11degC across Boiler, why?

[Tom]

Most manufacturers seem to recommend setting flow rates in CH systems to achieve a rise of 11deg across boiler, why is this? is it the most efficient setting for heat transfer? Tom

[Ian Stirling]

It's a compromise. For condensing boilers, you want the exhaust output temperature to be as low as possible, otherwise the efficiency will drop as it won't condense properly.

On the other hand, you've got to get temperature high enough to heat radiators and central heating. So, the smaller the temperature rise, the higher the efficiency. Even for non-condensing boilers, for non-pressurised systems, the boiler can't get past some 90C or so, otherwise you get localised boiling, which does all sorts of nasty things.

Both of these mean that you want the temperature drop across the boiler to be relatively small.

[Andy Hall]

This is the case for conventional, non-condensing boilers and represents the optimum for the heat exchanger. In fact the general design rule is to have 82 degree flow and 70 degree return. This also avoids the risk of the boiler entering condensing mode, which for a conventional boiler would result in corrosion.

For condensing boilers the heat exchanger can work with a temperature drop of 20 or even 25 degrees and is optimal at as low a temperature as possible -i.e. efficiiency is maximised at lower temperatures. .andy

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[Tom]

Thanks for that, The background to my question is that I'm contemplating redesigning my CH system in a bungalow I moved into some 6 Months ago; since

4 of the 6 rooms are subjected to high solar gain at different times of the day I'm attracted towards using TRVs on the rads in the 4 rooms which make up approx 80% of the total heat load. I'm further minded to install a Grundfos Alpha pump, however, I'm undecided as to the control mechanism i.e. a flow switch as advocated by IMM in a previous thread or to rely on the boiler stat set to 82degC as you stated. I'm anxious to avoid short cycling of the boiler. I'd be gratefull for your counsel. Regards Tom .

[IMM]

The prime aim in regular boilers is to avoid condensation in the burner box. Regular boilers should always be set as high as possible, and "never" below

65C flow temp. Some regular boilers only had a high limit stat and the flow temp pre-set.

[IMM]

It is the most efficient for the heat exchanger design which will be a low priced one. Having the heat exchanger within a tight temp range is cheaper and quicker to produce. That is why the expensive condensing boilers have

25C across the heat exchanger as the it is made to a higher spec and with superior manufacture and materials. They can stretch the temp range across it.

[Andy Hall]

With an Alpha pump together with TRVs the water flow aspect will be taken care of.

You would need anyway to put a room thermostat in a room or space where the radiator has no TRV (or you could fit one and turn it fully on). If this is to be a living room, then it mustn't have another source of heat like a gas fire.

The aim is tha tthe TRVs in the other rooms should begin to close and reduce the heat load. Remember that these are quite analogue in nature - they don't just suddenly cut off. The room thermostat is then set just above this point, the idea being that the other rooms come up to temperature slightly before the heat source is cut off. You can tweak the flow to the radiator in the room where the thermostat is, to adjust all of this as well.

As the load requirement falls, the boiler will cycle or cycle more, inevitably. The trick is to make sure that this does not go on for too long.

It does take some experimentation, so one good way if you are not sure of which room to use for the thermostat is to use an RF one and move it around.

The flow switch idea does have its uses in that it could be used to shut down the boiler and pump when the flow reduces. However, the inexpensive ones with a magnet and reed switch are not very accurate. You can adjust their effect a bit by putting a bypass around the valve, but really they are intended for sensing little flow or no flow. For example I have one for my secondary heating circuit for the workshop. The pump and controls are in the workshop and I didn't want any electrical signalling connections to the house. The workshop circuit is heated via a heat exchanger from the main system and has a zone valve to allow water from the main system to the primary side of the exchanger. There is a flow switch in the secondary side in the house which detects when the pump is running (at all) and opens the valve. I'm not trying to sense low flow with it.

So this could be useful, but I would try the thermostat idea first.

.andy

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[Tom]

Much obliged for your detailed reply,

I've been mulling over this flow switch idea and had mused on using it to shut down the boiler and not the Alpha in the hope that when a TRV next opened, the Alpha would increase it's flow against the reduced resistance, the flow switch consequently switching the boiler back on. I assume the Alpha uses little power when throttled down so to speak. I realise that this would require careful balancing of the system perhaps incorporating a permanently connected pressure gauge on the pump outlet and manual throttling valve(s) to facilitate balancing. What do you think? Interesting use of your flow switch to separate the electrics from the shed, I like it. Thanks Tom

[Set Square]

As others have stated, many systems are designed to run with a flow temperature of 82 degC and a return temperature of 71 degC.

But why these particular values, I hear you cry? Well, it just so happens that these correspond to 180 and 160 degF - with a 20 degree drop - all nice round numbers!

So the whole thing dates from the time when temperatures were measured in Fahrenheit, and energy in good old Britsh Thermal Units. [Wait for the reaction to that one!] The values have simply been converted to Celsius.

[Malcolm Reeves]

You should have TRV on at least all the bedroom rads which is the new rules AFAIR. Ideally all rooms should have TRVs except one (the lounge?) and that room should have the thermostat that turns the system on/off.

The optimum system is where the water comes back to the boiler at just above the condensing level for non-condensing boilers or below for condensing. If condensing boiler return temperature is above then it doesn't condense and so the extra money you paid for it is wasted as it's no more efficient than a standard boiler. If the temperature is too low on a non-condensing boiler then you get condensing which corrodes the heat exchanger. I have been told that 56C is the condensing threshold.

The normal flow and return is 80C out, 70C back (in round numbers,

82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above room temperature). So rads will be at 75C, about 55C above room temperature. If you run which a larger drop, say 80C out, 60C back, then the rads are at 70C. That 5C difference is significant (see rad makers for curves). The rad could be 10% down on output. Hence for a condensing boiler system you need larger radiators.

I wouldn't go with an alpha pump with a standard boiler. Think about it. An alpha pump slows down the flow so as the TRVs cut in the flow back to the boiler is reduced. Water spends more time in the rads so comes back colder. The risk is you get condensing. What I would go for is a standard pump + an automatic bypass (which is cheaper anyway). Then as the TRVs cut in the hot water gets fed back with the cold from the rads and the return temperature flow goes up - no risk of condensing.

Of course as the return flow temperature rises the efficiency goes down but the trade off is that you can set the normal flow for a larger drop (assuming you have the rad area to cope) and know that the return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is constant so temperature rise across the boiler is constant. It's the return temperature that rises. Exceed the limit before the thermostat cuts in and the boiler short cycles. For an alpha the flow drops so the rise across the boiler increases. Again exceed the limit and the boiler short cycles. It's all down to the boiler outputting X kW and the house only needing

[Tom]

Fine, Ok to that, existing thermostat in hallway, a non-TRV area. Lounge has additional heat source(SWMBO requirement) + high Solar Gain ,hence TRV.

Agreed, I remember in the dim and distant past a HVAC consultant quoting that figure at me in work.

Agreed, no problem with that, makes sense.

Yessss, I hadn't thought about that, getting too old in the tooth I think!!!

OK.--The automatic bypass, do these work in an analogue fashion, do they have differing pressure/resistance curves or are they on/off devices.

Implying analogue characteristicts?

Agreed again.

Unfortunately I don't have a modulating or condensing boiler.

Much obliged to you for your detailed critique, I will be looking at bypass systems I think. I inherited the boiler (Gloworm Hideaway 60B FF) with the bungalow I moved into 6 Months ago, I shall use the existing pump which will save me a bit.

Best Regards Tom

[Andy Hall]

That isn't true. Non-condensing boilers are designed to operate most efficiently at 82/70 flow return.

That isn't quite true either.

A condensing boiler's efficiency doesn't suddenly improve as if a Holy Grail has been found at the dew point.

WHat happens is that the efficiency increases with decrease in temperature both above and below the dew point. When condensing commences at the dew point, all that happens is that the *rate* of increase of efficiency increases with falling temperature.

At a flow of 82 degrees, a condensing boiler will be more efficient than a conventional one anyway because the heat exchanger is larger.

That is also untrue.

You can replace a conventional boiler with a condensing one. When required, in the depths of winter the condensing boiler can run up to the 82 degree level of the conventional one and still be more efficient. For most of the heating season, the condensing boiler will modulate down to a lower output and into an even more efficient working range.

It is true that system efficency can be improved by using larger radiators, but it isn't essential.

In terms of derating, the manufacturers tables are too high for typical UK boilers.

The normal derating factors are to multiply by 0.89 for an 82/70 boiler and by 0.6 for a 70/50 design with a condensing boiler.

This is precisely where an Alpha pump is intended to be used. An automatic bypass is intended to deal with the case where both CH and DHW demand has ceased altogether and the boiler has been firing at full tilt. It simply prevents the water from boiling and nothing more.

The TRVs do not stop the flow completely if adjusted correctly in relation to the radiator in the non-TRV room. The object is to turn off the boiler via the room thermostat as all rooms have arrived at set point, not for the TRVs to have closed completely. Provided that the boiler thermostat is adjusted correctly, operating temperatures will also be correctly in range.

.andy

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[Andy Hall]

I doubt if you need to make it that complicated. As long as the boiler output is not hugely larger than the house requirement, simply using a room thermostat, TRVs and an Alpha should give a good result.

If you get the radiator without the TRV adjusted to the correct operating point such that the TRVs are not completely closing as the room thermostat operates, you will get the desired effect.

Another approach would be to get one of the optimising room thermostats which controls the boiler and prevents temperature overshoot - e.g. CM67 or TP75. This will give a more controlled result than a simple thermostat.

.andy

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[Andy Hall]

You might find that you get smoother results if you put it on the upstairs landing. It will be less suscepitble to the effects of opening the front door.

It's around 52-54 degrees.

See separate note on derating of radiator specs.

They are pretty much on/off and are intended to deal with the case where DHW and CH both stop demanding and the boiler is in full burn. The setting should be arranged so that the valve is closed in all cases exept where this happens. The idea is *not* to allow any feedback of flow water to the boiler except under these conditions when the only purpose is to keep a flow going and to prevent boiling.

You can do that provided that the pump is set to compromise with the requirements of full heat demand and at the point when the TRVs are closing. You also need to arrange that the room thermostat is turning off the boiler when there is still some flow left through the TRVs.

.andy

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[Tom]

It's a Bungalow, and because of the layout of the dwelling we hardly ever use the front door in practice.

Since my query I found the specs on the honeywell DU145 and whereas they claim that it is designed to operate as a boiler bypass *or* where radiator thermostats are fitted, the curves they publish are very flat indicating that they open over a very small pressure change, in the absence of detailed info on the actual pressure variations which occur in a typical TRV domestic CH system it's difficult for me to make a reasoned judgment. Are you aware of any other Automatic Bypass Valves which have steeper curves.

Thanks Andy, would you agree that the Alpha might induce condensing conditions in my standard boiler, assuming that thermostats and TRV are susceptible to frequent adjustments in my household :-(

Cheers Tom

[Malcolm Reeves]

Not according to HRM technical department. It also stands to reason that all other things being equal the lower the boiler average water temperature the more energy you get from the fuel, i.e. the cooler the flue gas. SEDBUK cap standard boiler efficiencies specifically to stop boiler makers designing standard boilers on the edge of condensing and thus improving their efficiency at the risk of shorter life (due to condensing causing corrosion). Hence the sedbuk figures need to be viewed in the light of being capped when comparing condensing and non-condensing boilers :-/.

So what you are saying is that a larger difference than 82/70 does improve efficiency. Like I said. The specific latent heat extracted at the dew point is a lump of energy, either you have it or you don't. If you are running above the dew point you don't have it and the boiler is working in standard mode not condensing.

True a larger heat exchanger will get more heat out of the fuel and thus be more efficient. However, condensing boilers are $$$ more money and if the only benefit is the larger heat exchanger (i.e. not condensing) would the savings be worth it?

I'm just changing my heating to oil fire standard boiler not a condensing since on consideration of the extra boiler cost, problems (smells, plume etc.) vs the potential savings (perhaps GBP20 pa) I have chosen a standard oil boiler. I'm fitting this now in case they change the rules in April 2005.

How does a non modulating boiler modulate? The water comes out of the boiler at 80C goes through the radiators which cools it by 10C (if they are exactly right for the room loss) and returns to the boiler at

70C. If you want it to return cooler the radiators have to be bigger since the heat they give up to the room is dependent on their size and the (fixed) room temperature. You could run with a lower flow temperature but then you'd have less drop across the boiler which as we have already agreed is less efficient. In the case of a condensing boiler the condensing gain would probably exceed the standard heat exchange section drop in efficiency but it still won't be as good as if it was running at the design figures.

No. An automatic bypass says it is for TRVs. In any case what will a bypass do in the situation you have above. If you are trying to get heat out of boiler after the fuel has cut off you need a pump over run. That is something that runs the pump for a few minutes after the thermostats have cut off and the boiler stopped firing. Usually a thermostat on the boiler I believe rather than a timer but the effect is the same. Some boilers don't need it as the cut off quick enough that the water can absorb the extra heat.

A three way valve must be open to at least one way and you must have a path (non TRV rads or tank). What stops the flow is the pump not running. If you have flow but not enough then that's where an automatic bypass comes in. Pump head (pressure) rises and the valve opens.

If the TRVs are even partially closed then the flow is reduced. An alpha will keep the head constant-ish resulting in less flow and thus lower return temperature. A standard + bypass will result in constant head (the setting the bypass is set to), and a constant-ish flow

[Andy Hall]

OK, so I would suggest positioning the thermostat away from areas where there can be sudden increases or decreases of heat such as the kitchen or if you have supplementary heating, the living room.

I am pretty sure that all are fairly similar. Danfoss is another make.

I think that it's unlikely provided that you set the boiler thermostat high enough.

I would take it one step at a time.

- Give the system a good clean out as you fit the TRVs. This is a good opportunity to take the radiators outside and give them a good hose through. Don't forget to note the lockshield valve positions.

Afterwards a flush with a system cleanser is a good plan - run hot for the recommended period of time.

Flush, Refill and add inhibitor.

- Balance the non TRV radiator so that the TRVs are starting to close as the room thermostat cuts off the boiler. You will know the rough point because if the boiler is cut off too early, the other rooms will be too cool. If much later, the radiators with TRVs will start to cool and the boiler will run for shorter periods.

I would leave the pump unless you find that the system becomes noisy as the TRVs are starting to close. You will know if the existing pump is a problem if you find that it gets noisy and you have to turn it down but that on that setting when all TRVs are open, the boiler runs for short periods.

The other thinkg is that you won't really be able to do this until the weather turns cooler.

.andy

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[Malcolm Reeves]

Does it matter? The combined system has a pump driving the head and the flow. Even if the bypass valve has a very sharp response the pump doesn't. The sharp or soft response of a bypass combines with the pump curve. Or, if you like, a sharp response bypass will keep the maximum head at say 3m (if is it set to 3m), whereas a soft response one will keep the maximum head at say 2.5m to 3.5m depending on how much it has to open. In either case the bypass will open enough to let the flow stay constant-ish.

BTW a bypass is just a spring loaded valve. A knob on the top (or similar) allows you to set the maximum head. When that is reached the valve opens, just enough so that the head stays at that level. The increase in flow then allows the pump head to decrease and the system settles to a stable value.

[Andy Hall]

The operating range possible with a conventional boiler is so narrow anyway that this makes very little difference. The heat exchangers used are relatively small.

The SEDBUK figures really confuse the issue because of the factors used. For the purposes of looking at boiler behaviour it is much more useful to use the measuring methods used in the rest of Europe.

These specify certain temperatures and firing rates.

I don't think you did, but the point is that a larger temperature difference together with a larger heat exchanger certainly does help.

You are missing the point. While it is true that latent heat is extracted below the dew point and results in a greater efficiency, this is not a step function as people seem to believe -it is an increase in the rate of change of efficiency with falling temperature.. You also have to account for the burn rate of the boiler and the heat delivery to the load.

The answer is that both aspects are useful, as is the control system running the boiler. Generally these are more sophisticated than with conventional boilers and are able to match the heat generated with the load and to minimise the temperature of operation. All of these factors improve efficiency.

The difference in cost is becoming less and less, and from next year the whole issue will be academic since changes to the Building Regulations will mandate condensing boilers anyway.

In terms of cost saving, I have found it to be in the predicted band of 25-30%

You must be spending very little on energy if £20 represents 25% of your energy cost.

There are no real problems with modern condensing models in terms of pluming. Much improvement has been made over early designs. Where you get the idea that there are smells, I have no idea.

You are missing the point. The temperature difference across a heat exchanger is limited by the heat exchanger itself. So on a conventional boiler, arranging a greater heating load by having more emission will simply result in the flow being cooler. It doesn't buy you anything.

For a modulating condensing boiler, there is no specific "design temperature". They will work at 82/70 if put into a standard system and the level of heat demand is high. It is typical to run at 70/50 with new radiator systems but you don't have to do so. You can run UFH systems at 50/30 if the boiler will do it, and some will. The lower the temperature, the greater the efficiency and below the dew point the rate of change increases.

You are assuming that TRVs are digital devices that suddenly cut off the flow when the temperature is reached. They are not, and are simple wax expansion devices that gradually reduce the flow.

The value in a bypass is if you get to a situation where all radiators have been turned off.

Yes, I'm fully aware of that.

No because when this happens the heat emission will have dropped. The purpose is to reduce the power to the pump as the flow resistance increases.

No it won't because the bypass opens in effect digitally at a certain head. After that, it has a pressure/flow characteristic.

.andy

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[IMM]

Nothing says it is madatory in the regs.

The stat should be in the coolest room which is usualy the hall.

Condensing boilers are far more efficient as they have a larger heat exchanger, condensing or not.

Not so.

Modern condensing boilers have load compensation control which is a great be benefit on part load. Most systems are on part load for most of the time.

I would use a condesning boiler.

You only get condensing when the return temp is far to low.

And difficult to get right. Do not use one of these with a condensing boiler.

You need to know more about regular v condensing boilers and much, much more.