Radiator Sizes

Hi

I am currently looking into replacing my radiators as part of a central heating upgrade, we currently have single panel and single convector of assorted sizes around the house, the style that has been chosen only comes in single convector and above. I have work out that these can be accommodated depth wise, and would like to keep close to the existing width as they sit neatly under the window sill.

Q. What are the drawbacks if I oversize a radiator (fitted with TRV) based on room calcs?

TIA

Reply to
Mark
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None particularly. If you are going to do this, it would be a good idea to take a look at the manufacturer derating tables.

If you are using a conventional boiler at 82/70 degrees, the usual figure is to take the actual heat output as 90% of the data sheet value (which is based on 90 degrees).

If you are going to go for a condensing boiler, then you can gain some efficiency advantage while you are doing this exercise by running the system at 70/50 degrees. To do this, you would use 60% as the factor on the radiators.

In other words, let's say your calculations show a heat requirement in the room of 600W, you would use a radiator of 1000W nominal data sheet rating.

I did a similar exercise to this a couple of years ago and it's very effective. In fact, the boiler flow temperature seldom needs to go above about 55-60 degrees.

Reply to
Andy Hall

Same here. Actually, I can run my heating with a flow temperature of 45C once the house is heated up (which means there was a fault in the program I used to do the calculations, Myson's Java calculator when in beta, and it did warn about this I must admit).

However, on reflection this is great. I can heat the house up from stone cold in about 1/4 hour by setting the boiler up high, and run the boiler (Keston C25) in super-efficent mode at at 45C flow temp at all other times.

Reply to
Andrew Gabriel

As a matter of interest, how do you heat your DHW to an acceptable temperature?

Reply to
Roger Mills

I'm not sure what the Keston does, but on mine the boiler receives room temperature and cylinder temperature information. When a HW reheat is required, it opens the valve for the cylinder coil, closes those for the CH and winds up to full power. Flow temperature is up to 85 degrees in that mode. The HW temperature setting is also on the boiler controller and it stops when that is satisfied.

After that, it returns to CH mode and responds to room temperature from room sensor, weather compensator and max temperature setting for CH flow.

Reply to
Andy Hall

Thanks. Sounds like a fast recovery cylinder is fairly necessary.

Does this method of control have a 'plan' name? It pretty certainly isn't standard S-Plan because that would allow HW and CH to run concurrently.

Reply to
Roger Mills

I have that, but the reheat cycles are fairly fast anyway. The temperature detection on the cylinder is done with an analogue sensor connected directly to the boiler controller. Therefore the boiler knows the actual temperature as opposed to being told when to fire as would be the case in a typical thermostat/programmer setup. If the temperature is gradually drifting downwards because of the use of small amounts of water, then nothing happens until it reaches a configurable number of degrees below the DHW set point. If the temperature drops more rapidly because the shower is turned on, for example, then the boiler fires much earlier.

The boiler controller does all the work. It has analogue temperature inputs from the cylinder and outside temperature sensor.

The room controller signals to the boiler using PWM (effectively providing the boiler with analogue room temperature information as well). It has the program time settings, desired setback and so on. However, the boiler is doing the calculation of when to fire up for optimised start etc.

There are also internal sensors for flow and return temperatures in the boiler.

The controller is able to control the power level of the boiler, the pump speed and to open motorised valves.

There are a whole variety of different soft programmable operating modes that can be set. For example, I could have opted to have conventional external controls running the boiler with a switched live and using an external programmer opening the valves in Y or S Plan. I could also have the boiler operate one diverter valve and do Y plan.

It would also be possible to configure for S Plan with both valves open, but that would create the issues with temperature that you were alluding to because either the radiator temperatures would have to be run all the time at 80-odd degrees or during water heating or the cylinder would not be able to be heated to required temperature. It would also have been possible to force the boiler to behave more like a conventional one and always run at 80 degrees or off.

In essence what I have is most closely described as S-plan plus (since there are several heating zones) with HW priority.

Reply to
Andy Hall

Presumably you've got a boiler with two call for heat inputs and two stats? The Keston Celsius only has one.

Reply to
Tony Bryer

It's really the other way round, Tony.

The boiler has sensor inputs for the cylinder and outside temperatures and a digital PWM input for the (special Siemens) room controller.

This means that it figures out what needs to be done and opens valves and runs burner and pump as required.

I suppose that to achieve something equivalent on a Celsius, one would need to modify the control electronics in order to be able to set two different flow temperatures.

Even better would be if manufacturers had two switched live inputs and each had a separate control setting inside the boiler

Better still would be if manufacturers would adopt a standard interface between controls and boilers etc. Opentherm seemed to be an attempt at that but I'm not sure where that has gone.

Reply to
Andy Hall

A separate Multipoint Gas water heater. It was almost brand new (having been replaced in an emergency -- see FAQ Humour section) when I installed the central heating. Also, there was a lot less choice of good quality condensing combi's back then than there are now. So I kept the hot water heating quite separate.

Keston C25 doesn't come with any such facility. However, I have built a replacement facia PCB which allows remote control of the temperature setting and remote monitoring, and I could easily do it using that. (Actually, other than some initial prototype testing, I haven't had time to write the software and deploy this yet.) Someone posted a much simpler mod to the facia board just to switch output to max temp by addition of a relay and resistor IIRC. Obviously, such mods are not going to be within the capabilities of most people, and if you need such a feature, you would be better advised to get a boiler which includes it as standard.

Reply to
Andrew Gabriel

In article , andrew@a17.? writes

Yep, the Celsius 25 isn't that clever in this respect, I found out the hard way after installing mine. The mod you mentioned was mine, the post ref is below but I've quoted the content too as I'm not sure how google will handle the ascii art and most newsreaders will offer a fixed pitch font option which is what's needed to make sense of it.

As Andy says this mod isn't for everyone and it's a pity Keston didn't think of doing it for themselves, their larger 40 has it built in.

formatting link
Ed Sirett and anyone else who's interested, here's the mod I made to the control panel of my Keston Celsius 25 system boiler to create full heat demand for hot water operation.

The circuit is not the simplest one possible but for reliability it avoids having a relay in the high impedance signal path to the boiler demand level. The 4k7 resistor means that there is always a small current flow on the relay contacts which is something I prefer. The worst that can happen if the relay contacts fail is that the mod ceases to function rather than the whole c/h packing up.

The relay is mains powered and must have gold plated contacts (which is rare on mains relays), I used RS model 394-9116.

The mod is straightforward to implement but requires some soldering skill. The 10k resistor needs a cut in the boiler demand level signal which I made by cutting the control pot wiper lead (middle connection) and putting the 10k res in its place. The diode & the 4k7 resistor can then be soldered to the pot terminals on the back of the board and the relay connections wired up.

Full demand for the Celsius 25 is when the demand control level is at

0V; I have not found that the diode drop affects the operation significantly.

Control board mod:

+5V --- | ctrl / 10k modified panel \ demand pot / | level (100k) \ +5V | _|_ --- | 0V | | \4k7 | h/w / | demand >----- \ 1N4001 | 240V~ | |--|---------> boiler 240V RTN demand demand

Against my better judgement, here's a version that doesn't involve a mod to the control board but places a cut in the loom instead. Lots simpler, just a relay and wiring, but with the relay in the signal path so beware of dirty contacts affecting operation in the years to come. Again, relay contacts must be gold plated.

Loom based mod:

Control Board Loom

+5V --- | ctrl / RL1a modified panel \ demand pot / \o----| level (100k) \ o |---------------(#---------|-------------> _|_ 0V

h/w demand >----- 240V~ | --- - | | | o RL1 | | RL1b / coil | | o - | | | --- c/h >---------> boiler 240V RTN demand demand

I both cases, be careful to separate the wiring on the two relay contacts as one is low voltage and one is mains, neat soldering is required and best to sleeve the joints or gloop with hot melt glue.

I posted some pics at the same time, let me know if anyone needs these again.

HTH

Reply to
fred

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