does balancing increase efficiency?

[bengasman]

I can see one glaring error to start with. The return from the hot water cylinder must be the last tee before the boiler; it must come after all radiator return tees.

All radiators in one zone are parallel, so there is no real first or last.
The dhw circuit runs as a separate zone parallel to the ch zone/circuit.
Assuming for the moment that 21st century controls are used allowing independent settings, they would not even be on at the same time under normal circumstances.

Even if ch and dhw were on at the same time, the resistance over dhw zone is around 10 times lower than your average radiator and as such will have a much larger "drain" and thus still absorb much more heat than any radiator even if the cylinder was piped up as another radiator in the heating circuit.

 

[UpgradeME]

I can see one glaring error to start with. The return from the hot water cylinder must be the last tee before the boiler; it must come after all radiator return tees. As drawn the lounge, kitchen, utility, bed 2 and bed 4 all have their returns connected after the cylinder return.

The reason for connecting the cylinder return last is to prevent reverse circulation. This can happen when the heating valves are closed and the HW one is open. Water travelling down the return pipe from the cylinder will get diverted up the return pipes to the rads, thus heating them up. This would be noticeable in the summer when the rads, which are supposed to be off, feel warm.

Aha. I'd heard the term reverse circulation but didn't know the symptoms... Before we had the ABV & TRV's fitted, if the valve marked "unused" was opened (by me.. before I knew much about the system), the "Landing" radiator would get warm after the HW had been on for a while and possibly Bed 2 (I can't remember now) so it now makes sense this was likely reverse circulation?

It doesn't happen any more (that I've noticed) but suspect all three of the rads fed off the right hand pipe (Bed2,4 & landing) maybe after the cylinder!?

Have you checked that the 10mm drops are actually off the main pipe and not fed from a central manifold?

No. Due to the physical locations, after drawing and looking, I guessed that logically (possibly naïvely) they would just be tee'd off the main pipe. Is there any easy way to tell without pulling all the floorboards/ceiling off?

I'll check around the ceilings next time the heating's been on with the IR Thermometer, I assume a manifold would show up as a large heat source!? - The plumbing is mainly 'original' from when the house was built around 17 years ago (as part of a housing estate) if it makes a difference?

 

[UpgradeME]

The assumption is that the boiler flow rate is set according to MI. Then the temperature rise is directly proportional to boiler output. The manual states 18 kW at 12.9 l/min gives a temperature rise of 20°C. Keeping the flow rate constant, 12 kW gives a temperature rise of 20°C × 12 kW ÷ 18 kW = 13°C.

Using different flow rates, you can calculate the temperature rise across the boiler as:

ΔT (K) = power (kW) ÷ flow (l/min) ÷ 4.1 (kJ/l/K) × 60 (s/min)

Okay. thanks!

Re-arranging the formula, could I therefore use this calculation to determine my actual (current) flow rate?

Flow (l/min) = Power (kW) ÷ 4.1 (kJ/l/K) * 60 (s/min) ÷ ΔT (K)

With d.0 set at 11 kW (max boiler output), pump speed set at II, I am currently getting 16C between flow/return at the boiler.

Using the above calculation my flow is calculated at 10.06 l/min (=11÷4.1*60÷16) - If I understand this correctly, in theory if I change d.0 to 14, keeping the same flow rate, the temp rise at the boiler would increase to 20.36C (=14/10.06/4.1*60)?

 

[D_Hailsham]

So at 80°C the volumetric heat capacity of water would be 0.9718 * 4.198 = 4.080

But that's only the flow; what about the return water which may be 20 degrees lower.

Using the data in the link I gave, if you average over the temperature range 20°C to 80°C the volume heat capacity is 4.13 kJ/litre per K. Using three decimal places seems pointless to me, as we are only guessing most of the time.

 

[ajrobb]

So at 80°C the volumetric heat capacity of water would be 0.9718 * 4.198 = 4.080

But that's only the flow; what about the return water which may be 20 degrees lower.

Using the data in the link I gave, if you average over the temperature range 20°C to 80°C the volume heat capacity is 4.13 kJ/litre per K. Using three decimal places seems pointless to me, as we are only guessing most of the time.

The density and the specific heat capacity go on opposite directions with temperature, so the change of volumetric heat capacity will be moderately slow. At about 60°C it crosses 4.1, which is probably close enough for the boiler. Anyway, the volume coming out of the boiler is higher than the volume going in. (I suspect that boiler manufacturers might use litre and kilogram interchangeably for water.)

As you say, it is only an approximate value to help select pump and by-pass settings etc.

 

[UpgradeME]

Re-arranging the formula, could I therefore use this calculation to determine my actual (current) flow rate?

Flow (l/min) = Power (kW) ÷ 4.1 (kJ/l/K) * 60 (s/min) ÷ ΔT (K)

With d.0 set at 11 kW (max boiler output), pump speed set at II, I am currently getting 16C between flow/return at the boiler.

Using the above calculation my flow is calculated at 10.06 l/min (=11÷4.1*60÷16) - If I understand this correctly, in theory if I change d.0 to 14, keeping the same flow rate, the temp rise at the boiler would increase to 20.36C (=14/10.06/4.1*60)?

Test complete. d.0 set to 14, d.5 set to 60C (output temp) and as per the formula, I get 60C flow (d.40) and 40C return (d.41) at the boiler! (So nice to see mathmatical theory confirmed in practice!)

So, as my total kW requirement from rads is around 9.5kW + 2kW for cylinder = 11.5kW, which of the following would be most efficient:

• 1) Set boiler to 14kW, 10.06l/min flow and get 20C difference between flow and return
  2) Set boiler to 12kW, 10.06l/min flow and get 17C difference between flow and return
  3) Set boiler to 10kW, 10.06l/min flow and get 14.5C difference between flow and return (assuming I never have HW and CH on at the same time)

Is there a way to calculate that one ajrobb? Alternatively, which one would you (anyone else) go for?

Other info which may be relevant:

a) My cylinder 'absorbs' around 6C between flow and return when tank temp was already warm (I've not had the chance to test from cold yet)

b) I will soon be fitting new Vaillant controls (VRT392/VR65) which will allow me to set HW & CH flow temp's independently and I can set HW priority or allow both to come on concurrently (I currently have S-Plan wiring with 2xValves).

 

[ajrobb]

First check that boiler isn't expecting W-plan with separate flow temperatures for CH and DHW. I think it should be (otherwise you won't comply with minimum cylinder temp of 60°C for legionella control). If it is, you don't need to add anything to the radiator output to calculate boiler output as it will either be CH OR DHW and not both.

Edit: just spotted you are going W-plan.

If you are happy with the calculated flow of 10.6 l/min, you might as well set boiler output to 14 kW and get your temperature rise up to 20°C. This will warm the radiators quicker. It might also warm the DHW quicker. As soon as the radiators warm up, the boiler should modulate down to 10 kW and then lower as the TRVs kick in.

In mild weather, turn down the CH flow temperature to improve condensing efficiency or fit automatic weather compensation.

Given than condensation efficiency depends on average boiler temperature and radiator output depends mostly on average radiator temperature, it doesn't matter a huge deal how you get the average water temperature down, increasing ΔT or reducing flow temperature.

On S-plan, the situation would be more difficult as the flow temperature is governed by HWC temperature and cannot be reduced. This only leaves ΔT.

 

[UpgradeME]

First check that boiler isn't expecting W-plan with separate flow temperatures for CH and DHW. I think it should be (otherwise you won't comply with minimum cylinder temp of 60°C for legionella control). If it is, you don't need to add anything to the radiator output to calculate boiler output as it will either be CH OR DHW and not both.

Edit: just spotted you are going W-plan.

The Vaillant controls use their own wiring plan (eBUS) with a seperate control centre (VR65) for the valves/cylinder. With the VR65 fitted, I can change d.70 at the boiler so I can have "Warm water priority" (CH OR DHW) alternatively, I can set it to "Enable mid position" which allows me to have both CH & HW on at the same time.

Would "Warm water priority" be the preferred option? Does it give me any advantages?

EDIT: Just read your edit - I think I understand now, it will allow the boiler to reduce the flow temp for the CH whilst heating it hotter (60C) for the HW!?

In mild weather, turn down the CH flow temperature to improve condensing efficiency or fit automatic weather compensation.

So far as I can tell, the VRT392 will modulate down the CH flow temperature based on internal temperature readings. I think* it works similar to the VRT430 (Weather compensation) but based on internal temp rather than external.

* I say "think" as Vaillant technical support don't appear to actually know that much detail about their product, appart from what is printed / displayed online! (There's settings for "modulate" or "on/off thermostat" on the VRT392, but no-one I've asked appears to understand how it calculates the modulation!)

 

[D_Hailsham]

All radiators in one zone are parallel, so there is no real first or last.

That's debatable. All rungs on a ladder are parallel, but most people would refer to the bottom one as the first and the top one as the last.

The dhw circuit runs as a separate zone parallel to the ch zone/circuit.

That's what should happen, but not according to the OP's diagram, which you obviously have not studied.

Assuming for the moment that 21st century controls are used allowing independent settings, they would not even be on at the same time under normal circumstances.

1. You cannot make that assumption.
2. Reverse circulation is normally only noticeable in the summer, when the heating is off.

Even if ch and dhw were on at the same time, the resistance over dhw zone is around 10 times lower than your average radiator and as such will have a much larger "drain" and thus still absorb much more heat than any radiator even if the cylinder was piped up as another radiator in the heating circuit.

You seem to be saying that it does not matter if the HW return is not the last Tee. Who re-wrote the rule book??

 

[ajrobb]

I think you might have the option of a standard room thermostat switch or a room thermometer (there are different links on the controller). With a room thermometer (thermistor), the boiler can reduce flow temperature as the set temperature is approached based on how quickly the room warmed up. With a standard room thermostat, it might make some educated guesses based on the onff ratio.

Be aware that as flow temperature is decreased, radiator output will be reduced. At a given flow, as radiator output is reduced, so is ΔT reduced across the radiator. So if the boiler modulates output temperature and power together, everything stays reasonably balanced.

In W-plan, the boiler flow temperature can go much higher than 60°C to heat the water cylinder quickly. (Once the return temperature reaches 55°C it can't condense anyway.)

In a small domestic system, the first radiator on the flow is often the last on the return. On larger systems, the first radiator on the flow might also be the first radiator on the return.

edit: Have you also checked the temperature rise with all the TRVs turned off (just one radiator on)? This checks the ABV setting.

 

[D_Hailsham]

Anyway, the volume coming out of the boiler is higher than the volume going in.

Surely the volume is the same, it's just the density which changes? It's a closed system! There will be an initial increase in volume (taken up by the F&E tank or expansion vessel), but after that the volume of water in circulation will be constant.

Think of the "gravity circulation" hot water heating systems. They worked because the density of hot water is lower than cold water.

 

[D_Hailsham]

So, as my total kW requirement from rads is around 9.5kW + 2kW for cylinder = 11.5kW

If you are changing over to W Plan, there is no need to allow the extra 2kW for the HW cylinder, though it may allow the cylinder to heat up faster.

You said earlier that your rads total 2kW more than the calculated boiler size. Do they total 11.5kW or 13.5kW?

 

[UpgradeME]

So, as my total kW requirement from rads is around 9.5kW + 2kW for cylinder = 11.5kW

If you are changing over to W Plan, there is no need to allow the extra 2kW for the HW cylinder, though it may allow the cylinder to heat up faster.

You said earlier that your rads total 2kW more than the calculated boiler size. Do they total 11.5kW or 13.5kW?

The total radiator output is 9.5kW. I was allowing 2kW for the cylinder as I am currently on a S-Plan where there are times in the day when they are both on together. Therefore I set d.0 to 11 or 12 (9.5+2=11.5)

I have the new Vaillant controls here (VRT392/VR65), they will likely be fitted tomorrow moving me over to the eBUS wiring (independant HW/CH control). Once fitted, I will set the HW to come on prior to the CH in the morning so my total demand will then reduce to 9.5kW (or less as the TRV's close) at any one time.

Have you also checked the temperature rise with all the TRVs turned off (just one radiator on)?[/b] This checks the ABV setting.

Currently it's set at 3 turns (Honeywell DU145) which I calculated originally to be the correct setting for 12.9l/min (boiler min flow rate). I will re-check the ABV once all the new controls are fitted and I know the final flow rate.

 

[ajrobb]

Surely the volume is the same, it's just the density which changes?

No, it's the mass that stays the same. Because the density decreases with temperature rise, there is slightly more volume coming out of the boiler than going in. I don't think it make a blind bit of difference though (as long as the expansion vessel's working). As I said before, I suspect litres and kilograms of water are used interchangeably.

Once fitted, I will set the HW to come on prior to the CH in the morning.

That should be fine. However, if you have decent lagging on the HWC, I'd opt for DHW continuous. The option of DHW or CH priority comes down to whether you run out of hot water on CH priority.

 

[ianniann]

1) Set boiler to 14kW, 10.06l/min flow and get 20C difference between flow and return
2) Set boiler to 12kW, 10.06l/min flow and get 17C difference between flow and return
3) Set boiler to 10kW, 10.06l/min flow and get 14.5C difference between flow and return

As I have suggested before, it isn't this simple. You can calculate the difference between the flow and return temperatures by using the heat lost through the radiators and the flow rate. At a given temperature, this is constant and independent of the boiler power and so the temperature drop does not depend on the boiler output!

This paradox is caused by the assumption of steady state conditions. When the radiator output is, say 12kW suggesting a 17C temperature drop, and the boiler output is 14kW suggesting a 20C temperature drop, then the temperature of the water will be increasing. Very roughly in this case, you can guess that the water temperature will increase by 3C in the time it takes to circulate all the water right round the system (guess that one!). So your boiler will never actually be running at a flow temperature of 60C and a boiler output of 14kW (OK, maybe for a few seconds).

Clearly, if the radiators only output 9.5kW (at 60C) then you will not be able to maintain a 20C temperature drop for any length of time no matter what your boiler is set to. When the flow temperature reaches 60C, the boiler must modulate down to 9.5kW (give or take some for hot water) and the temperature drop will be around 14C. Increasing the flow temperature will increase the output from the radiators and so increase the temperature drop but it will reduce condensing efficiency.

Since the boiler is going to modulate down anyway (hopefully low enough that it doesn't have to cycle), you may as well limit it to the highest power that doesn't cause short cycling at your pump setting. With very high power settings and low pump speeds heat will not be shifted from the heat exchanger quickly enough even though much of the water is still cool. The boiler will exceed the set flow temperature and shut off, then very quickly have to fire again as cold water is pumped to it. So long as your boiler is able to smoothly heat to the set temperature and modulate down without this sort of cycling, then the higher the power the better because things will heat up faster.