Central Heating controlls

[Andy Wade]

Yes:

guess the threat to short-range devices (SRDs) will be mainly from the

4G/LTE base transmissions, rather than the closer (in frequency) but lower power, mobiles. The frequency spacing between the top of the LTE base transmit band and the lower edge of the SRD allocation is quite wide - 42 MHz - so it ought to be possible to design new SRD kit with sufficient immunity. Existing products might not have adequate filtering and dynamic range though... That said, the LTE base transmissions will be quite powerful - up to around 4 kW EIRP if all three frequency blocks are in use - so it's by no means a trivial EMC problem.

UHF radio mics are moving from ch 69 to ch 38, are they not? And 433 MHz SRDs haven't been free from receiver immunity problems either (from TETRA and 70 cm amateur radio).

[tony sayer]

So thats the base transmit power is it?..

[Andy Wade]

Yes, it can definitely be in the kilowatt range. That's EIRP of course, not TX output power.

gives the "base station in-block emission limit" as +61 dBm (~1.26 kW), measured in 5 MHz bandwidth. In the full available 30 MHz that's around

7.5 kW, worst case. In practice max. EIRPs will apparently be 2-3 below that, i.e. 4-point-something KW.

So on-beam, a field strength of ~35 V/m at 10 m range or 3.5 V/m at 100 metres...

Mobile power is only 200 mW per 5 MHz, so isn't such a worry.

[Doctor Drivel]

High flow combis are available. Things have moved on.

[Doctor Drivel]

Powerful? A domestic gas meter can cope with approx 62kW. so a 35kW (approx

15 litres/min) is only using half the available capacity.

[Doctor Drivel]

Boiler make? model? 3-way zone valve. Two zone valves. Room stat.

[Doctor Drivel]

Maxie, you are so wise. Putting a Chav firmly in his place. You are a fantastic man indeed.

[chris French]

From memory, (we had something like a 28kw combi in the old house, maybe

24KW) that sounds about right.

Sure it was sufficient for sinks and showers (good shower in fact, we had plenty of mains pressure), but as Blair says takes a ges to fill a bath

[Doctor Drivel]

It is small for combi, which is rated on DHW performance.

28kW does about 11-12 litres/min, depending on model.

[Doctor Drivel]

35kW is becoming the norm. You can get a 50kW plus and the flow rate belts out.

[charles]

That is classed as "industrial" not domestic.

[Doctor Drivel]

Sounds like a BoilerMate. They suffer from old-wives tales. Jobbing plumbers do not understand them - they are good at toilet changing - and call them sludge buckets. It is best to have the CH run off a coil in the store then sludge occurs in the store.

Make sure the store is topped up every 3.5 years with 3 to 4 cans of inhibitor - essential in a direct store.

As you noticed when the mid section gets below 60C the boiler cuts in dumping "ALL" the boilers heat at the top of the store. So if a shower is taken, a beefy boiler can be reheating faster than the draw-off. Their selling point was that a smaller boiler can be fitted to save costs. But now larger boilers are not expensive and many one size fits all are around as they modulate.

As long as the flow and return pipes are the right size, you cannot fit a large boiler onto a directly heated thermal store - it dumps all its heat into the store. A very large boiler can reheat in few minutes and no adverse effects. An over-large boiler on a direct rad system will result in boiler cycling and cause premature failure of controls and maybe be the heat exchanger.

[John Rumm]

Having lived with a 35kW combi in the past, I would count that as about a realistic minimum if you need bath filling. Great showers, but still fairly feeble for volume delivery in the winter.

It will be better but not spectacular. You are also getting to the stage where you are going to run out of flow capacity on a standard domestic meter and governor if it has to share the supply with much else.

[John Rumm]

Not sure what your point is...

Lots of installers still slap in 24kW combis for moderate sized households, and their DHW performance is fairly feeble, and hence it does the reputation of the breed as a whole a disservice.

I would class 35kW and up as a moderately powerful combi, in the sense that is about the point where it becomes tolerable to live with in a house that uses the bath reasonably often. Many makers don't do anything much above 40kW for the domestic market.

With the fairly decent cold main feed we have in this place, even a 60kW combi would be marginal.

If you were selecting a PHE for a heatbank fed from a decent mains supply, you would probably stick a 100kW unit in there to make sure you always got adequate performance from it.

[Doctor Drivel]

It will approach what the cold mains can deliver.

62kW can go down the pipe past the meter.

[Doctor Drivel]

But many do.

That will deliver about 27-28 litres/min.

The idea is to ensure a cool return from the plate so it is over-sized and to ensure it works well if scaled up inside.

[gremlin_95]

Won't having a combi with such a high output use a huge amount of gas?! We have a 37kw storage combi which is perfectly adequate but given the choice, we would have gone for a heat only boiler + unvented cylinder. We went for a combi simply because we haven't the room for an unvented cylinder.

[Andy Champ]

It shouldn't use any more than a smaller one.

If you need to heat 100l for a bath it's not going to take any different amount of gas whether you do it in 2 minutes or 4. Unless of course the bath is cold by the time you've filled it!

Andy