Pump overrun is killing my heating!

[Milleniumaire]

Okay, maybe the subject is a little dramatic, but hopefully I've got your attention for a few minutes

After a recent upgrade of my CH system, which included a LLH and a new circulation pump, my trusty old Vaillant ecoTEC 637 Plus system boiler seems to be spending a lot of time in pump overrun mode, especially in the mornings, when it first comes on.

It now spends the first 50 minutes heating up the water (DHW priority) before it starts to even think about heating the radiators. I've noticed a lot of that 50 minutes is spent in pump overrun mode, which I think means the hot water is cycling round the boiler circuit and is not getting to the unvented hot water cylinder to heat the water. During pump overrun the new circulation pump is off.

Am I correct in my assessment of the situation? Is there anything that can be done to resolve this issue so that the hot water is heated faster allowing the radiators to be heated, just like it used to do?

 

[muggles]

What was the reason for having the LLH fitted? Do you have any more info you could provide about the controls setup installed? It sounds like something hasn't been set up correctly to me - the system side pump should run at all times when heat is being called for, and 50 mins is far too long to be heating a cylinder unless it's got a tiny coil in it.

 

[Milleniumaire]

Hi muggles and thanks for your quick response.
The system has been expanded over the last 6 years and the recent changes included an addition UFH zone, making it 3 zones (radiators, kitchen UFH, bathroom UFH) and DHW.
To achieve this a VR 71 wiring centre, VRC 700 and 2 X VR 91 remote thermostats were added. The LLH and new pump off the LLH flow was added to improve the efficiency of the different heating zones.

 

[aesmith]

Can you give a diagram or description of how these elements are controlled? Pump "over run" normally refers to the pump running on after the heating load goes off, to keep water moving through the boiler while it cools down. Are any of the controls triggered by pipe thermostats, with a low temp cut off? I wonder if the issue here is that the zones won't come on until the boiler/LLH circulation is up to some preset minimum temp.

 

[Milleniumaire]

Hi aesmith, thanks for posting but thankfully I have now got to the bottom of the issue and resolved it via a different forum.

The pump overrun issue I thought I had was a red herring. The real problem was my 20 year old son having secret midnight showers, thereby depleting the hot water in the cylinder after the boiler had turned off (at 23:00), plus my cylinder temperature being set to 65 deg C. So, first thing in the morning, the boiler was taking nearly an hour, sometimes more, to re-heat the cylinder and because the system gives priorty to DHW the heating wasn't coming on until an hour later than expected, by which time we awoke to a very cold house!

 

[Mottie]

FWIW I leave my DHW permanently on. It’ll only heat it when needed and I’m always sure of a cylinder full of hot water. We have two long showers a day - sometimes the odd bath plus we have a gas hob. Looking at my gas usage in July when the heating is not used, it’s only pennies a day anyway.

 

[Milleniumaire]

FWIW I leave my DHW permanently on. It’ll only heat it when needed and I’m always sure of a cylinder full of hot water. We have two long showers a day - sometimes the odd bath plus we have a gas hob. Looking at my gas usage in July when the heating is not used, it’s only pennies a day anyway.

View attachment 217495

Hi Mottie, when you say you leave it on, I guess you mean during the night. During my monitoring of the system, I determined that the cylinder looses around 4 deg C between 23:00 when it turns off and 06:30 when it turns on. That's with the hot water demand temperature set to 60 deg C, and I may even reduce this to 55 deg C and turn on the anti-legionella settings. A 4 deg C reduction overnight (in Winter i.e. last night) wasn't even enough to cause the boiler to re-heat the cylinder (as a priority) when it turned on this morning and it went straight to heat demand.

I can therefore imagine, as you say, it's probably not going to use much more gas to leave the hot water on all night, which would then allow for my son to have his late night showers, especially as it's unlikely he'll be returning to university any time soon. They only issue with the boiler operating during the night is that we can hear it as our bedroom is directly above the garage where it is situated.

 

[Mottie]

Yes, my DHW has been on 24/7 since we moved into this house in 1990! I've always had heat only boilers - Y plan on my old boiler and S plan on my current one - so I’ve never had hot water priority as if both the heating and DHW were calling for heat, they both got it. You must have a very well trained son - taking voluntary showers - we had to wrestle ours into the shower!

 

[aesmith]

It still doesn't sound right taking 50 minutes to bring the HW cylinder up to temp. I assume it would be stone cold in the morning, unless your son really ran his shower until it ran cold.

 

[Milleniumaire]

It still doesn't sound right taking 50 minutes to bring the HW cylinder up to temp. I assume it would be stone cold in the morning, unless your son really ran his shower until it ran cold.

He does spend a lot of time in the shower, but you're right, it shouldn't take that long, which is why I started looking at settings etc. I had assumed the issue had been caused by the upgrade that occurred at the back end of November, which included a LLH, system pump, new controllers and new wiring centre. The controls and new wiring obviously introduced a lot of unknowns, but now that I've worked through some of them, I'm feeling much happier it is performing as expected (having changed some settings), although I'm still waiting to hear back from Vaillant on a number of questions I've asked.

 

[sxturbo]

It still doesn't sound right taking 50 minutes to bring the HW cylinder up to temp. I assume it would be stone cold in the morning, unless your son really ran his shower until it ran cold.

mine takes around that amount of time from 25 degrees (when we've all had our showers/baths etc)

i have to have mine come on an hour before the heating to ensure we have hot water and cosy house when we wake up.

 

[Milleniumaire]

mine takes around that amount of time from 25 degrees (when we've all had our showers/baths etc)

i have to have mine come on an hour before the heating to ensure we have hot water and cosy house when we wake up.

That was one of the options I considered when the issue started occurring, but I wanted to understand why the house was so cold when I got up in the morning, despite the boiler being on for an hour before. Initially, I thought I was seeing the boiler in pump overrun mode a lot of the time, but this does appear to be a normal part of the heating process and TBH I've not noticed this as much since I started monitoring it. The boiler is 10 years old and has always been set to come on at 06:00am and this has mostly never been an issue, even in Winter, so clearly, something had changed.

 

[ScottishGasMan]

He does spend a lot of time in the shower, but you're right, it shouldn't take that long, which is why I started looking at settings etc. I had assumed the issue had been caused by the upgrade that occurred at the back end of November, which included a LLH, system pump, new controllers and new wiring centre. The controls and new wiring obviously introduced a lot of unknowns, but now that I've worked through some of them, I'm feeling much happier it is performing as expected (having changed some settings), although I'm still waiting to hear back from Vaillant on a number of questions I've asked.

You may be waiting a while, given the time of year and the fact that many companies have understaffed offices etc at the moment due to covid.

What questions have you got and I'll try answer some in the meantime?

 

[Milleniumaire]

You may be waiting a while, given the time of year and the fact that many companies have understaffed offices etc at the moment due to covid.

What questions have you got and I'll try answer some in the meantime?

Yes, they have usually been reasonably quick in the past. I did wonder if the delay may also be due to the number and "complexity" of my questions, especially as they seem to be giving me/my installer conflicting information, which I'm still waiting for them to clarify.

It's very kind of you to offer to help answer my questions, so I'll include the more straight forward ones below, to avoid having to give you lots of background information about my system.

1. What is the difference between:
a) Circulation Pump
b) External Pump
c) Storage Charge Pump
As defined in the installation instructions for the VR 40. (I tried to attach a pdf of this manual, but kept getting a security error, so have included a link to the manual instead).
I now understand that a "Storage Charge Pump" is a pump for the cylinder only i.e. pumping water "TO" the cylinder. Still not sure of the difference between a) and b). My new Grundfos pump is obviously external to the boiler, but isn't it also a circulation pump i.e. it pumps the flow around the system?

2. Why does the VR 71 wiring centre constantly make a clicking sound? It’s like there is a relay constantly opening and closing. This happens irrespective of the zone valves opening/closing i.e. when I know there is a demand or no longer any demand from one of the zones. Is this constant clicking normal?

3. What should the values of the following boiler parameters be set to:
a. d.75 (currently set to the default of 45 mins)
b. d.77 (currently set to the default of 37kW). Should this be lower?
c. d.78 (currently set to 80 deg C)
d. Are there any other settings I need to be aware of that could possibly cause the hot water cylinder to take so long to heat up?

4. Can you tell me the minimum output of the ecoTEC 637 plus system boiler?

5. On the VRC 700, when programming times, I have the option to program each heating zone (radiators, UFH1, UFH2), DHW and “Circulation Pump”. What exactly does the “Circulation Pump” program control? Is this the internal boiler pump or does it relate to the newly added Grundfos pump (I suspect neither)? I believe this was also available as an option on my old VRC 430.

6. There are two dials on the front of the boiler; the top one for hot water temperature and the bottom for heating temperature. As these dials are turned, the boiler display shows the adjusted temperature setting. In the VRC 700 configuration menu there are MANY temperature settings shown, some of which can be changed, some of which are for information only. Which VRC 700 menu items relate to the boiler dials i.e. hot water temperature and heating temperature? For example, on the VRC 700, in the Desired temperatures, DHW circuit menu, I can set the desired Domestic hot water temperature (currently set to 55 deg C), however, this value does not match the hot water temperature shown on the boiler, when the top dial is changed! For the bottom dial used to change the heating temperature on the boiler, I don't know which VRC 700 menu options relate to this, if any.
So, it would appear the VRC 700 must be used to set the desired temperatures, so what is the relevance of the temperatures set on the boiler using the dials? Should they be matched i.e. changed on both the boiler and VRC 700? Is this an example of the VRC 700 not being completely integrated with the 10 year old ecoTEC 637 plus system boiler?

7. On the VRC 700, in the Configuration menu under DHW circuit, is a menu called Buffer cylinder, which has two options; Cyl temp top and Cyl temp bottom. On my system, only the Cyl temp top ever has a value.
a. From where does it get this value as there is only one cable (the temperature sensor) running from the cylinder and I assume this cable,
which is wired into S9 on the VR 71, provides the current cylinder temperature?
b. Why does the Cyl temp bottom never have a value (it always shows “---“)?

8. There are a number of temperature sensors that were installed along with the VR 71 wiring centre and these are attached as follows:
a) S1 - system flow side of LLH.
b) S2 - on flow pipe immediately after zone valve for zone 1 (Radiators).
c) S3 - on flow pipe immediately after zone valve for zone 2 (Kitchen UFH).
d) S4 - on flow pipe immediately after zone valve for zone 3 (Bathroom UFH).
e) S9 - cylinder temperature probe.

It would appear (as already mentioned in 7a above) the reading for the cylinder temperature probe is shown as the "Current cyl. temp." option of the DHW circuit menu under System Configuration, so I can see at any time what the cylinder temperature is.

Where are the other sensor values reported in the VRC 700 menu system? Are these sensors used for anything i.e. calculating target temperatures, or are they just for information only?

My other questions relate to the way in which the new Grundfos "system pump" has been wired and I've had conflicting responses from Vaillant, so I won't include these questions for now.

Any answers would be useful as I'm trying to understand how the new controllers/wiring centre and the old boiler are working.

 

[ScottishGasMan]

Righto, I'll answer what I can just now.

1. One is a heating pump, another is a Cylinder charging pump, and normally another is a secondry circualtion pump (the one that moves the hot water for taps round the taps in the likes of a hotel so the hot waters always near the taps) Basically if your system heats then its at the right setting for you.

2. Thats something Im going to pass onto technical to have a look at, mine does the exact same, Im 99% sure its the relay opening and closing to control an electronic mixing valve, but I assume it should be able to be stopped when a mixing valve is not in use, pretty annoying, although taking the VR10 sensor off the pipe (the one on the heating flow) stops the clicking after a few moments which makes me think it is a mizer relay.

3.a, N/A that is now set on the VRC700

b. depends on the coil rating of the cylinder, if the cylinder coil is rated at 15kw then not much use having the boiler run at 37kw, would have it a bit higher than cylinder coil rating just to keep the recovery time faster.

c. N/A set at the VRC700

d. will read the rest before comming back to this.

4. Your on an old version of the boiler so probably about 12kw

5. Yep, circualtion pump is the type mentioned before on large systems some places have a pump that circulates the hot tap water round a loop so that its always hot near the tap to reduce the lenght of time it takes to be hot at tap when you open it.

6. N/A the VRC700 takes over this, you just leave both dials at max.

7. You almost certainly dont have a buffer cylinder, a buffer cyl is not for hot water, its like a store for heating water, be interesing to know what your System Diagram number is under that same config menu?

8. They can be found on the confiiguration menu under the info for each zone. They only actually take effect if you have electronic mixing valves on the system, but they are required to be fitted for the system to work regardless.

What make/model/size is your DHW cylinder? heat up times will vary considerably depending on the installation and cylinder fitted but with a little more info we could maybe tweak it to suit

hopefully that clears up a few of the questions, on my old laptop so its a pain typing into, anything else just ask and I'll get back when I can get onto the other computer