Pump overrun is killing my heating!

[Murdoch]

run the water at 5am .......

Water heating is generally "slower" if the system is also doing radiators if I recall things correctly

I must admit I'm a bit puzzled by the depth you are going to - can't you just get the engineer back?

 

[John.g]

Yes, re pump overrun but it shouldn't IMO be ever on pump overrun with any demand for CH or HW, I read that the minimum output of this boiler is 12 kw? so if the coil is not absorbing this 12 kw then the boiler will cycle, maybe this is when the pump overrun is showing up. If the coil is absorbing 12 kw then that should heat a 260 litre cylinder from say 25C to 65c in exactly 1 hour, obviously if a 20kw coil, in 36 minutes. If the coil inlet temp is < say 68/70C then the cylinder will never reach setpoint temperature. You can monitor the flow/return temps on the temperature gauges which will tell you something and if you had/have access to a energy monitor you can measure the secondary pump power in watts from which the flow rate can be derived and then you will know exactly what's happening re boiler/coil power.

And presume this is a heat only boiler and not some form of combi!!.

 

[milleniumaire]

run the water at 5am ******.

Water heating is generally "slower" if the system is also doing radiators if I recall things correctly

I must admit I'm a bit puzzled by the depth you are going to - can't you just get the engineer back?

I could get the engineer back, but to what end? He has setup the system as it is, to the best of his knowledge, and it doesn't appear to be working "as well" as it used to be before the upgrade. I know he was on the phone to Vaillant at least three times while setting it up, which doesn't exactly inspire me with confidence! It is possible that introducing the LLH, external pump, VR 71 wiring centre and new VRC 700 controller has changed the way it works such that it doesn't work "as well" as it used to and I'll just have to accept that.

By "as well" I mean the hot water heated up and the house warmed up within an hour of everything starting up in the morning.

Yes, I could start the water earlier at 5am and this would probably resolve the issue, but isn't it best to first determine if there is an issue with the way things are working rather than just working around the issue?

 

[John.g]

Before installing the LLH the system flow temperature would always be the same as the boiler flow (setpoint) temperature but with the LLH, depending on (any) mixing then the system flow temperature may be lower but easy to see this by your temperature gauges.

 

[milleniumaire]

Before installing the LLH the system flow temperature would always be the same as the boiler flow (setpoint) temperature but with the LLH, depending on (any) mixing then the system flow temperature may be lower but easy to see this by your temperature gauges.

Yes, I can see this through the analogue temp gauges I have on the pipes. The photos I've posted don't show that I have also fitted a couple of gauges to the boiler flow and return as well, so I now have 4 gauges showing the temps between the boiler and LLH and the LLH and system circuit. You are correct, the flow and return around the system, from/to the LLH are always slightly lower than those on the boiler side, but I wouldn't have thought they were low enough to make a huge difference.

Currently, there is heating demand from all three heating areas (radiators and 2 x UFH zones), as has been the case all morning and the thermostats are showing:

LLH flow from boiler: 58 deg C
LLH return to boiler: 48 deg C
LLH flow to system: 56 deg C
LLH return from system: 46 deg C

I confess, I'm a little surprised the return from the system isn't lower, given it is going through 14 radiators, kitchen/sitting/dining UFH and bathroom UFH!

I have seen the boiler side temps get up to 75 deg C.

 

[John.g]

Is the boiler setpoint 75C ?, and was it the same prior to changes?. if the system flow temp never reaches 65C then it doesn't matter how long the boiler is on as the cylinder will never reach its SP, you could temporarily reduce it to say 55C.
Also most rapid heat recovery coils are designed with flow temperatues of up to 85C and with very high circulation rates to get their rated output.

 

[SJB060685]

With LLH if the system flow is equal to boiler flow then there should be know mixing and an equal temperature maintained, likewise if the system flow was less than boiler flow. If the system flow was greater than boiler flow then there will be a mixed lower temperature going to system circuit/circuits.
The connections in the picture are correct. All a LLH header is an empty tube effectively giving a means of separating two systems with a low pressure loss and low velocity zone. They are also know as a low velocity header.

 

[milleniumaire]

Is the boiler setpoint 75C ?, and was it the same prior to changes?. if the system flow temp never reaches 65C then it doesn't matter how long the boiler is on as the cylinder will never reach its SP, you could temporarily reduce it to say 55C.
Also most rapid heat recovery coils are designed with flow temperatues of up to 85C and with very high circulation rates to get their rated output.

Sorry John, I'm not sure what you mean by setpoint.

On the boiler, the dials are set such that hot water is at its maximum setting of 65 deg C. The radiators are set to 70 deg C. Is this the setpoint?

What is confusing is that the VRC 700 allows the desired hot water temperature to be set more than 65 deg C, but the boiler dial never shows more than this. The VRC 700 hot water setting and boiler hot water setting don't seem to be in sync. Not sure what this is about, possibly an example of where the 10 year old boiler isn't as compatible with the new VRC 700 controller as it could be.

When scrolling through the menus on the VRC 700, there are lots of temperatures mentioned, but the only one's I've changed are the "desired" temperatures, so 21 deg C for the radiator circuit (controlled by VRC 700, which is in the hallway - the coldest part of the house) and 20 deg C for the two UFH circuits.

 

[John.g]

It seems that this controller, for whatever reason is telling your boiler to run at 58C, based on this and your return temps and assuming room temperatures of 20C then these rads will only be emitting 56% of their rated output which would account for the slow heat up. This controller may be some smart form of outside temperature compensation, must have a read up of it.

 

[milleniumaire]

It seems that this controller, for whatever reason is telling your boiler to run at 58C, based on this and your return temps and assuming room temperatures of 20C then these rads will only be emitting 56% of their rated output which would account for the slow heat up. This controller may be some smart form of outside temperature compensation, must have a read up of it.

Yes, the system does use weather compensation and each zone has its own heat curve. While this is supposed to make the system more efficient, it also makes it more difficult to understand what or why things are happening e.g. why is there no heat demand for a zone when the desired temp is set to 21 and the current temp for the zone is showing 19 and the rooms are cold!

This kind of scenario usually ends up with me increasing the heat curve for the zone, making the system less efficient! Of course, the outside temperature also has a large impact and usually, at this time of year, when it is much colder outside, the heating system achieves the desired indoor temperatures more often than not.

 

[John.g]

Just had a quick scan through it and it does compensate for the outside air temperature with a outside temperature sensor, you can see on page 12 that you can change the heating curves, for example if currently set on curve 2, it will call for a boiler SP (reqrired boiler temperature) of 65C at OT of 0C, 55C at 5C, and 45C at 10C. If you select heating curve 2.5 then the numbers are 73C at 0C, 63C at 5C, and 52C at 10C and so on. The HW required temperature seems to be set by default to 60C but don't see any corresponding boiler SP, if working properly, I would expect to see the boiler SP temperature rising with the actual cylinder temperature. You obviously need a cylinder sensor connected.
It also mentions somewhere about switching off the heating pump with no heating demand, maybe this is S.0?

Just saw your post.

 

[milleniumaire]

Just had a quick scan through it and it does compensate for the outside air temperature with a outside temperature sensor, you can see on page 12 that you can change the heating curves, for example if currently set on curve 2, it will call for a boiler SP (reqrired boiler temperature) of 65C at OT of 0C, 55C at 5C, and 45C at 10C. If you select heating curve 2.5 then the numbers are 73C at 0C, 63C at 5C, and 52C at 10C and so on. The HW required temperature seems to be set by default to 60C but don't see any corresponding boiler SP, if working properly, I would expect to see the boiler SP temperature rising with the actual cylinder temperature. You obviously need a cylinder sensor connected.

I just beat you to posting!

The cylinder does have a temperature sensor, which is wired upto the VR 71 wiring centre, so I assume this is being used to get the cylinder to the desired temperature of 65 deg C.

I've been regularly checking the boiler today, since 7:00am and haven't once seen it go into pump overrun. Typical!

 

[John.g]

Re zone temperature control, Presume if a number of zones in service then it should base the boiler SP on the highest heat curve and if only one, on that zones heating curve.

I know you have HW priority and the reason you can't have HW & CH on together is probably the conflicting boiler SP requirements? although one might think that the controller could be configured to select the boiler SP based on the HW requirement or something like this to allow both systems on together.
When HW next selected can you note the actual cylinder temperature, the boiler SP temperature and the actual (thermometer measured) flow&return temps, this will give you a very good feel for what's going on there.

Can you also state the minimum output of your boiler.

 

[EvilDrPorkChop]

What size is the cylinder, 65oc seems a high set point tbh and may explain your reheat issues in the morning.

 

[milleniumaire]

What size is the cylinder, 65oc seems a high set point tbh and may explain your reheat issues in the morning.

It's a 260 litre Vaillant unistor unvented cylinder.
60 Deg C is obviously the lowest setting to avoid legionella.
Since the upgrade I'm fairly sure it has been necessary to turn up the shower temperature. I had assumed the engineer may have turned down the hot water setting, so I turned it up to the maximum of 65 Deg C using the boiler control. The shower temperature setting still needs to be set higher e.g. 7 out of 10, instead of 5.

 

[EvilDrPorkChop]

It's a 260 litre Vaillant unistor unvented cylinder.
60 Deg C is obviously the lowest setting to avoid legionella.
Since the upgrade I'm fairly sure it has been necessary to turn up the shower temperature. I had assumed the engineer may have turned down the hot water setting, so I turned it up to the maximum of 65 Deg C using the boiler control. The shower temperature setting still needs to be set higher e.g. 7 out of 10, instead of 5.

The Vaillant system has an Anti Legionella Setting that you can turn on, personally 60oc is too hot for me mines set at 55oc. I'd be careful as many showers have a Max inlet temp of 65oc also, so if your probe is measuring 65oc at the bottom it'll be more like 70oc at the top of the cylinder.

This is probably your problem too, getting a 260l cylinder to 65oc, and with the boiler possibly anti cycling because your HW output isn't set correctly then your going to be taking a good 30/45 minutes to get to the set point. Also if you're drawing off in this period it isn't going to help.

 

[John.g]

Spec Sheet:
the unistor260 contains 250 litres of water, it has 22.3kw heating performance at a circulation flow rate of 22.3 LPM (no heating temp given) with a 35 min heating time, primary heating surface 0.75M2.

So presume the controller is looking at the rate of temperature rise and adjusting the boiler flow temp, I think it was mentioned that 75C was noted sometimes?.
It may be of some help to increase the heating output from 16kw to 22kw which may help to get the temperature up within a hour. Possibly some fouling of the heating surfaces as well which won't help.

 

[EvilDrPorkChop]

Spec Sheet:
the unistor260 contains 250 litres of water, it has 22.3kw heating performance at a circulation flow rate of 22.3 LPM (no heating temp given) with a 35 min heating time, primary heating surface 0.75M2.

So presume the controller is looking at the rate of temperature rise and adjusting the boiler flow temp, I think it was mentioned that 75C was noted sometimes?.
It may be of some help to increase the heating output from 16kw to 22kw which may help to get the temperature up within a hour. Possibly some fouling of the heating surfaces as well which won't help.

It was my understanding that the coil was 16.6w output? (According to Vaillants Website).

On HW priority the cylinder will reheat with a flow temp of 80oc.

 

[SJB060685]

Spec Sheet:
the unistor260 contains 250 litres of water, it has 22.3kw heating performance at a circulation flow rate of 22.3 LPM (no heating temp given) with a 35 min heating time, primary heating surface 0.75M2.

So presume the controller is looking at the rate of temperature rise and adjusting the boiler flow temp, I think it was mentioned that 75C was noted sometimes?.
It may be of some help to increase the heating output from 16kw to 22kw which may help to get the temperature up within a hour. Possibly some fouling of the heating surfaces as well which won't help.

But an output of 22kw with a DT of 20°c which my understanding is is what the boiler is looking for is 15.8 lpm. Boiler pumps are burner linked in modern gas boilers?

 

[John.g]

Don't know what the EN standard is but assuming heating 250 litres from 20C to 65C with 22.3kw, 23.3LPM in 35 minutes gives, by calculation, a deltaT of 8C, rapid heat recovery systems don't care what the deltaT is they use the highest possible boiler temps with the greatest flow rates to achieve their object.

Can't attach files for some reason.

 

[John.g]

EN12897.....temperature difference between water (65°C) and ambient (20°C) of 45K

The spec above is for the "VIH GB 260/2/S"

Total capacity litres 120 155 180 210 260 310
Actual capacity litres 119,2 149,1 176,6 211,2 250,3 298,5
Hot water capacity litres 104,0 136,7 166,4 203,9 247,5 270,5
Maximum supply pressure to pressure reducing valve MPa (bar) 1,2 (12)
Rated pressure of cylinder MPa (bar) 0,7 (7)
Maximum operating pressure of heating coil MPa (bar) 0,35 (3,5)
Operating pressure MPa (bar) 0,35 (3,5)
Pressure reducing valve MPa (bar) 0,35 (3,5)
Expansion relief valve MPa (bar) 0,6 (6)
Temperature and pressure relief valve °C,
MPa (bar)
90,
0,7 (7)
Charge pressure of hot water expansion vessel MPa (bar) 0,4 (4)
Maximum temperature of heating circuit °C 85
Maximum temperature of potable hot water °C 85
Standing heat loss kW/24 h 1,26 1,53 1,66 1,89 2,07 2,26
Heat up time according to EN 12897 mins 18 23 24 28 35 42
Recovery time (70% capacity) mins 16 18 17 20 25 30
Primary heat exchanger performance kW 18,6 18,8 22,0 22,6 22,3 20,4

 

[SJB060685]

Don't know what the EN standard is but assuming heating 250 litres from 20C to 65C with 22.3kw, 23.3LPM in 35 minutes gives, by calculation, a deltaT of 8C, rapid heat recovery systems don't care what the deltaT is they use the highest possible boiler temps with the greatest flow rates to achieve their object.

Can't attach files for some reason.

How are you arriving at those figures?
I'm right with you right up until the DT

 

[EvilDrPorkChop]

Interesting, the older UniStore is 22.3kw coil and the newer model is 16.6kw. They must have changed who manufactured the cylinders.

 

[John.g]

How are you arriving at those figures?
I'm right with you right up until the DT

I think you spotted my deliberate mistake SJB.

KW = LPM*deltaT*60/860 so deltaT = Kw*860/LPM/60 = 22.3*860/23.3/60 = 13.7C.

 

[SJB060685]

That was the figure I had !