Do I need an External Bypass Valve

[hendersonm66]

Does anyone here have experience fitting the Evohome System to a POTTERTON PROMAX COMBI 28 HE PLUS A?

I've bought an Evohome system with eight TRV's, but what seems like one simple bit of information is thwarting the installation.

* The boiler has an Internal Bypass, some say its good enough, some say I should fit an external Bypass? Potterton won't help and neither would Honeywell.
* If I need to fit an Automatic External Bypass (Honeywell DU144), then I'm struggling to get the information on minimum flow and pump speeds as they are not stated in the manual and the pump is a single speed not adjustable. This makes following the setup instructions for the DU144 a bit difficult.

So, does any here have experience with this type of boiler?

Evohome kit still sits in its box waiting

Mike

[Richard]

Hi Mike,

A built in boiler bypass is normally sufficient to provide minimum flow rates required by the boiler manufacturer.

An external bypass valve is to reduce system pressure or where a boiler manufacturer doesn't have an internal bypass valve fitted or the internal bypass is insufficient under certain flow conditions to meet minimum flow rate of the heat exchanger.

Normally a boiler manufacturer will advise, but you need to ask them the right question... 'If my boiler is fitted on an S Plan Plus system, does it require an external bypass'.

I hope that helps?

Thanks,

Richard

[alexgray]

Conventional systems need a good bypass or permanently open radiator, normally in the same room as the thermostat, to make sure that some water can circulate if the thermostat calls for heat but all the other rooms are warm with their TRVs closed.

But in an Evohome system the boiler will only ever be asked to switch on if at least one of the controlled radiators is calling for heat and so has opened its valve, so there should never be a situation where the boiler is fired up with no radiator open.

However, it's a handy 'belt and braces' backup to have some bypass in place, and I should thing the one built in to the boiler would be enough. I have no bypass on my oil-fired boiler system and have had no problems. If the worst comes to the worst the boiler will also have an overheat thermostat or trip that will cut it out if no water is circulating.

I should sayI am not a qualified heating engineer, but I am a chartered engineer and have done a number of successful evohome installations.

[Richard]

Conventional systems need a good bypass or permanently open radiator, normally in the same room as the thermostat, to make sure that some water can circulate if the thermostat calls for heat but all the other rooms are warm with their TRVs closed.

But in an Evohome system the boiler will only ever be asked to switch on if at least one of the controlled radiators is calling for heat and so has opened its valve, so there should never be a situation where the boiler is fired up with no radiator open.

However, it's a handy 'belt and braces' backup to have some bypass in place, and I should thing the one built in to the boiler would be enough. I have no bypass on my oil-fired boiler system and have had no problems. If the worst comes to the worst the boiler will also have an overheat thermostat or trip that will cut it out if no water is circulating.

I should sayI am not a qualified heating engineer, but I am a chartered engineer and have done a number of successful evohome installations.

Welcome to the group and thanks for your comments.

Some of your detail however is incorrect. For instance, what happens to the central heating water between the period of time the HR Radiator Controller closes and the boiler demand relay (or heating zone actuator relay) turns off, which can be up to 4 minutes? Oil boilers are very different to a gas boiler with a very low water content heat exchanger. If minimum flow rates are not maintained correctly with a LWC heat exchanger you would probably cause damage to the boiler.

As I stated above, the boiler manufacturers/installation instructions must be consulted.

[alexgray]

Thanks for pointing that out. It's true my installations have all been on larger oil-fired boilers, and a 4-minute overrun would be no problem to them - and if it's more prolonged the boiler water temperature thermostat would simply switch off the boiler until it cooled again.

But Mike said his boiler had an internal bypass, and I would have thought if they fitted that at all Potterton would have made that suitable for protecting the boiler minimum flow to allow its thermostat to respond in time to prevent boiling or other damage.

But in any event your advice to stick to the boiler manufacturer's specifications is of course absolutely right.

Alx

[Richard]

Thanks for pointing that out. It's true my installations have all been on larger oil-fired boilers, and a 4-minute overrun would be no problem to them - and if it's more prolonged the boiler water temperature thermostat would simply switch off the boiler until it cooled again.

But Mike said his boiler had an internal bypass, and I would have thought if they fitted that at all Potterton would have made that suitable for protecting the boiler minimum flow to allow its thermostat to respond in time to prevent boiling or other damage.

But in any event your advice to stick to the boiler manufacturer's specifications is of course absolutely right.

Alx

Having fitted oil and gas boilers since the early 2000's, some appliances are forgiving and others are not. Normally with gas boilers it is nuisance overheat faults that happen by not having an external bypass fitted, because minimum heat exchanger flow rates are not achieved. Take Intergas boilers for instance, they don't have an internal bypass so provision must always be made to ensure minimum flow rates externally from the boiler.