radiator size calculations.......my first post - be gentle!

[TreeFellaPete]

Morning All,

Tearing my hair out here doing rad calcs and all the online methods come up different. Surely someone's gonna get a prize for being the millionth person to ask this soon. Anyway, I've got all super nerdy and used the Kermi calculator and I've measured all the windows, correctly entered the walls types and thicknesses, same for the floors and ceilings, adjusted each room temperature according to the wishes (orders) of my missus. With this info the software accounts for heatloss/gain from adjoining rooms, and I've allowed for an outside design temp of -3 instead of -1. And I've added 10 percent for fast heat up.

So my question is - Because of the high level of detail involved is it more reliable than the simple calcs and "rule of thumb" methods? The temptation is to go large on the rads firstly cos if I mess this up the aforementioned missus will make me suffer and secondly cos there are some great package deals out there which are better value if you pick big rads.

I'll be fitting TRVs to all rads and using a room stat so does that mean it doesn't matter and get the biggest rad that looks right for the room as the TRV will manage it?

Loads of people say that big rads mean the return temp to boiler is cooler and the boiler is then more efficient, but surely modern combis with electronic wizardry inside can deal with higher return temps!

For info - there is no chs in the house, just a gas heater for the taps. And I will be using a RGI for the boiler install - I'm not even going to take it out of the box! it's a 1939 house, solid walls, semi detached, with cold roof space, bare timber floors on joists

I'd really really appreciate some wise words on this asap -I need to get a crack on with the chs or I'll be in the doghouse (which is also unheated!).

Cheers in advance,
Pete

 

[D_Hailsham]

the software accounts for heatloss/gain from adjoining rooms, and I've allowed for an outside design temp of -3 instead of -1. And I've added 10 percent for fast heat up.

So far, you have done everything "by the book", but practice and theory are not always the same thing. For example: different temperatures in the rooms implies that the doors are kept closed.

So my question is - Because of the high level of detail involved is it more reliable than the simple calcs and "rule of thumb" methods? The temptation is to go large on the rads

A detailed calculation should be better as it takes into account the actual construction, provided you get the info correct and not guessing. It might be worth checking against an online calculator. However, mos of them are not very "scientific". This Radiator Calculator is better than most.

If you decide to oversize, do it by a constant factor, e.g 20%, for each radiator.

Loads of people say that big rads mean the return temp to boiler is cooler and the boiler is then more efficient, but surely modern combis with electronic wizardry inside can deal with higher return temps!

Unfortunately electronics can't overcome the laws of physics: a condensing boiler only condenses if the return temperature is below 55C.

Boilers are therefore designed to work with a flow temp of 75C and return of 55C - a 20C difference.

Now radiator outputs are measured using a 10C difference, so when they are installed in a system with a 20C difference the output of the rad will be lower - about 85% of the quoted figure. You therefore need to oversize the rad by about 20% to get the required heat output.

 

[TreeFellaPete]

Great advice! It's not just about "knowing stuff"- it's about conveying it to someone else in a way that can be understood - which you've done. Thanks! Funnily enough, me and Her have discussed doors open/closed etc and that's the sort of variable that I guess we'll have to live with (preferably with she and me on either side of a closed door!)

Totally agree that guesstimation makes the scientific method meaningless so I have been really careful with all measurements inc wall depth and type, window sizes inc type of glazing etc and even researched the u-values of the ground floor- timber floor on joist above void with flippin cold concrete slab below.

I started with the online ones and it was the variation that made me seek something maybe more precise. Not sure but perhaps the online ones especially for the large diy store chains go for big oversizing so that customers get big hot rads that look like they can do the job and everyones happy. this kermi one has highlighted that my hallway needs more heat than the online calcs suggest due to two external uninsulated walls and it suggests lower than the others for a room with 3 internal walls to warm adjacent rooms and one external wall - half of which is double glazed which I found out is better insulation than the solid wall!

So I'll take your advice and oversize by 20% instead of 10% and I'll change all the room temps to a "missus pleasing" 21 deg to simulate the balancing effect of having all doors open and see how that changes the suggested rad requirements.

Then I guess it'll be a case of see-how-it-goes and some fine-tuning of the rad flows etc. beats cutting down trees.

THANKS AGAIN

 

[D_Hailsham]

So I'll take your advice and oversize by 20% instead of 10%

It's not either 10% or 20%; it's both.

The 10% is there to give faster warm up, useful if the house is unoccupied most of the day. If, like me, you are retired and the heating is on all day from 0630 to 2300 there's no real need for the 10%.

But if you need the 10% warm up, so a 1kW requirement becomes a 1.1kW rad, the 20% will add another 0.22kW, so you would install a 1.32kW rad (or nearest size).

 

[TreeFellaPete]

right, makes complete sense - and fortunately I didn't place an order for the rads yesterday so I could double triple check the calcs again today, glad I checked in here again today.

Yeah me and the Boss are out at work during the week so we'd need the heat up. Just out of interest, it's generally accepted that you get the heating to switch on either just before you get home or when you get in (if you don't mind waiting for the warm, but...in terms of energy use..is that more efficient than keeping the heating on all day at a "holding" temperature of, for example, 10 degrees? Or even 5 degrees? It's turn my brain into scrambled eggs trying to compute that but what's your view?

I'm sure I remember being told at school that when you are boiling water the energy input required to raise the temperature is less as the temperature gets higher. e.g. 20 degrees to 30 degrees needs more than 40 - 50.

perhaps this is only relevant in a modern super insulated house not the old solid brick walled house I'm in.

Thanks again - it's great that you're still giving out advice even though you're retired. Also confirms my growing suspicion that this subject matter is addictive!