees.

So I stand by MY opinion. You are perfectly entitled to yours.

greenbee

Before going to the expense of changing radiators, it is also worth making sure that they aren't full of air (if they're cold at the tops bleed them), and check for sludge/corrosion (when they're cold at the bottom, in which case you need to clean and drain them).

As least this way the rads you have got will be working as efficiently as possible.

Cardew

flashnazia wrote: »

No need to get personal.

I'm talking about my system that only switches off when the thermostat in hall reaches a certain temp.
So in my situation the boiler will fire for longer as the rads will never allow the room to reach temp.

I've asked a heating person and he agrees.

So I stand by MY opinion. You are perfectly entitled to yours.

I can only assume that your 'heating person' misunderstood the question.

It really doesn't matter about the room thermostat - it can 'demand' heat all day.

It is the temperature of the water in the system that will stop the boiler firing. So with undersize radiators the boiler will fire for less time than with the correct size radiators.

As said in post 2 if the boiler fires for longer, where do you think the heat goes?