Central heating modifications

What really matters is not the differential per se but the Delta-T (dT) - i.e. the difference between the mean rad temp and the room temp. *That's* what determines the rad's output. If you're running at 70/50, the mean rad temp is 60. If the room temp is 20, dT is 40. If you have a rad whose output at a dT of 60 is X kW, the output at a dT of 40 will only be about 60% of X. So, when choosing rads, you need to look at their stated output at whatever dT they're spec'd at and downrate them for whatever dT you're using in your system design. [There are published downrating factors which you need to consult when doing the calc].

You're right about needing to do a heat loss calc - simply to work out what max heating requirement you have for each room. You then need to find a rad which has sufficient output *after* you have downrated it for reduced dT.

Maybe. Can you increase the capacity of the 700w rad without taking up much more space? For example, if it's a single without fins, you could replace it with one with fins, or even with a double.

One final comment. If you size your rads such that the boiler can run at its max efficiency on the coldest day, you'll end up with massive radiators - particularly if you go for a dT of even lower than 40. Bearing in mind that - last winter excepted! - we don't normally get *that* many very cold days, it may make more sense to design for using condensing mode *most* of the time, and to accept that you may have to resort to 82/71 non-condensing mode when the need arises.

In what way is the fan convector inadequate? Is this water heated or electric?

How big is the kitchen?

Chris

David wrote: ...

Not even under the kitchen units?

That's OK as long as you know how dangerous 80C is. I prefer to make them really big and run them at a low temp like under floor heating.

I think that is the more efficient way to run things as well.

Big and warm beats small and hot any day. Heating is more even too.

Other option is fan blowing. That moves warm air around very fast.

Dangerous to whom? I've been running my systems at 80+ for over 40 years, and *I'm* still here!!

You must be one of the lucky ones then:-)

Adam

Thanks.

Yeah I get you :)

It is a P+ radiator, very small and behind the door, anything larger and the door will constantly hit against it.

But if the system is configured as a 70/50 won't the highest possible output be at 80/60?

Water heated

I'll have to measure up, it is extended so one half has cavity wall insulation whilst the other has solid walls.

An elderly person I knew died from the burn resulting from falling asleep against a radiator which then came on hot.

If you are concerned about cost of new radiators, you might be able to make savings by reusing some. To a first approximation, chuck out the smallest ones, move them all such that a larger one replaces a smaller one, and then just buy even larger replacements to go in the positions the larger ones moved from. In practice it's never that simple, but some reuse may be possible. Don't reuse any corroded ones though.

If you do that for an outside temp of -3C (which is the normal way to calculate), then in practice you'll be able to run even cooler (more efficiently) most of the time.

A taller one, or additional one somewhere else?

There are two factors here. Highest radiator output will be at 82/71 (mean 76), verses 80/60 (mean 70). However, the boiler might not be powerful enough to drive all radiators at max output.

My boiler will in theory deliver a flow of up to 83C, but at that temp, the radiator outputs will exceed the boiler power rating, so it won't ever manage to get up to 83C in practice. (It normally runs at 45/40, so running at 83C would represent somewhere around 3 times the heating power the house needs.)

Its not uncommon for that to happen, why do people always assume its kids that are the vulnerable ones?

When my dad had a system fitted under the warmsure scheme I had the rads oversized and set to 50C, they didn't mind the extra few quid on rads, the boiler would go down lower than that but the space for rads is not infinite.

Just about everything in life is 'dangerous' to some degree or other, and there will always be examples of people who have encountered such dangers.

But there has to be a balance which allows people to accept minor risks for the sake of convenience. If you were to legislate for radiators being no hotter than 50 - or even 60 - on the coldest days, you'd be in danger of ending up with your entire walls being covered by radiators, with no space for any furniture. Ok, maybe I exaggerate a little - but you'll get the point!

You might end up with UFH on the walls. That shouldn't be a problem in a well insulated house.

Thanks for the tip, I doubt any will be corroded as they are fairly new which is actually annoying as the guys that installed the system didn't do any heat loss calcs and kept the existing 11C differential..

Yes, that makes sense :)

I will have to look into a taller one.

The boiler is 28kw FYI so I think it should be adequate.

No-one's mentioned it yet, but insulation would achieve the same aim, but also with reduced bills. I'd look first at any insulation options.

There are other options too.

NT