Wasted heat - what can be done

[anonymous-user]

guindilias said:

(except if you do need an "overnight boil" for legionella,

no such thing, combi boilers have no storage of hot water its all instant.

No legonella risk.

[Sheepshanks]

Learn2MergeInTurn said:

guindilias said:

(except if you do need an "overnight boil" for legionella,

no such thing, combi boilers have no storage of hot water its all instant.

No legonella risk.

Several posters were talking about systems with stored hot water.

[anonymous-user]

Sheepshanks said:

Learn2MergeInTurn said:

guindilias said:

(except if you do need an "overnight boil" for legionella,

no such thing, combi boilers have no storage of hot water its all instant.

No legonella risk.

Several posters were talking about systems with stored hot water.

Even so a thermostat will only switch on the required temperature.

[Andehh]

zeewrath said:

so as I'm about to get a new heating system I may as well ask as it's a minefield.

I'll have ufh downstairs, rads upstairs and an unvented cylinder

for a 5mx4m bedroom 1st online calc says 2300BTU, second 3200BTU.

1200 600 single convector is 4000BTU so I thought that would suffice as the consensus seems to be overspec

I'm guess the rad BTU is for a large temp difference, how do you size them so that everything works efficiently?

Just be aware the larger the radiator, the longer it takes to heat up....but also the longer it keeps heating. So if you set the thermostat to 20 degrees in the morning, that radiator will gain a sort of thermal momentum, and when 'switched off' at 20 degrees, will keep on outputting until your room hits 21,22,23 degrees etc, it will then cool down, then take longer to heat up again, and again over shoot your desired room temp.... leading to a potentially large oscillating room temperature.

It isn't as easy as going ''ill add 20% to ensure I am never cold''. I would try and calculate it as accurately as possible.

edit: As for sizing them up, best you can do is use several ''online calculators'' that measure your rooms BTU needs, make any obvious corrections (front door opens into the lounge/have an open chimney, balcony doors, solid tiled floor etc) and then average them together. IMO anyway.

Edited by Andehh on Thursday 2nd February 08:45

[Sheepshanks]

Andehh said:

So if you set the thermostat to 20 degrees in the morning, that radiator will gain a sort of thermal momentum, and when 'switched off' at 20 degrees, will keep on outputting until your room hits 21,22,23 degrees etc, it will then cool down, then take longer to heat up again, and again over shoot your desired room temp.... leading to a potentially large oscillating room temperature.

In a normal domestic environment I can't for the life of me imagine overshooting by more than a degree. Obviously you could create a set of circumstances where it might happen, but it's not going to happen in normla use.

Anyway, even the most basic thermostats have accellerator heaters in them so the 'stat shuts off early.

[andy43]

zeewrath said:

so as I'm about to get a new heating system I may as well ask as it's a minefield.

The way a radiator is sized is using delta T - the difference between flow and return temps. Most still assume 70 degree flow.
You could argue that a huge rad at 45 degrees flow will have the same heating effect and/or overshoot as a smaller rad at 70 degrees - it's getting the sizes exactly right that's the tricky bit - as above, make them too big and it's not as efficient or as responsive as it should be, too small for your flow temp and it's too cold.
If you can choose a rad by kw per hour vs flow temp you're sorted.
If you're trying to combine rads, ufh and a complete low temp boiler system where it just ticks over to stay within it's condensing range you need conversion tables to work out how much to oversize the rads by.
Myson have a calculator that allows you to tell it what your flow temp is - http://www.myson.co.uk/useful_services/heat_loss_m... - so it will size the rads for you using any flow temp. No idea how accurate it is

[guindilias]

And that's why people employ energy consultants!
We use Hevacomp/CAD at work, it's a program that takes into account solar gain/loss from windows, type of blocks the house is built from, absolutely everything - but it costs an arm and a leg and takes a 2-day course to be certified in it, plus regular "update" courses when they add even more features.
It basically imports a 3d model of your house from a CAD program, then you specify materials, location, orientation etc. and it produces a data sheet for each room.
There might be something free or cheap out there that does roughly the same thing, but if I wanted my house to be "just right" on a new system installation, I'd be going the expensive way!

[guindilias]

Learn2MergeInTurn said:

Even so a thermostat will only switch on the required temperature.

It's common on a system at risk for legionella to have a timer on the immersion heater, so it clicks on for an hour or so at midnight to kill anything off. Rarely seen in domestic houses though, as I said.

[Sheepshanks]

guindilias said:

There might be something free or cheap out there that does roughly the same thing, but if I wanted my house to be "just right" on a new system installation, I'd be going the expensive way!

Designing for a UK house has surely got to be a massive compromise though?

It's +10C here today, it could easily be 0C or even -10C another time. One system can't be optimised for all of those temeperatures.

[guindilias]

That's why it wants your location - there is a fairly complicated calculation, and a huge table of typical min/max temps per year, which it carries out to ensure you have maximum heat output needed for the typical "coldest day". Then you have "Degree Days", just to complicate things further...
http://www.degreedays.net/introduction