Bypass valves for TRVs

[AndyDavis]

I'm having TRVs (Honeywell VT200) fitted to all my radiators, been told I need to use automatic bypass valves to maintain constant pressure across the system. Can anyone tell me what its for, doing and do I need one?

Also, is it a good idea to have TRVs on all radiators, or just the ones upstairs?

 

[ChrisR]

First , what boiler? How many motorised valves? how many rads?

 

[AndyDavis]

The boiler is a condensing combi (Baxi 133HE Plus) and theres about 15 rads (8 downstairs, 7 upstairs). I'm not fitting a motorised valve, unless theres one in the boiler.

 

[ChrisR]

I don't have the MI handy on that one - you should so have a look and see what it says. If it needs one it MUST have one fitted. Some of us plumbers who think about things put them at the far end of the system, not by the boiler/pump. I'm sure you can work out why!

 

[corgiman]

if you want trv's on every rad you MUST fit a bypass, if you were to leave the one in them same location as the boiler interlock you DONT need one


Section 6.2 Of the MI

 

[Steve Hart]

I'd be interested if ChrisR could expand upon what he means please. Not sure I can work out why to locate an auto bypass at the far end of the system makes more sense? I too am in the process of replacing a lot of my rad valves with TRVs. My boiler instructions say fit an auto bypass OR a rad with no TRV to ensure a clear path for boiler over-run, whereas the instructions with the Peglar TRVs says fit an auto bypass if using TRVs in a system. As I understand it as the system gets up to temp and the TRVs start to close down the auto bypass opens. Not sure exactly what function this is performing though? If the TVRs start to close surely the water is flowing around the system even faster because not as much is being fed to the rads so what purpose does the auto by-pass provide?
Cheers
Steve

 

[htgeng]

I don't religiously install bypasses more than two metres from boiler itself. More down to practicality.

You have to dissipate a certain amount of heat otherwise the boiler will be happy. A bypass fitted on the other side of the house will give a much longer path. Although in practice I think its hardly an issue since the boilers I install have already modulated right down by the time the call for heat has gone.

 

[corgiman]

The purpose of a bypass is to ensure that there is circulation available to for the boiler pump overrun (the pump remains running for a period of time once thermostats, programmer are satisfied repeated boiler fire up which shortens the life expectancy of the boiler and is very efficient on fuel usage) the main reason it is best practice to install the bypass as far away from the boiler as practicable is to dissipate as much heat from the boilers return as possible.

An automatic bypass, for want of a better way to explain it, sense the resistance within the system caused by the pump straining against a closed circuit and opens accordingly, this is more cost efficient than have a gatevalve or such like permanently open as this will cause a heat sink whilst all thermostats are demanding

It is also good practice to install an automatic bypass on a system even when one radiator has no TRV's install on it ass this radiator can be, and often is, turned off by the end user.


A TRV is not a boiler inter lock as it has no way of informing the boiler that a satisfactory heat has been attained and it no longer needs to fire up, in theory with the programmer calling for central heating on a hot day when all the TRV's are closed the3se boiler will fire up for central heating, it goes with out saying that this is a waste of fuel.

On a conventional system (y or S plan) the auto matic bypass is sited on the flow between the boiler and the zone valve(s) and the primary flow to achieve the above.

It is my opinion that an automatic bypass should be fitted in the appropriate place on ALL modern boilers.

i hope this explains it ok but am sure everyone else will be all over me like a bad suit disagreeing

 

[croydoncorgi]

What nobody has said yet is WHY you need a bypass - fixed or (better) automatic spring type.

The problem is that many modern boilers have 'low water content' and not much 'thermal mass'. When the demand for heat is satisfied (zone valve(s) close) the burner will switch off but there will still be a lot of heat left in the boiler. If the pump stops immediately or runs against closed valves, the boiler will boil! Pump overrun is a feature of many boilers in which the pump is powered from the boiler and continues to run after the burner has been turned off until the boiler has cooled down (a couple of minutes usually).

TRVs present a completely different problem which is NOT necessarily overrun-related. On a badly designed system (which, incidentally, will not comply with latest Building Regs) there is often no 'master' room thermostat to control the boiler. So it cycles on and off controlled by its own internal thermostat and if all the TRVs are satisfied and therefore shut it will potentially be running its burner with no (or very little) water flow. Unhappy boiler.

For maximum economy and a happy boiler, there should be a room thermostat AS WELL AS TRVs, AND a bypass arrangement to provide circulation to cool the boiler on overrun.

 

[ChrisR]

On a conventional system (y or S plan) the auto matic bypass is sited on the flow between the boiler and the zone valve(s) and the primary flow to achieve the above.

Now what the drivel are you driving at? Perhaps you mean ".. and the primary RETURN to achieve ..."?


Nope, none of you guessed what I was on about - bypass with trv's is supposed to stop them making too much noise . Which is where this thread started. Not the same as the need for a bypass for dissipating heat. Problem if you put the automatic (ie preset to some "head" level) ABV near the boiler/pump is that you limit the effect of the pump. If you set your ABV to say 3 metres, you can only get a limited amount of heat out of the boiler. Can't be bothered to do the sums but it'll be around 50,000BTU/hr I expect. But if you put the ABV at the far end of the run, it will only open when most everything else is shut. When all the rads are OPEN the flow resistance in getting to the ABV would be so high that it wouldn't have 3m head across it. So your pump can work to its full capacity. It's only when the pressure goes up and there isn't much flow, that the abv opens.



Maybe nobody answered Steve's question:
"As I understand it as the system gets up to temp and the TRVs start to close down the auto bypass opens. Not sure exactly what function this is performing though? If the TVRs start to close surely the water is flowing around the system even faster because not as much is being fed to the rads so what purpose does the auto by-pass provide? "

Ye probleme, is that if just one trv say is a wee bit open and all the rest are shut, the full head (pressure) of the pump will be forcing water fast through that trv - and it'll make a song and dance about it as it closes. Valve closes, pressure goes up so valve opens and pressure goes down, up, down = a noise. If there's another path the water can go through that so the oscillation is damped. I wonder if the damping effect is more important than the pressure-reducing one.

 

[corgiman]

Well ChrisR there is no need to be rude?

The drivel I was on about was connecting a ABV between the primary flow and return (that was a type O I grant you)

And reading your post I feel like we are all barking up the right tree but from different sides

so we are all right

 

[croydoncorgi]

At risk of seeming to put on anorak and get out numbers book....

Noise from TRVs can be a problem when they're mostly shut but I've never heard that the requirement for a bypass was PRIMARILY noise reduction.

ANY low-water-content boiler will get upset if water flow stops whilst it's still hot. That's why some manufacturers put a bypass inside the boiler (although obviously it will be short and not as effective at dissipating heat as an external one). If you have no bypass, how will pump overrun actually achieve anything? If TRV noise was the only problem, we'd all select constant-head pumps from that Nice Mr Grundfos!

 

[evdama]

Reading all this from the dark edges of the arena, for my own info ;

The pump overrun is managed by which device?

 

[bripl]

normally the boiler but a pump over run can be run by other controls depending on size or controls for example relays in a panel.a auto bypass is primarily used to dissapate the heat when controls ie trv cly stat have reached temp hence the need for boiler interlock to conserve energy,and yes it can help reduce noise on trvs but its main function is dissapate heat

 

[blahblah]

Is this not a new (or nearly new) boiler which should therefor be fitted in accordance to part L? So you shouldn't have trvs on ALL rads. (Did you mean 'all'?) Do baxi combis not have a built in bypass? I haven't checked but surely a look at MI's will answer all.