TRVs noisy

[Alex100]

Hi all

System: Worcester Bosch 38CDi combi, 10 rads on two floors, all on same circuit, all with TRV except large towel rail in bathroom. System and boiler are new, in 28mm copper with 15mm branches to Stelrad Compact rads. The TRVs are Danfoss RAS-C2 combi.

Question: When a few rads have got turned off by the TRV (room reached temp), the remaining rads get noisy (rushing water sound). The noise is definitely at the TRV. This noise at the rads still on (e.g. ones set to max) increases as more rooms reach their TRV temp and turn off, presumably because the same volume of water is being pushed through fewer rads. The boiler is supposed to have a fully modulating pump (and the pump speed is at standard setting, which installer advised not to adjust), so is this noise at the TRvs, as other rads turn off normal/unavoidable?

Also, another question: I find these TRVs tend to turn off at much lower room temps than Danfoss suggests. e.g. setting IV (highest one before max) is supposed to be about 24 degrees but tends to shut off at more like 17-19 degrees....meaning they aren't very useful.

Thanks
Alex

 

[plumber0808]

Has it got a auto bypass fitted?
Trvs are there to control the room temp but are never going to be exact.

 

[Whyme]

Also depends where they are ie if the trvs in a corner behind a wardrobe etc that can make the local area around the valve warmer than the room.

 

[Dannypipe]

Have all the TRV's been put on the flow side of the radiator pipe work?

Even though most valves these days are bi-directional. In my experience they are a lot noisier when on the wrong side the return).

 

[doitmyself]

System: Worcester Bosch 38CDi combi, 10 rads on two floors, all on same circuit, all with TRV except large towel rail in bathroom. System and boiler are new, in 28mm copper with 15mm branches to Stelrad Compact rads. The TRVs are Danfoss RAS-C2 combi.

1. Did the installers balance the system?

If you don't know, a quick check is to find out how far open the Lockshield valves are. (They are the one at opposite end from TRV). On a correctly balanced system the valves will be open less than one to one-and-a-half turns and some may be open only a quarter of a turn or less. Remove the plain cap and turn the spindle clockwise to shut the valve.


2. Are there really TRVs on all rads (apart from towel rail)?

If Yes,
3. Do you have a thermostat on the wall in hall or living room?

If Yes, there should not be a TRV on the stat in that room. You can remedy this by setting the TRV to Max.

The 38CDi produces 10kW to 30kW for heating, and you have 10 rads which will probably produce no more than 15kW. So your boiler is oversized for heating. Presumably it was chosen based just on HW requirements. You should ask the installer to reduce the maximum output so it is closer to the actual output of the rads.

 

[lame plumber]

heat output and flow noise are two separate items with no real effect on each other. if the boiler is short cycling, ie on and off during ch mode all the time then a reduction in output may be required. however flow noise as the trys shut down is more than likely due to 1. no autobypass fitted, 2 trvs in wrong direction, but this just make them clatter normally or 3 the pump speed is too high. my bet would be no auto bypass on the heating circuit.

 

[CM25]

Agreed with lame plumber if that helps

 

[doitmyself]

heat output and flow noise are two separate items with no real effect on each other.

Flow rate is directly proportional to the heat output. Flow noise depends on the velocity, which is inversely proportional to the square of the diameter of pipe. Reducing the heat output will reduce the required flow rate and hence the velocity.

flow noise as the trys shut down is more than likely due to 1. no autobypass fitted, 2 trvs in wrong direction, but this just make them clatter normally or 3 the pump speed is too high.

I agree that 1 and 2 are possibilities. As for 3, the OP's boiler has a modulating pump, which is set, by default, to Proportional Pressure (High). Maybe the Installer forgot to set it to the appropriate setting for the OP's installation.

 

[HTG-Apprentice]

No auto by pass fitted to system, knock a trv off,
or the system isn't balanced properly?

 

[lame plumber]

Flow rate is directly proportional to the heat output. Flow noise depends on the velocity, which is inversely proportional to the square of the diameter of pipe. Reducing the heat output will reduce the required flow rate and hence the velocity..

If you reduce the heat output of the boiler, your rads wont warm as quickly in basic terms, the flowrate of the system will mean it warms up quicker the higher the flow rate, which can in turn make more noise as a water velocity increases. Cant understand why reducing the heat output reduces the flow rate, one is tied in to the burner output the other to the pump speed setting. Most of all the only reason you have a nice big combi is to heat the HW at a bigger flowrate, nothing to do with central heating! which modulates down for central heating automatically as mentioned

 

[thompsonbrown]

system needs balancing. turn of system, let cool, then balance all rads on lockshields. change trv's to rad flow if still noisey.

 

[lame plumber]

system needs balancing. turn of system, let cool, then balance all rads on lockshields. change trv's to rad flow if still noisey.

easier to turn down pump speed first as the water velocity is the causal factor

 

[leethegasman]

Hi all

System: Worcester Bosch 38CDi combi, 10 rads on two floors, all on same circuit, all with TRV except large towel rail in bathroom. System and boiler are new, in 28mm copper with 15mm branches to Stelrad Compact rads. The TRVs are Danfoss RAS-C2 combi.

Question: When a few rads have got turned off by the TRV (room reached temp), the remaining rads get noisy (rushing water sound). The noise is definitely at the TRV. This noise at the rads still on (e.g. ones set to max) increases as more rooms reach their TRV temp and turn off, presumably because the same volume of water is being pushed through fewer rads. The boiler is supposed to have a fully modulating pump (and the pump speed is at standard setting, which installer advised not to adjust), so is this noise at the TRvs, as other rads turn off normal/unavoidable?

Also, another question: I find these TRVs tend to turn off at much lower room temps than Danfoss suggests. e.g. setting IV (highest one before max) is supposed to be about 24 degrees but tends to shut off at more like 17-19 degrees....meaning they aren't very useful.

Thanks
Alex

I think ras-c2 are poss one of the best on the market and always use. but you do need to set the valves up to installation instructions as they use revolver technology real simple to do they can some times be noisy if direction of flow not set on valve.have sent picture if inside of valve if you need copy of instructions for valves let me know and I will send picture.

 

[doitmyself]

If you reduce the heat output of the boiler, your rads wont warm as quickly in basic terms, the flowrate of the system will mean it warms up quicker the higher the flow rate, which can in turn make more noise as a water velocity increases.

If you have 15kW of rads and a 30kW boiler will still get only get 15kW of heat produced by the rads. OK; the boiler will modulate down but there is plenty of evidence that, if the boiler output is much greater than the rad output, the boiler will go start cycling very quickly because the return temperature is higher than expected, so the flow temperature rises faster than the boiler can modulate down. The net effect is that the average water temperature is low, so the house takes a long time to heat up. Reducing the boiler output to match the rad output removes this problem. Why do you think the manufacturers provide a facility to reduce the output?

Can't understand why reducing the heat output reduces the flow rate, one is tied in to the burner output the other to the pump speed setting.

Sorry, my explanation should have been more detailed.

Assuming the same temperature differential, the flow rate of a 30kW boiler will be twice that of a 15kW boiler. So, reducing the output from 30kW to 15kW means that you can run the pump at a slower speed.

Most of all the only reason you have a nice big combi is to heat the HW at a bigger flowrate, nothing to do with central heating! which modulates down for central heating automatically as mentioned

Assuming it can modulate low enough!

 

[leethegasman]