Central heating bypass circuit

I'm in the throes of (slowly) fitting a new central heating system. The house is a 2 bed terrace in North London. The boiler is a Keston Celsius with the system kit and is located in the first floor bathroom. I've removed the middle section of the chimmey breast to about head height to create a cupboard and run the flue and intake up the chimmey. I've got seven rads sized according to the Myson Heatloss calculator (although I've reduced the size of the kitchen rad as the program can't take account of all the cupboard volume). The two bedroom rads will have thermostatic valves but the main living area (living room/dining room) will be controlled by a room thermostat. I'm setting it up as an S-plan with a Landis & Staefa RWB9 programmer set for 5+2 operation.
In any case I was looking around the Honeywell site as a result of another thread (Central Heating - Controls) and I noticed this:-
"If a bypass circuit is fitted, an automatic bypass valve must be used. (Slumber or Bypass radiators are only acceptable on solid fuel systems)."
on this page:- http://content.honeywell.com/uk/homes/Regulations.htm
I was intending to use the bathroom rad with a couple of lockshield valves as the bypass but it now seems I'll have to scrub that. :-( Is that correct?
Looking at the BES site (thanks Ed and why isn't it in the FAQ?), is the Differential By-Pass Valve (12161) the approiate part?
Looking at the diagrams the bypass valve should be fitted from the flow to the return between the boiler and the zone valves. But according to the Keston installation instructions the flow/return temperature differential should be 10 to 15 C. In the layout I was planning the zone valves are only about a meter from the boiler. I can't see that short run of pipework producing that much of a differential. Any thoughts?
Any other thoughts/observations would be welcome.
Thanks
Paul
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it will only open when all (or most of) the TRVs on the system rads have closed and are up to temperature. When the bypass operates, it stops the boiler seeing and 'open circuit' giving rise to pump and boiling problems. When the boiler sees the return water rising in temperature, it will modulate down and eventually turn off, which is what we want as the house has reached the desired temperature. It will restart again when the temp drops.
There is nothing to stop you having a towel rail on bypass and an automatic bypass in parallel.
I experienced a problem (with a near identical system to yours) where the bypass was close to the boiler and would tend to 'pop off' rather than gently increase in flow as it was coming into operation. This resulted in a nasty, but slow, pressure oscillation when the system was running on bypass. To cut a long story short, I chose to limit the capacity of the bypass by placing a 15mm lockshield valve in series with it. This was enough to damp the oscillation.
HTH
--
fred

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<SNIP>

I've now mailed and spoken to Keston and I'm still not clear about how to deal with this issue. What is normal practice since Part L came in? Are installers simply ignoring the boilers requirement for a temperature drop across the by-pass circuit? Or are they creating an (artificially) larger loop for the by-pass circuit? Or are they still using a rad as a by-pass?
Thanks for your responses.
Paul
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bypass not coming into operation at all when the heating is at full demand. Its purpose is to provide an uninterrupted water path _only_ when the TRVs on the rads are shutting down, which is a transient state just before the boiler says 'right, the house is hot enough, I'm shutting down', so don't worry about it. The absence of a temp drop across the bypass will tell the boiler that it is time to shut down.
It is actually more complex than that as at first the bypass will come on a little and the return flow will be a mixture of rad return water (at low temp) and flow water ((hot) from the bypass). This will have the effect of increasing the return temp a little, which will cause the boiler to throttle back a bit. This is not a fault or a contravention of regs 'cos it is the normal control loop coming into play and (eventually) shutting down the boiler when the house is up to temp.
Hope that makes it a little clearer.
--
fred

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writes

The main radiators in any house are in the hallways or reception rooms (front lounge) so this radiator should, in actual fact, be your by-pass loop. The proper thing to do with any heating system, is to place a thermostat in the same room or area as the by-pass radiator. This way the system is actually controlled by the loop and not from any radiator willy nilly telling the system that it is to hot and should close down everything.
TRV's are there to tell the radiator that it is fitted to, that the area that it is heating is up to temperature, that way each room, or valve controlled zone, because that is what it literally becomes when you install a TRV, will independently control its own temperature.
The main area of the house needs as much heat as it can to stop draughts and colder air moving from room to room when doors or windows are opened, so the radiator installed in the main area should be controlling the boiler and other main parts of the system through an independent thermostat fitted at the opposite end of the hallway from the radiator. That way the whole room is up to the correct temperature setting and is now helping to keep the whole house warm. No one actually sits in the hallway, but it is an integral part to keeping the whole house heated.
I still think that underfloor heating is the best way to control the temperature in a house. The heat is actually spread more evenly throughout each room, and when a 'stat is installed in the main area of the house, then you know that each area is at a comfortable and liveable heat.
So if you think of what the manufacturer is saying about a drop in temperature over the by-pass loop, and think of the size of radiator you've fitted in the main area of the house, then I think you'll have spotted where the loop should be made. Along with a room 'stat if you want even better control.
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I'm a bit confused by this. If the bypass valve is normally shut your primary rad will be mostly out of action until such time as all the other rooms are up to temp as specified by the TRV's. Because the room that the primary rad is in is still cold the boiler still has a heat demand which then flows through the bypass to heat that area until the room stat says ok and shuts the boiler down?
Correct? What actually causes the bypass valve to function? The Heating and Hot Water Guide to Part L states that:
"If the boiler manufacturers instructions advise that a bypass is to be fitted, as a requirement ot the new boiler, then an automatic bypass valve must be used in conjunction with any requirements for a minimum pipe length specified in the manufacturers instructions."
Now if Keston specify an explicit temperature gradient for the loop then it implies that whatever length of pipe for thet to happen is necessary. It also doesn't specify that a rad cannot be used to achieve this. However, if the bypass isn't normally an open loop now then there is a potentially large functional loss to the system
I'm concerned by this as I am about to replace my boiler with a Celcius 25 and convert to a fully pumped system <probably W plan> with an open loop to feed towel rads as a bypass. It seems I may not effectively be able to do this any more and will be in the same boat as the otehr Paul.
cheers
Paul
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A by-pass loop is totally different from a by-pass valve. An unrestricted flow of heated water from the boiler must be allowed to travel through at least one radiator circuit to prevent the boiler from over heating in the event that a fault occurs. The manufacturer states that a minimum temperature loss must be lost in that circuit to make sure the boiler doesn't have water it has just heated, flowing straight back into it. The unrestricted loop is for your safety and in that case, it is in your own interest to have the system fitted correctly.
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But your interpretation bans the use of zone valves. When the room thermostat is satisfied and the zone valve closes, where's your bypass loop now? It needs some sort of automatic bypass loop to take the water as the boiler pump overrun happens. Losing 10-15C over an automatic bypass loop does seem somewhat inachievable, and probably illegal. Of course, the boiler should have shut down as the zone valves started to move, so it shouldn't be adding more heat to the circuit.
Part L does not allow any heating appliance to be uncontrolled. Every heat outlet has to have either a TRV or thermally controlled zone valve in its circuit, so can't be relied on as a bypass.
Christian.
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Thinking about it, it could be achieved by using the automatic bypass valve to bypass a zone valve. You could use the hot water cylinder circuit (unless it is unvented) or a heating circuit (assuming there is at least one radiator physically locked on with no user controls).
Christian.
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Yes !!! Someone's got the idea.
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Pain in the butt though.... If it's just 4 metres it's not so bad; I've just got visions of this stuffing great pipe looping it's way round the house to burn off the required heat...
;->
Paul
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But if there is a stuffing big radiator within the loop, then heat is burned off easily and then the pipework doesn't have to be all that long. All a manufacturer asks is for you to make an unrestricted loop of some kind, which will dissipate a required amount of heat so that if a fault occurs on the boiler itself, the heated water is not circulating directly back into the boiler to quickly and is loosing a required amount of heat to stop it steam venting or splitting through the build up of pressure. I've actually seen it done with a heated towel rail on the other side of a wall from the boiler and no more than four feet away in the actual run of pipe, but it was enough to satisfy the requirements for that system.
Because it asks for a loop, the loop itself doesn't have to be a loop of pipework running all around the house. All it is, is an unrestricted circuit which will dissipate the heat required and do it by what ever means possible for the situation. A valve doesn't have to be used either, as a normal open unrestricted loop will more than usually suffice for a domestic system.
How I was taught to do it was, place a circuit on the flow and return pipe, but make it so that, that one particular circuit heated when a call for heat request was given to the boiler even if the motorised valve or pump failed to activate. The easiest way to do this, is to install a three port valve that when de-energised, defaults to room heating closed and water heater open. This makes the circulating coil for the hot water storage tank your unrestricted loop. It is the perfect unrestricted loop between flow and return with a perfect medium for dissipating heat from the boiler. Yes, you got it. The "Water" in the storage tank.
Tell me how many times you've turned a conventional central heating system on and not wanted it to heat the water you use at the sink or in the bath. You've probably wanted hot water without heating the radiators in the house, so you would fit a motorised valve to the radiator circuit to stop the flow from circulating around the house, which is what it is supposed to do. But I'd really like a show of hands on how many people call for room heating and don't want the hot water tank heated to go with it.
The thermostat on the hot water storage tank is fixed in line with the three port motorised valve and when it call for heat when only hot water is required, then the valve is already in the default open position for that circuit, so the valve doesn't have to move at all. When a call for heat is made by both the room heating circuit and the hot water, then the valve moves to mid position to give both circuits a flow of water. When the thermostat on the tank reaches its setting, it then moves the valve to close only the hot water side to stop the flow to the tank. This would only happen when both the heating and hot water setting is selected on the programmer or timer system by the connections to the switching of the valve itself. When only the hot water is on, then the tank 'stat tells the boiler to shut down.
We have a water flow underfloor heating system and it works perfectly by the method I've just described above. It also exceeds the manufacturers requirement of heat dissipation by a few degrees using the open loop through the hot water storage tank, and our tank is only a couple of yards away from the boiler. The main test on our system, consisted of the engineer by-passing the thermostat from the boiler and allowing the system to run itself silly for about half an hour with only the unrestricted hot water loop in play, no heating circuit.
I'm glad to say that the water returning to the boiler never got hotter than 83 degrees centigrade, which I'm now reading from the pass certificate from Scottish Gas, so it never allowed the boiler to boil already boiling water, but we did have clouds of steam blowing out through the expansion pipe of the hot water storage tank and the floor did heat slightly, and only slightly, under the first half of the kitchen floor. The system was given a good pass certificate and has been running merrily and economically for many years.
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spamguard@_spam_guard.com says...

BTW did my diagrams come out OK or do I need to repost with a narrower page setting?
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This wouldn't comply on an unvented cylinder, though. These have to be zoned off and capable of shutting down their heating circuit independently. As the bypass is particularly likely to be used when there is an overheat situation, using a circuit that can be independently shut down by the cylinder's safety systems would not be a good idea.
Christian.
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Well, I did mine before Part L came into effect, but I thought it conformed to the draft. I use the rads in the room with the thermostat as the bypass loop. They have lockshield valves only, and are controlled by the thermostat which cuts off the boiler when there's no call for heat.
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Andrew Gabriel

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