Noisy bypass valve.

Discussion in 'Plumbers' Talk' started by ukdiyboy, Jun 19, 2020.

  1. ukdiyboy

    ukdiyboy Member

    Hi all,

    I've got a noise problem with our heating system which is driving me mad!

    We've been in our house a year now and have had a few problems with the heating system generating a lot of noise. I had put much of it down to a leak in the system; 4 out of my 5 auto bleed valves had positive signs of leakage and having replaced them all the pressure in the system has remained rock solid for the past 6 weeks.

    Unfortunately the heating system continues to be very noisy with a loud vibrating hum coming along the pipes. I've tried to locate exactly where it comes from and think I have identified it as the auto bypass valve. If I have any of the CH zones open and heating then the system is as quiet as I would expect but when they are closed and it is just heating the hot water cylinder then the loud humming noise kicks in. Unfortunately this is exasperated by the fact all the boiler, pumps etc are in the garage immediately blow the main bedrooms, so it wakes us up whenever it comes on.

    The fact that the noise is there when heating the hot water with the CH zones closed and then goes away when they open suggests to me that the pump is over supplying the hot water only circuit. The ABV was would all the way in to .5 bar but even winding it out a bit to around 0.2 bar the noise is still there. Any ideas what might be up? Does that sound like a faulty or just generally noisy ABV that needs changed? Do I need to get someone in to set up the heating system properly?

    My system (broadly) consists of:
    Vaillant EcoTec Plus 428
    Grundfoss UPS 15-60 130 Pump
    Ferroli hot water tank.
    4 separate heating zones, 3x zonal CH and 1x hot water.
     
  2. Mike83

    Mike83 Screwfix Select

    Are there any restrictions on the HW flow or return pipe.
    Even with just the hot water on the bypass shouldn’t be doing much especially if it’s an unvented cylinder.
    When the zone valves close does the pump still put water through the bypass and is there any noise.
     
  3. Heat

    Heat Screwfix Select

    Also consider that the coil inside the unvented hot cylinder might be causing a vibration noise. This does happen on unvented cylinder stainless steel coils.
    If you can’t find the cause of noise I would just replace ABV to test
     
  4. ukdiyboy

    ukdiyboy Member

    Thanks for the replies folks.
    I don't think there are any restrictions in the HW flow or return pipe, but both seem to heat up pretty rapidly, so it doesn't feel like it.
    The pump definitely pumps water through the bypass when the zone valves are closed as the bypass is very warm and with the UPS 15-60 set to two or three I get a good amount of noise - sticking a piece of wood to the ABV I can hear and feel vibration through it (but then I can in the connecting pipes too, just to a much lesser extent). With the pump set on the lowest setting the system is all nice and quiet for the HW only, but according to the performance chart it has less than 2m head on that setting so can't imagine it is going to have the oomph to send the CH water through 5 downstairs and 6 upstairs rads and some towel rails (we have UFH in the kitchen too, but it has its own Grundfos pump to supply that) without being turned up.

    Maybe a variable output pump like a Grundfos Alpha is a better way of matching flow to system requirements rather than relying on the ABV and return?
     
  5. sam spade

    sam spade Active Member

    My previous house had 13 rads and the system ran perfectly with the pump (Grunfos 15-50) on 1. When I moved in, it was on 3. So I rebalanced the system and got it down to 1.

    The heat exchanger in the Vaillant 400 series is designed to work with a 20C temperature drop. If you balance for a 11C drop the hex resistance increases by a factor of 4. So the pump has to be turned up just to get the water through the hex. Increasing the temperature drop does mean that the output of the radiators will be reduced by about 15%, but that shouldn't matter as most rads are oversized.
     
    ukdiyboy likes this.
  6. ukdiyboy

    ukdiyboy Member

    That's interesting that you successfully ran the whole system on the lowest setting. I note that the manual says "Set the pump so that the temperature difference between the flow and return is no more than 20 °C when the maximum flow temperature is set." so are the 400 series designed to work with a 20 deg drop, or designed for 11 ish degrees but have a tolerance for up to 20 deg? I've never checked what the drop is on ours since moving in, so might have to have a look at that.
     
  7. sam spade

    sam spade Active Member

    Old boilers, made of cast iron, were designed for an 11C drop (actually 20F, hence the unusual number) as it was essential to keep the return hghenough to ensure the hex did not disintegrate due to condensation. The channels through the hext were relatively large so the pressure loss was low and only a small pump was required. The hex on a modern condensing boilers has very restricted waterways so the pressure loss is much higher. Here is the pressure loss chart for the 428 boiler.

    ecotec 428.JPG
    As you can see, the loss at full output with a 20C differential is nearly 2.5 metres. If a 10C differential was used, requiring a flow rate of 2440 l/hr the pressure loss would be much higher (estimated 7.5m to 8m), A much larger pump would be required - more expensive to buy and run.

    So yes, you can use a lower differential (mine balanced at 15C). The 20C is a maximum, any greater could stress the hex too much. The actual temperature drop you achive will depend on the pressure loss in the heating system as a whole.

    However, you may have a 28kW boiler, but do you actually use all 28? Eleven rads and a couple of towel rails suggest not, maybe 20kW (My 13 rads totalled 13kW).I suspect the 28kW boiler was sized to include the HW cylinder; but that's not normally necessary - unless everyone baths/showers when the heating is running at max early in the morning. Timing the hot water to come on when heating demand is non-existant or very low, e.g overnight, or setting the system to give hot water priority (combi boilers do this), means that the boiler could be turned down to 20kW or less, requiring a lower flow rate.
     
    ukdiyboy likes this.
  8. ukdiyboy

    ukdiyboy Member

    Sam, thanks for your detailed reply - unfortunately it has really highlighted how much I'm still yet to understand about how these things work! To really show my lack of understanding, how does the auto bypass circuit effect the temp differential? If a differential of 15-20 deg is the way ahead, what happens to that when the CH zone valves are closed and the pump is only supplying the DHW so the auto bypass opens and send a bunch of water near output temp back into the boiler?

    In terms of the boiler rating, no, we certainly don't need 28kw. The previous owners might have when they had three kids, all the rooms in use and hot water used at a rate only teenagers can manage, but it is just the two of us for the time being so our needs will be much lower than 28 KW. What does turning down the boiler to 20 KW do? Would it not modulate down to a figure lower than that when there is little demand?
     
  9. ukdiyboy

    ukdiyboy Member

    Any recommendations for a replacement angled ABV? Any particularly good quality, quiet and reliable? I presume the one on there has lasted 12+ years, so don't mind paying extra for decent one rather than the cheapest no-name brand!
     
  10. sam spade

    sam spade Active Member

    The simple answer is: the boiler will modulate down to maintain the required flow temperature. This will continue until the boiler reaches minimum modulation. If the flow temperature continues to rise, the boiler will start on/off recycling.

    The boiler will modulate down, eventually. But, if you don't need more than 20kW, why ask it to pump out 28kW every time it lights? You can adjust the output using parameter d. 0 (or d.000). The 400 series do have a reputation for not modulating very quickly.
     
  11. ukdiyboy

    ukdiyboy Member

    Thanks for your ongoing help Sam. I will look to lower the output to circa 20KW over the weekend. Hopefully that might also help with the short cycles. I think I also need to do a test with the cylinder thermostat and make sure the call for heat from the cylinder ceases when the requested water temperature is reached, rather than just effectively demanding heat whenever the programmer is timed to be on. I also need to reduce the amount of time it's set to heat, as the previous occupants had it on for 2 hours in the morning and 4 hours odd in the evening which seems somewhat excessive for a decent quality sealed ferroli boiler!

    I need to do some more reading, but sounds like the way ahead ultimately is to rebalance the whole system to give 20 deg C drop across each radiator and then adjust the pump flow rate to ensure that delta is also 20 deg between outflow and inflow?

    Is replacing the standalone pump with a smarter solution like an automatically variable speed pump such as a Grundfos Alpha 3 a better solution than relying on the auto bypass, or is there a risk that it reduces flow below that required by the boiler?
     

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