BYPASS

[SortedPlumbing]

Just with the automatic bypass valves i take it that the higher the number the greater the flow of water through it?
haha silly Q but just want to make sure!

the boiler is only a couple of weeks old and installer isnt going back so some how or other i have been roped into this job.

system all has Trvs and heats up fairly quickly. about 13/14 rads, customer says installer flushed it but i cant take her word for it!

i have reduced the gas rates so that system would take longer to heat up but this hasnt solved prob.

 

[Gasguru]

Just with the automatic bypass valves i take it that the higher the number the greater the flow of water through it?

No. The higher the number the stronger the spring pressure on the valve seat so a higher water pressure is required to open the valve.

ABVs such as the Honey DU145 only need to be set just up from the minimum setting on domestic installations with 5 or 6 meter pumps.

TRV's should not be fitted to all radiators unless the boiler incorporates a flowswitch (eg. some horrible Pottertons).

What boiler make/model?

 

[SortedPlumbing]

ravenheat 150 low nox

 

[gas4you]

I always use the Honeywell and usually find it needs setting on 3 -3.5 for most standard systems.

 

[Agile]

TRV's should not be fitted to all radiators unless the boiler incorporates a flowswitch (eg. some horrible Pottertons).

Surely Geoff, that does not apply when an auto bypass has been fitted?

Tony

 

[D_Hailsham]

TRV's should not be fitted to all radiators unless the boiler incorporates a flowswitch (eg. some horrible Pottertons).

Surely Geoff, that does not apply when an auto bypass has been fitted?

I don't understand your reasoning

An auto bypass does not act as an interlock. All it does is to redirect some of the flow back through the boiler when the head increases in the radiator circuit due to the closing of the TRVs; thus ensuring that the flow rate through the boiler is correct.

 

[Agile]

The energy efficiency requirement for an auto bypass is to allow all the rads to be fitted with TRVs rather than leaving one permanently on to ensure some flow through the boiler.

I agree that that does not sit well with the requirement to have no TRV in the room with the room stat.

But then I dont write the regulations, I just try to abide by them!

Tony

 

[D_Hailsham]

The energy efficiency requirement for an auto bypass is to allow all the rads to be fitted with TRVs rather than leaving one permanently on to ensure some flow through the boiler.

Where does it say that?

There is nothing to that effect in either the Domestic Heating Compliance Guide, CHeSS 2005, Domestic Heating by Gas: Boiler Systems (CE30) or the Good Practice Guide for Controls.

 

[Gasguru]

TRV's should not be fitted to all radiators unless the boiler incorporates a flowswitch (eg. some horrible Pottertons).

Surely Geoff, that does not apply when an auto bypass has been fitted?

Tony

I can't believe you said that but then at 2.35am perhaps you meant otherwise.

Auto bypass's are specified by TRV manufacturers to reduce noise due to high differential pressures across the valve seatings (especially as TRV's succesivly shut down). In addition many boilers require external ABV's (especially on S plan with pump overun) and to maintain minimum flow rates for the sensors to operate and prevent kettling.

The room stat requirement has been good practice for 30 years (for quality installers) and results in a longer lasting more efficient system. It is also a legal requirement if the boiler does not incorporate a flowswitch.

My view is that Potterton screwed up the design (nothing new there) with their boilers that include flowswitches. It was a blatant atttempt to win boiler sales by removing the requirement for a boiler interlock in the form of a room stat. We all know that many systems (most flats etc) do not have room stats fitted so a significant additional cost is involved for a fully compliant boiler replacement ie wiring + stat or wireless stat.

Pottertons combis are fitted with single speed 6m pumps. Having TRVs on all the rads will result in very high differential pressures across the valves especially on small systems (eg 3 or 4 rads in a flat) where it is quite likely that only one radiator will be on at times. Of course a fully modulating pump should have been fitted in the boiler but this would be at cconsiderable extra boiler cost. Bean counters shave pennies of a boiler cost....they would never agree to adding a tenner for a modulating pump to do the job properly. AFAIR Potterton conveniently forget to mention a modulating pump might be in order for conventional heat only boilers.

Of course this is immaterial since many Potterton Promax's and the other junk with flowswitches will be swapped out not long after installation when the owners are sick of the continual breakdowns.

 

[Agile]

OK Geoff, if the ABV is not required to enable TRVs to be fitted to all the rads then:-

Why are they necessary when the traditional bodge of having no TRV on the bathroom rad maintains a flow at all times through that rad at a saving of one TRV and an ABV ?

Why is an ABV now required?

Tony

 

[Gasguru]

I'm assuming the intentions of Part L were to have TRVs fitted to ALL radiators (except the uncontrolled rad coupled with the room stat), hence the requirement for the ABV to reduce flow pressures with standard fixed speed pumps. Most systems before Part L would probably only have TRVs on bedroom rads ie there is more likelyhood that several rads will still remain open so pump flow pressure would stay at a lower rate across the TRV'd rads.

However surely a fixed bypass (gate valve etc) effectively de-rates the boiler even though there may be a full heat load requirement by the radiator circuit. An auto bypass can remain closed allowing full boiler output to the rads when necessary but opening up as the heat load diminishes (as TRVs close) providing sufficient flow through the primary heat ex. to avoid kettling, adequate temperature sensing and reduced diff. pressure across TRVs.

It's not perfect system, perhaps the ultimate solution (within reason) would be a modulating pump (sensing load) coupled to a boiler with a very high turndown ratio with both flow and return temperature sensing. Currently T.D ratios of only 3 or perhaps 4 at best are available which is hopeless on high output combis installed in the average flat or semi. Several types of burner can cope with T.D ratios of 10, perhaps we are waiting for a proper hot wire/gas injection systems rather than clunky gas valves.

 

[Agile]

I'm assuming the intentions of Part L were to have TRVs fitted to ALL radiators (except the uncontrolled rad coupled with the room stat), hence the requirement for the ABV to reduce flow pressures with standard fixed speed pumps.

Yes, exactly! Thats why I said "Surely Geoff, that does not apply when an auto bypass has been fitted?" after you said "TRV's should not be fitted to all radiators unless the boiler incorporates a flowswitch (eg. some horrible Pottertons)."

There are a few boilers which do modulate the pump although most seem to use a standard pump and a triac control.

Tony

 

[D_Hailsham]

I'm assuming the intentions of Part L were to have TRVs fitted to ALL radiators (except the uncontrolled rad coupled with the room stat)

Part L says nothing about TRVs. All it says is:

38. Reasonable Provision for the performance of heating and hot water system(s) would be

• the use of an appliance with an efficiency not less than that recommended for its type in the Domestic Heating Compliance Guide; and
• the provision of controls that meet the minimum control requirements as given in the Domestic Heating Compliance Guide for the particular type of appliance and heat distribution system.

All the DHC Guide asks for is (Table 2):

• Dwellings up to 150M² should have two space heating zones with independent temperature control, one of which should be the living area
• Dwellings over 150M² should have two space heating zones with separate timing and temperature controls
• Separate temperature control of zones within the dwelling should be provided using:

1. room thermostats or programmable room thermostats in all zones; or
2. a room thermostat or programmable room thermostat in the main zone and individual radiator controls such as TRVs on all radiators in the other zones; or
3. a combination of 1 and 2.

Nowhere does it envisage TRVs on all radiators. It even warns: "The use of TRVs alone does not provide interlock"

The instructions for the Danfoss AVDO Valve are worth reading as they explain the different ways an ABV can be installed.

 

[doitall]

The energy efficiency requirement for an auto bypass is to allow all the rads to be fitted with TRVs rather than leaving one permanently on to ensure some flow through the boiler.

Where does it say that?

There is nothing to that effect in either the Domestic Heating Compliance Guide, CHeSS 2005, Domestic Heating by Gas: Boiler Systems (CE30) or the Good Practice Guide for Controls.

Best practice CE 30 page 18.

The use of an automatic bypass is important when the system includes a large number of TRV's blah, blah, maintaining the appropriate water flow through the boiler. It is also likely to reduce noise in the system caused by excess water velocity.

Not in the book.

It also stops TRV valve hammer due to excess pressure differential.

 

[D_Hailsham]

Best practice CE 30 page 18.

The use of an automatic bypass is important when the system includes a large number of TRV's blah, blah, maintaining the appropriate water flow through the boiler. It is also likely to reduce noise in the system caused by excess water velocity.

But that is not the same as saying:

The energy efficiency requirement for an auto bypass is to allow all the rads to be fitted with TRVs rather than leaving one permanently on to ensure some flow through the boiler.