Is there a built in bypass in Worcester Bosch Greenstar 28cdi Compact ErP?

[alan333]

Hi, Does a Worcester Bosch Greenstar 28cdi Compact ErP LPG combi boiler have a bypass built in for the heating side or do I need a dump radiator?
I've recently moved home and the heating is appalling and expensive. I have some single rads which hardly make any difference to room temp even tho they're hot to touch, and some rads with TRVs and some without etc. I'm considering installing TRVs on the ones without for a starter for now, and come back to fitting replacement rads once we decide where they should go and what size/output they should be.
Thanks

 

[Madrab]

do I need a dump radiator

Isn't such a thing any more in CH design, an auto bypass is now used.

Check the MI, it will tell you in there.

 

[ericmark]

I seem to remember with mothers old house, there was a by-pass inside and the installers had put one outside the boiler, not a clue why. And they had also not fitted a TRV to towel rails and hall radiator, again not a clue why not fitted.

I read instructions and it did seem to say don't fit a TRV in room with the wall thermostat, but it also said fit the wall thermostat in a room without any independent heating, with no outside doors, and on lower floor, and there was simply no such room or area.

So I fitted a TRV onto the hall radiator and the heating was transformed, OK took some setting up, but when front door opened the heating would trigger, and hall radiator heat up, but before it switched off the wall thermostat the TRV would turn down the hall radiator giving rest of house chance to heat up.

However tried the same with this house, and it has not really worked, as to if just lucky with mothers, or if with non modulating boiler that idea does not work, I don't know, but considering getting a wall thermostat which can link to the TRV's, I thought the one I have would, a Nest Gen 3, but no, it seems the support for Energenie MiHome TRV's has been removed.

I have 9 electronic TRV heads, but non link to the wall thermostat, I have looked at the likes of Hive, Wiser, and EvoHome, but my boiler does not modulate, and I wonder if they will be any better?

I got the 4 Energenie MiHome TRV's when in mothers house, new owners did not what them, so they came here, other 5 are eqiva eQ-3 which were far cheaper, (£15 each in 2019) and seem to work better. But only show target not current temperature.

But you need an end plan, OK mine did not work, but if you get at least 2 TRV heads which will link to a hub/thermostat then you can add the thermostat latter.

I think my problem here is micro bore piping and the lock shield valves are not really required, as the pipes have enough resistance to flow, but mothers was the reverse, the 15 mm pipes were sending hot water back to boiler and the boiler was modulating, when some rooms still cool. Setting the lock shield valve seemed to be the key in her house.

I thought I could set times to help here, idea was on returning home, first kitchen, then dinning room, then living room, and finally bedrooms would heat up, however the radiators are not big enough to sink the 19 kW output of my boiler, so my theory did not work, also major problem getting my wife to close doors, last house was open plan, and she can't get the idea of doors are for closing into her head.

 

[alan333]

Isn't such a thing any more in CH design, an auto bypass is now used.

Check the MI, it will tell you in there.

Thanks for your reply mate. I've had a read of the instructions per your recommendation, they state:

"The boiler is equipped with an internal by-pass.
The internal bypass is not intended to be a substitute for an external system bypass.
An external automatic bypass should be used if the system flow can be significantly adjusted or stopped by zone valves and thermostatic radiator valves (TRV)"
They also say that TRVs should be fitted to all rads except bathrooms and the room with the thermostat.

My theory is to have TRVs everywhere, with a Hive thermostat which moves rooms (office daytime, lounge evenings etc). Ensure the TRV on full in whichever room the thermostat is in so the heating will turn off when not needed, and we can have other rooms cooler by TRV settings. This would go against the instructions tho, is it a really bad idea?

 

[alan333]

I seem to remember with mothers old house, there was a by-pass inside and the installers had put one outside the boiler, not a clue why. And they had also not fitted a TRV to towel rails and hall radiator, again not a clue why not fitted.

I read instructions and it did seem to say don't fit a TRV in room with the wall thermostat, but it also said fit the wall thermostat in a room without any independent heating, with no outside doors, and on lower floor, and there was simply no such room or area.

So I fitted a TRV onto the hall radiator and the heating was transformed, OK took some setting up, but when front door opened the heating would trigger, and hall radiator heat up, but before it switched off the wall thermostat the TRV would turn down the hall radiator giving rest of house chance to heat up.

However tried the same with this house, and it has not really worked, as to if just lucky with mothers, or if with non modulating boiler that idea does not work, I don't know, but considering getting a wall thermostat which can link to the TRV's, I thought the one I have would, a Nest Gen 3, but no, it seems the support for Energenie MiHome TRV's has been removed.

I have 9 electronic TRV heads, but non link to the wall thermostat, I have looked at the likes of Hive, Wiser, and EvoHome, but my boiler does not modulate, and I wonder if they will be any better?

I got the 4 Energenie MiHome TRV's when in mothers house, new owners did not what them, so they came here, other 5 are eqiva eQ-3 which were far cheaper, (£15 each in 2019) and seem to work better. But only show target not current temperature.

But you need an end plan, OK mine did not work, but if you get at least 2 TRV heads which will link to a hub/thermostat then you can add the thermostat latter.

I think my problem here is micro bore piping and the lock shield valves are not really required, as the pipes have enough resistance to flow, but mothers was the reverse, the 15 mm pipes were sending hot water back to boiler and the boiler was modulating, when some rooms still cool. Setting the lock shield valve seemed to be the key in her house.

I thought I could set times to help here, idea was on returning home, first kitchen, then dinning room, then living room, and finally bedrooms would heat up, however the radiators are not big enough to sink the 19 kW output of my boiler, so my theory did not work, also major problem getting my wife to close doors, last house was open plan, and she can't get the idea of doors are for closing into her head.

Thanks for your reply
I think my end game is somewhere along the same lines as you. We currently have Hive and I'm considering their smart TRVs so there would be less walking around the house turning this rad off and that one on every morning/evening.

 

[bsr]

Hi

The idea of the bypass is that if all the TRVs close, the bypass opens to allow some water to circulate so the boiler doesn't overheat and boil.

Therefore you want an external bypass at some distance from the boiler so that the water can circulate and dump some heat.

If the bypass is internal then the water will still get pumped round and round the heat exchanger so I would be concerned about overheating. Therefore I would look to maintain an external bypass.

 

[vulcancontinental]

Yes, the internal bypass is a boiler protection device to ensure there is always an adequate flow across the heat exchanger. Frankly the boiler manufacturer has little control on what happens outside the product but generally one radiator of decent size with only lockshield valve control preset will stop annoying whistling on a system otherwise complete with TRV's. Not as good as a self adjusting and correctly adjusted bypass device but adequate.

 

[alan333]

Thanks for all your help guys, it seems I'd be best with an external bypass fitted. For now I'll probably fit TRVs to all except one rad, and get a bypass fitted when we upgrade/add radiators.
Many thanks again

 

[Madrab]

My theory is to have TRVs everywhere, with a Hive thermostat which moves rooms (office daytime, lounge evenings etc). Ensure the TRV on full in whichever room the thermostat is in so the heating will turn off when not needed, and we can have other rooms cooler by TRV settings. This would go against the instructions tho, is it a really bad idea?

What instructions?? As the Hive is a smart control then yes you need to work with it to understand how it works best for you. Always have the room where the stat is located on full otherwise they can fight against reach other, usually ending up with the boiler cycling and therefore wasting energy. TRV's can't turn the boiler off but the stat can keep the boiler on even when the TRV in that room has shut the rad down.

An external bypass, properly calibrated, will ensure that the pump and main HEX are looked after as much as possible. Of course a radiator with only lockshields will also act as a bypass but they stopped advising that as an acceptable option as there is still the option to be able to shut it down therefore effectively removing it. An ABV removes that ability.

 

[ianmcd]

What instructions?? As the Hive is a smart control then yes you need to work with it to understand how it works best for you. Always have the room where the stat is located on full otherwise they can fight against reach other, usually ending up with the boiler cycling and therefore wasting energy. TRV's can't turn the boiler off but the stat can keep the boiler on even when the TRV in that room has shut the rad down.

An external bypass, properly calibrated, will ensure that the pump and main HEX are looked after as much as possible. Of course a radiator with only lockshields will also act as a bypass but they stopped advising that as an acceptable option as there is still the option to be able to shut it down therefore effectively removing it. An ABV removes that ability.

Although I completely agree with everything stated by @Madrab , this is what current regs stipulate, my opinion and simply that, when you have an ABV that circulates heat straight back to the boiler, of course it works, but my own personal opinion is this heat is better used in domestic situations as a towel rad or similar where the excess heat is used rather than just dissipated in pipework, every woman loves warm towels

 

[ericmark]

Always have the room where the stat is located on full otherwise they can fight against reach other, usually ending up with the boiler cycling and therefore wasting energy.

I must question that, purely down to mothers house, I stuck with no TRV in the hall and struggled to get the heating to work as required, then I decided to take bull by the horns and fit a TRV in the hall, same room as the wall thermostat.

The transformation was huge, opened up the lock shield a little so radiator could heat up a little faster, but what you need to remember is a mechanical TRV has an operating temperature range of 2ºC or more, where the wall thermostat is likely only 0.5ºC between on and off, not used a modulating thermostat so can't say how they work.

So the scenario, front door opened, hall cools down, in her wheel chair it would likely take me 5 minutes to get her in/out of the house, but same applies when unloading a car. So the TRV and wall thermostat turn on, but before the wall thermostat opens, the TRV starts to close, so hall heats up fast from 10ºC to 17ºC, but then it slows down as the TRV starts to close, this stops the boiler switching off prematurely, and allows it to heat rest of house, and in the heart of winter boiler will never switch off.

So jump to how the boiler works, each manufacturer has a different algorithm, so it will vary, but whole idea is the boiler measures the temperature of the return water, and the hotter the return water is, the more the boiler modulates, (turns down) so as each TRV starts to close, first the water is diverted to radiators where the TRV is still open, and then it will start to lift the by-pass valve, so hot water is returned to boiler so boiler turns down more.

Until that is the boiler reaches minimum output, now the only way to reduce the output further is to use a mark/space ratio, and slowly the mark time decreases, and the space time increases, but the important thing is when the boiler restarts, it does so at minimum output, so most economic point, however there is a problem with this, it can never turn the boiler off fully, as unless the water is circulated the boiler can't know when no longer needed.

So some where we need a wall on/off thermostat so when summer arrives, the boiler will switch fully off, also we tend to want the home cooled at some parts of the day to others, so if we want the boiler to only heat home to 17ºC at night, 19ºC during the day, and 21ºC in the evening, the wall thermostat and TRV need to both change.

Also the TRV is often mounted lower than the wall thermostat, so in winter my TRV is reporting 16ºC (just read it) and the wall thermostat 17.5ºC (again just read it) on a cold day there is more of a difference, at moment 15.9ºC outside.

So to have different temperatures through the day, both the TRV and the wall thermostat need to be programmable, or the TRV needs to connect to the wall thermostat.

Now Hive is an odd one out, after 22ºC it stops accepting demands for heat from the TRV's, so the wall thermostat needs to be in a room kept normally cool, this actually helps, as it means it can switch off the heating earlier as summer arrives, if the room with wall thermostat is normally kept at 17ºC then the heating is turned off earlier when we are likely to get a warm day.

However without linking it is hard to set the TRV and wall thermostat to work with each other, specially if using an electronic TRV so the slewing range (difference between fully open temperature and fully closed temperature) is smaller, and also the response time is important, my electronic TRV when it exercises at mid day every Saturday takes around 3.5 minutes to complete. Even longer to respond to a temperature change, like sun coming out and heating room through windows, so the lock shield needs setting so the radiator does not suddenly heat up, without the TRV having time to respond.

Some of my electronic TRV's show both target and current, so if the current during winter exceeds the target, then the lock shield needs closing a bit, and in mothers house once set, each room was spot on, except where the sun through bay windows caused the temperature to raise.

With linked TRV's this is not so much of a problem, as does not matter what temperature the wall thermostat is set at, but in winter only time boiler should switch off is when there is a change of target temperature, the whole idea of a modulating boiler is it does not keep switching on/off, until it can't turn down any more.

If a wall thermostat unless a modulating type also, turns off the boiler, then when the boiler is turned on again it does so flat out in most cases, doing this three times a day when there is a target temperature change is no real problem, but doing this 24 times a day wastes gas.

OK I know there is a thought that switching off the pump saves energy, but unlikely to be as much as allowing the boiler to modulate as it is designed to do.

 

[Madrab]

The way I look at it is - if there are 2 controls in the same space, one that controls its localised temp (TRV) and the other that is a global control (Stat) then if the local control shuts down first before the global control is satisfied then the global control will keep calling for heat. It is a long established premise that wherever the stat (hard wired) was located the radiator should never have a TRV fitted, more often than not that would be in the hall. Invariably the hall also had an upstairs and is always the last space to reach the set temp, with all the other rooms have TRV's to allow that room to be shut off once it's up to it desired temp. If every other TRV is up to temp and shut down and the global control is still calling for heat then the boiler will modulate down till it reaches set temp, shut off but keep the pump running circulating that hot water around the system until it cools enough for the boiler stat to fire it back up again. That cycle continues until the global control stops calling for heat, that could continue indefinitely if the global control is never satisfied. Not very efficient at all.

Of course, each system would be tailored to it's own specific requirements as is obviously the case for you or when the system design moves away from a simple setup and becomes much smarter with individual zone control (smart TRV's), weather comp, mixed rad/UFH, etc.

Ultimately though as a design standard then TRV's wouldn't be located on radiator(s) in the same space where there a room thermostat controlling the boiler is located.