11degC across Boiler, why?

[IMM]

They are not. The valve opens according to pressure.the higher the more it opens.

To save money that is fine. For anew system go Alpha and flow stitch and condensing boiler.

[IMM]

As long at the boiler is set to 80C this is highly unlikely when the system is warmed up.

You may require a minimum flow through the boiler. Then I would forget the Alpha a go for am open rad and room stat.

[Andy Hall]

In fact if you look at the data sheet you will see that for a given setting, they are roughly a constant pressure drop device over a range of flows.

.andy

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[Andy Hall]

You can set the nominal power level on the Alpha above or below nominal setting.

Even so, I agree that if the existing pump is OK, it makes more sense to try that first and only switch to an Alpha if there is a problem with noise in the radiators closed down case (i.e. reduce pump setting), but that that is not enough wit everything open.

.andy

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[IMM]

It is true that "generally" the lower the operating temperature the higher the efficiency. But regular boilers are designed to operate at maximum efficiency at higher temperatures. Any gains at running at lower temp is very marginal.

You could fit a blending valve on the flow return to ensure that only water higher than 55C is returned back to the boiler. Then you would need to set the boiler temp 11C higher to ensure the delta T. Exceeding the Delta T for long periods can harm the heat exchanger.

Sedbuk is being phased out.

These days not that much and in April they will be standard, except for the odd boiler here and there where pluming is a problem. Condensing boilers can be made not to plume.

Using a large quick recovery coil in a cylinder enhances condensing and in most newish homes DHW now used more gas than CH. When the new insulation regs bite in a few years CH will use far more than CH.

Standard oil boilers make more sense economically. They are much cheaper and less latent heat is produced by oil. As most people use n gas condensing makes much more sense as more heat is extracted and they are much cheaper.

When all boilers are condensers, prices will drop. You may have jumped the gun.

[IMM]

If all TRVs are close, no flow. It is best to get a flow switch rated at the minum boiler flow. below this flow the boiler burner is cut out. The pump can still pump through the boiler.

Even so, I agree that if the existing pump is OK, it makes more sense

[Andy Hall]

For example, the Crydom ones have a 2:1 or 3:1 switch on/off flow rate, and these are max and min figures.

One would have to put some valves around and in series with the switch to tune it to the correct behaviour.

I suppose a more accurate flow sensor that measures the flow coupled to some electronics sould be used, but this is going to cost a bit.

.andy

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[IMM]

That is the cheap way. Precise flow switchs, even adjustable, are available, but not cheap as they tend to be for commercial applications.

[Malcolm Reeves]

Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit independent temperature control in two separate zones: the living and sleeping areas. This can be achieved with a single room thermostat and thermostatic radiator valves."

Opinions on this differ. Mine is that I want the room control where I am, the lounge (being couch potato :-)). I don't care if the hall is at 17 or 18 or 19. The lounge is where I care about the temperature so that's what I want to control.

In any case, as you can see from above, the building regs now suggest the lounge for the stat.

We are probably talking about oil (me), gas (you). Yes condensing boilers have a bigger heat exchanger but "far" more efficient? For oil, a standard boiler on sedbuk rates at about 85% (capped). Condensing range is 90-95%. Saving is thus 5-10%.

In my case I want a wall mount oil fired and from sedbuk there is only one oil fired wall mounted (and it's not available anymore). That one is only 90% and a lot more than GBP200 more expensive! Sedbuk reckons running costs are 400 pa for my sort of house which I'd agree with. So 5% saving is 20 pa. Pay back is >10yrs

For a condensing boiler an alpha would be good, which is what I said. With an alpha the return temperature goes down. Bad news for a standard boiler but just what you want for a condensing.

What is difficult? As the TRVs cut in the flow resistance increases and the pump head rises. You just need to have the bypass set higher than the fully open condition. After that everything is automatic.

From someone who doesn't seem to know about part L or the performance figures it seems very patronising.

[Malcolm Reeves]

Perhaps we are talking about gas (you) and oil (me). Condensing boilers do have a larger heat exchanger. However, if you look at the sedbuk figures you'll see best standard is 85% (capped), condensing range 90-95%. So the efficiency difference is only 5-10% and with a lower return temp on a standard boiler, or a high return on a condensing, the difference is less again.

If the figures are available. The only database I know of with all the uk boiler. If you have a URL for another database please post it. Also, for oil, isn't it a problem that UK use kerosene and europe uses gas oil so europe figures will be different anyway.

A = Large Heat Exchanger + Large difference B = Large Heat Exchanger + Small difference C = Small Heat Exchanger + Large difference D = Small Heat Exchanger + Small difference

A is better than B. C is better than D. A is better than C (but boiler cost is more). B is better than D (but boiler cost is more).

Not clear if that will cover oil, but just in case I'm putting mine in now.

For oil that flies in the face of the sedbuk figures which suggest

5%-10% for floor mounted. 5% for wall mounted (which I need).

GBP20 is 5% of GBP400 my estimated fuel cost.

Oil smells. I suspect you are talking more about gas.

A condensing boiler with its lower flue gas temperature is much more likely to accumulate fumes at low level (that's why they plume after all). The advice on siting says to take account of the plume possibility. My flue will face my neighbour. Do I want to risk the possibility I might be dropping smelly fumes into his garden (and over his washing line). For, perhaps GBP20 pa when I won't break even on the boiler costs for >10yrs. NO.

We are talking about non-modulating boilers aren't we? I am since 99% of oil boilers are not modulating. I don't know how common modulation is on gas. So on a non modulating boiler how does the heat emission drop? Come to that since we were talking about short cycling then how does a modulating boiler short cycle since they would modulate before that and thus very unlikely to short cycle.

It's a valve with a spring. It will have some slope to its characteristic. See

which repeats what I said.

I would say the sunvic curve is soft. For my house I have calculated a head of about 1m (about 0.1 bar) and a flow of 0.8 m3/hr. So I'd set my valve at #3 (this assumes the pump matches this, in practice the head would be higher to get the flow as I can only restrict the flow). The valve would start to open at 0.15 bar (1.5m) and by 2 bar (2m) all my flow would be via the bypass. The head range is thus

1.5-2m. Not exactly digital.

[Andy Hall]

Predominantly I was.

The trouble is that the SEDBUK figures tend to obfuscate what is happening with the boiler.

The manufacturers should have them if they are selling their products outside the UK.

I'm not sure whether gas oil and kerosene would result in different efficiency figures - different calorific values possibly.

I was talking about gas.

It does anyway though, doesn't it?

Well again, gas ones don't have this issue or don't have to have this issue.

Nowadays very - on gas condensing boilers.

Right. I was referring to conventional gas boilers without modulation.

If you look at data sheets for devices like the Honeywell it shows it as a set of almost straight lines depending on setting. Once the valve opens it presents as a virtually constant pressure drop device.

.andy

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[Tony Bryer]

I would guess that what Andy is referring to the fact that the SEDBUK efficiencies are derived from formulae which can be found in the [free] SAP-2001 book Appendix D. For an on/off boiler this is

0.5(Efull+EPart)-2.5, for a modulating boiler 0.5(Efull+EPart)-2.0, less 4% if the boiler has a pilot light - other formulae apply to combis of various types. Efull and Epart are the efficiencies at full and 30% load.

Using 50/50 part and full load figures is reckoned to give a fairly accurate measure of likely efficiency under UK climatic conditions. It would be quite possible that some other EU country uses a different mix and if you were comparing two boilers with significantly different part and full load performance their relative assumed in-use efficiencies would be different to that shown by SEDBUK.

[Malcolm Reeves]

I'm not sure that they do. The oil difference is quite significant I think. In uk only kerosene can be low level flue so its all kerosene. Europe is all gas oil I think (and I guess they allow gas oil low level flues).

Lots of differences. The nozzle at least needs to change, perhaps more. Certainly a retest.

I'd have gas if I could :-). Anyway a good reason IMO to have a higher flue temperature so exhaust dissipates upwards.

Mine is a sunvic, perhaps they work better?

[Christian McArdle]

Unfortunately, relying on the boiler stat would not meet building regulations. You must have a flow switch, a room thermostat (in a room without a TRV) or other similar mechanism that entirely cuts the boiler when no heat is required.

Christian.

[Neil Jones]

*can* be achieved...

It can also be achieved by using zoning with 2 thermostats or (I can't believe I'm writing this) 2 boilers, one running the living areas, and one for the bedrooms.

Neil

[Malcolm Reeves]

True, but as I read it the post was saying you didn't have to do that, NOT, you didn't have to do that IF, you did something else.

It is mandatory to do some zone control - which for most TRVs are the easiest route. The original poster has TRVs AFAIR, I was just adding that he needed to keep them.

[IMM]

It is on about "logical" zones, not physical zones.

[IMM]

Where does it say that. A zone is an independently controlled, in time and temp, section of the heating system. A TVR does not converts a room into a zone.

[Malcolm Reeves]

Eh? I just quoted Building regs guide for part L above. That says you have to do it which makes it mandatory. They are only considering zones to be at different control temperatures not different on/off times. But of course you can go that far if you want.

[Andy Hall]

The Approved Documents to the Building Regulations are not of themselves legally binding. It says as much in the introduction of most.

They represent *a* way of complying with the statute but are not the

*only* way.

If you can propose an alternative to the suggestion, this can be just as valid, although it may be necessary to agree the point with the BCO.

.andy

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