System Balancing Help

[chrisfi]

I have an Ecotec plus 615 providing heat to 10 rads most of which are quite small. The flow and return temperature difference to the boiler and most of the rads is approx 3-4C. Is that OK?

I have tried to better balance the rads, but I have to almost completely close the LSVs to increase the drop on the rads to 6-7C. However the flow/return on the boiler remains at approx 4C.

I have reduced the pump speed, but that made little difference.

Any suggestions please?

Thanks.

 

[picasso]

Have you calculated the heating load?

 

[chrisfi]

Thanks for the reply.

No, I haven’t calculated the heating load and don’t know how to! Are you suggesting the rads may be too small or the boiler too large?

I had the old boiler (Potterton Ultra 2 12Kw) replaced in the summer. My plumber wanted to replace it with an Ecotec plus 618, but after I explained I had cavity insulated and doubled glazed since the original install, he suggested the 615. I understand this is fully modulating and will operated between 5-15kw.

The rads are those originally installed in the house (built 15 years ago). I recently replaced a couple, which is why I tried to rebalance the system.

What sort of temperature difference is acceptable? Would I need to calculate the heating load to find this figure?

Is it Ok to have the LSVs almost fully shut off or would it be best to open them up a bit more to allow a decent flow of water through the rads? I don’t have problems with cold rads.

Thanks again

 

[D_Hailsham]

No, I haven’t calculated the heating load and don’t know how to! Are you suggesting the rads may be too small or the boiler too large?

Use the EST online boiler calculator to work out the heating requirement for your house.

Then use the Stelrad Elite Catalogue to work out the total output of your rads.

Post the results of both calculations.

What sort of temperature difference is acceptable?

Your boiler works best when the temperature difference is about 20°C

Is it OK to have the LSVs almost fully shut off or would it be best to open them up a bit more to allow a decent flow of water through the rads?

It won't do any harm? How many turns open are your LSVs?

 

[chrisfi]

Thanks for the links.

The building calculator isn't working at the moment, but I have calculated the combined rad outputs to be approx 6KW.

I must say that a temp drop of 20C would seem unachievable for my system. Can this really be achieved with a condensing boiler set for a flow of 60C?

With ref to the LSVs - they are all set between 1/8 to 1/4 from fully closed. Consequently there is a water rushing sound coming from a few of the rads.

Thanks again.

 

[D_Hailsham]

The building calculator isn't working at the moment,

Just tried it and it's working now. Give it another go.

I have calculated the combined rad outputs to be approx 6KW.

And you have a 15kW boiler

OK, the boiler may modulate down to 5kW but, as you will only need 6kW when it is freezing outside (assuming rads are correct size), most of the time you will be needing even less than 5kW. As the boiler is unable to modulate below 5kW it goes into on/of mode. The problem with this is that every time the boiler comes on it goes to max output. As this is too much it quickly modulates down and then switches off. The net result is that the water never reaches the correct flow temperature.

You need to get the installer back and ask him to drop the max output down. Try 8kW and see what effect it has. You can always get it changed again.

I must say that a temp drop of 20C would seem unachievable for my system. Can this really be achieved with a condensing boiler set for a flow of 60C?

It's what the boiler will try to achieve when it modulates. The only problem is that rads set to have a 20C drop give off about 15% less heat. That's fine if your rads are oversized. This is why I suggested you use the boiler calculator. It will show if your rads are correctly sized.

With ref to the LSVs - they are all set between 1/8 to 1/4 from fully closed.

Do you mean 1/8 to 1/4 of a turn open, or 1/8 to 1/4 of the complete number of turns from open to closed?

 

[chrisfi]

I have found another calculator (still can't get the other working even on a different computer!) It has come up with 5.83 KW required, ignoring DHW charging.

And you have a 15kW boiler

Yep. Lucky I managed to convince the plumber to go for the smaller one! The smallest (612) operates 5-12 Kw.

You need to get the installer back and ask him to drop the max output down. Try 8kW and see what effect it has.

I forgot to mention that the boiler charges a hot water cylinder (Megaflow). Would it take longer to charge, if the boiler max output is dropped?

Also, I have a VRC430 weather compensator. Would this avoid the need to drop the max output?

Do you mean 1/8 to 1/4 of a turn open, or 1/8 to 1/4 of the complete number of turns from open to closed?

I mean 1/8 to 1/4 of a turn open.

Thanks yet again for the excellent advice.

 

[D_Hailsham]

I have found another calculator It has come up with 5.83 KW required, ignoring DHW charging.

Which is about the same as your rads.

I forgot to mention that the boiler charges a hot water cylinder (Megaflow). Would it take longer to charge, if the boiler max output is dropped?

No. Megaflow say allow 3kW for heating the cylinder so, if you have 6kW for heating, you should set the boiler to 9kW. This allows both CH and HW to run at the same time.

Also, I have a VRC430 weather compensator. Would this avoid the need to drop the max output?

Not really.
How has this been installed - in the boiler or mounted on the wall?
Has the correct heating curve been selected?
Has the external sensor been fitted in the correct place? i.e.
* on a N or NW facing wall
* no draughty location
* no direct sunlight
* no influence from heat sources

If you want to adjust the output the instructions are in the installation manual page 34. And if you don't have this, here they are:

Adjusting the central heating output (range rating)

• Press the “i” and “+” buttons simultaneously.
• Keep the “+” button pressed until “d.0” is displayed.

(Note: The display runs from “d.0” to “d.99” before restarting at “d.0”.)

• Press the “i” button. The “=” symbol is displayed. The part load setting is now displayed in kW.
• Use the “+” and “–” buttons to increase or decrease the value in steps of 1 kW. The displayed value will flash while the adjustment procedure is being carried out.
• Push the “i” button for about five seconds, or until the display stops flashing. The value is now saved to memory.

The display will now return to its normal state (current heating system flow temperature, e.g. 45 °C).

• Press the “i” and “+” buttons simultaneously to cancel adjustment mode.

Adjustment mode is also cancelled if no button is operated for a period of 4 minutes.

I mean 1/8 to 1/4 of a turn open.

That sounds about right. The effective working range of the typical LS valve is from closed to 1 or 1 1/2 turns open.

 

[chrisfi]

Thanks, you're a star!

I set the heating part load (d.0) to 8kW as I don't have the system set to charge HW when heating. Early signs are good - the boiler runs continuously until the house has reached the set temperature and it reaches the set temperature a lot quicker than before. Before the boiler would often cut out and go into anti cycling for 20 mins.

A point to note - from the manual I see that you can adjust the heating partial load (d.0) and storage partial load (d.77) on the boiler independently.

Ref the VRC430 - this is installed in the hall, weather sensor installed on north facing wall as per instructions. The plumber installed it, but left me to set it up as he found it a bit overwhelming!
Through trial and error, I have found a heating curve of 2.1 seems to work. I haven't decided whether to stick with switching or thermostat room control. I have noted that Eco mode doesn't work in combination with thermostat. The manual is very vague, confusing and contains some errors!

Finally, I intend to replace some radiators as I decorate. I would like to oversize them so to reduce the return flow temperature to better ensure the boiler is condensing more of the time. Is there a simple rule of thumb for oversizing?

 

[D_Hailsham]

I set the heating part load (d.0) to 8kW as I don't have the system set to charge HW when heating. Early signs are good - the boiler runs continuously until the house has reached the set temperature and it reaches the set temperature a lot quicker than before. Before the boiler would often cut out and go into anti cycling for 20 mins.

When will installers, and manufacturers, learn that you have to size correctly for heating and not rely on the boiler modulation.

A point to note - from the manual I see that you can adjust the heating partial load (d.0) and storage partial load (d.77) on the boiler independently.

That's only applicable if you have a Vaillant sensor on the HW cylinder and the VR65 Control Centre. If you just have the normal cylinder thermostat it does nothing.

Ref the VRC430 - this is installed in the hall, weather sensor installed on north facing wall as per instructions. The plumber installed it, but left me to set it up as he found it a bit overwhelming!

Not surprised. Came across two plumbers/installers recently who did not know what weather compensation was. Though it was like frost protection.

It's a bit of an anomaly that an installer has to be properly trained and tested before they can work for payment on any gas appliance, but anyone can call himself a "heating engineer" without any formal training or qualifications.

Is there a simple rule of thumb for oversizing?

The maths is quite complicated. Assuming you want to run the boiler at 75/55°C when the temperature is -1 and a room temperature of 21°C, you will have to oversize the rads by about 20%; and if you want to run at 70/50°C, you will have to oversize by about 40%.

 

[chrisfi]

That's only applicable if you have a Vaillant sensor on the HW cylinder and the VR65 Control Centre. If you just have the normal cylinder thermostat it does nothing.

That’s interesting. I have the Vaillant sensor and VR65, but didn’t fit the sesnor thinking the only advantage would be the ability to adjust the HW temp from the VRC430. What are the benefits of using the sensor and would I need to adjust some settings on the boiler ie storage part load?

Oh, and I almost lost sight of my original issue…..balancing the system…

We have since worked out that my rads output approx 6kW and my house requires a 5.8kW boiler. I have slowed down the pump and range rated the boiler to 8kW. However, the flow and return temperature difference (as indicated by d.40 and d.41 respectively) is 5-6C with the flow set at approx 70C or 3-4C with the flow at approx 60C. I think the system is a standard 22mm loop with 15mm tee’s off to the rads. A lot of the rads are narrow but tall ie 400mm/500mm wide. Some rads LSVs are almost fully closed to the point where they cause a ‘water rushing noise’. Based on this do you think I could realistically achieve anything approaching a 20C difference? Would it OK to aim a bit lower, say 10C? I do intend to oversize some of the rads as I decorate next year.

Thanks again.

 

[D_Hailsham]

I have the Vaillant sensor and VR65, but didn’t fit the sesnor thinking the only advantage would be the ability to adjust the HW temp from the VRC430. What are the benefits of using the sensor and would I need to adjust some settings on the boiler ie storage part load?

The sensor measures the temperature. The boiler can use this information to determine what output it needs to deliver. You can adjust the storage part load so it delivers no more than the max amount of heat to recover a tank full of cold water in an acceptable time.

We have since worked out that my rads output approx 6kW and my house requires a 5.8kW boiler. I have slowed down the pump and range rated the boiler to 8kW. However, the flow and return temperature difference (as indicated by d.40 and d.41 respectively) is 5-6C with the flow set at approx 70C or 3-4C with the flow at approx 60C.

At a temperature of 70/64 your rads will be giving off about 5700W and at 60/56 about 4300W. The house does not need the same amount of heat very day, it will depend on the outside temperature; the warmer it is, the less heat required.

What happens to the difference if you reduce the pump speed to 1?

do you think I could realistically achieve anything approaching a 20C difference? Would it OK to aim a bit lower, say 10C? I do intend to oversize some of the rads as I decorate next year.

10C sounds about right at the moment. If you set it any higher, there is a danger that your rads will not produce enough heat. At the moment your rads would only produce 5300W for 75/55 and 4500W for 70/50

When you replace your rads and upsize them by 20% is the time to aim for a 20C difference.

 

[chrisfi]

What happens to the difference if you reduce the pump speed to 1?

I have already reduced it from 2 to 1 and it made no perceivable difference.

When you replace your rads and upsize them by 20% is the time to aim for a 20C difference.

OK, will do.

Thanks very much for all your help.