External bypass valve with modern pump

[MNW67]

A overrun bypass is pretty essential for gas boilers as they have a very small HEX, 2 or 3 litres, vs ~ 20litres for a oil fired boiler, Also the boiler may be running at high output when the last zone valve closes so the HEX has a lot of residual heat. I got the installers of my daughter's now 9 year old Vokera Vision 20S to install a NO 2 port valve close to the HW cylinder which is a good distance from the boiler so the pipework dissipates the HEX's residual heat quite well during the boiler's 3 minute overrun period.

Worcester Bosch seem to have copied your idea! The attachment has the full document with more details, but I have copied and pasted a small section.

 

[Johntheo5]

I was wondering if the pump is running at constant pressure, would it slow right down in that situation. Maybe almost to a stand still. So that the pressure never actually rises above the level it is set at. Even when all the TRVs and zone valves have closed and it is dead heading. The horizontal pump curve at 3.0m shows it maintaining the same pressure all the way down to zero flow. I don't know whether that literally means zero flow! It is very confusing, because it is still using about 10W at zero flow at that setting.

View attachment 367051

The hydraulic power required by any pump at 100% efficiency is (LPM*M)/6.14, (if I have converted correctly from my old imperial units to SI units)
So a pump circulating 20LPM @ 3M requires 20*3/6.14, 9.77W, the pump hydraulic efficiency (see Link) is 61.44%, 9.77/0.6144, 15.9W, the motor+mechanical efficiency is 65.15%, total power, 15.9/0.6515, 24.4W. A pump circulating 1.0LPM @ 3M requires 1.0*3/6.14, 0.4885W, the hydraulic efficiency is only 7.66%, 0.4886/0.0766, 6.38W, the motor+mechanical efficiency is still 65.14%, total power, 6.38/0.6514, 9.79W.
Pump hydraulic efficiencies probably havn't changed in 70 years but the advent of these permanent magnet DC? motors has dramatically reduced the power required, the old type motor efficiency could be down to around 19% or so, the power demand between no flow & full flow scarcely changed.
The hydraulic (in) efficiency is absorbed in heating the water as it passes through the pump, on very large pumps, the impeller condition and the condition of the wearing rings etc is determined by measuring very accurately the dT between the pump suction and discharge, if you ran your own Wilo or UPS3 or whatever with the discharge valve shut then the temperature will rise and rise, the pump will then start cavitating and can eventually get damaged. Very high head pumps like multi stage boiler feed pumps with a discharge head of say (like the ones I had 30 years experience on) 600M, 60Bar, had a automatic leak off valve to allow adequate flow through the pump while running up (and stopping) until the discharge pressure was sufficient to overcome the 45bar boiler pressure and establish adequate flow. Even pumps of fairly modest discharge head of ~ 100m, 10bar will have a simple manual leak off valve, permanently locked open.

https://wilo.com/ie/en/Products-and-expertise/en/products-expertise/wilo-yonos-pico/yonos-pico-25-1-6

Grundfos will have similar data.

 

[MNW67]

Also the boiler may be running at high output when the last zone valve closes so the HEX has a lot of residual heat.

I have sometimes wondered how much residual heat they contain. How much do these modern heat exchangers weigh and how hot do they actually get in normal usage (when water is flowing through them)?

 

[Johntheo5]

Can't comment on a gas boiler HEX but the temperature on my oil fired boiler (no pump overrun) with its 20L HEX will rise from 70C to ~ 92C when all zone valves close, the hi limit stat doesn't operate on these boilers until 110/113C so no problem, however gas boilers probably have a hi limit HEX stat set to maybe 90C so no fun having to reset the trip on this every time all the stats close.

 

[MNW67]

Can't comment on a gas boiler HEX but the temperature on my oil fired boiler (no pump overrun) with its 20L HEX will rise from 70C to ~ 92C when all zone valves close, the hi limit stat doesn't operate on these boilers until 110/113C so no problem, however gas boilers probably have a hi limit HEX stat set to maybe 90C so no fun having to reset the trip on this every time all the stats close.

I was asking mainly because I have been reading the manuals for the Alpha boilers, and they only have a 10 second overrun. They have the same stainless steel heat exchanger that Vaillant, Ideal and others use. It made me wonder whether the long overruns used by other manufacturers are actually necessary with that particular type of heat exchanger.

 

[Johntheo5]

Does Alpha just market boilers like Immergas?.
Can you post a link to this 10 second overrun?.
What about anticycle time if the heat demand is lower than the minimum boiler output?

 

[MNW67]

Does Alpha just market boilers like Immergas?.
Can you post a link to this 10 second overrun?.
What about anticycle time if the heat demand is lower than the minimum boiler output?

Immergas own Alpha I think, and they seem to have provided slightly tweaked versions of their international boiler range for the UK.

It is on page 28 here of the heat only boiler manual. But it is exactly the same across all their boilers. It says:

If the primary sensor has not registered the pre-set temperature, but the room thermostat is satisfied, the burner is turned off. The fan continues to run for 30 seconds. In this instance there is no delay before the burner will relight. If the pump is connected to the boiler pump terminals a 10 second pump overrun is activated between requests.

https://alpha-heating.ams3.digitalo...121323/E-Tec-R-instructions-02-20-rev-001.pdf

With the combi and system boilers, if the boiler reaches the set point and the burner turns off, there is a three minute pump overrun.

When the set temperature is reached, the burner is turned off. The fan continues to run for 30 seconds and the pump continues to run for three minutes, after which the burner can relight if required.

For the combi boilers, the manual says that during DHW production, the pump shuts down immediately once the taps are turned off. So there is no overrun at all in DHW mode. So you could have been running a bath for ten minutes with the burner on full, and still there would be no pump overrun. I have never had a combi. Is that standard?

If the boiler is set for hot water only (Summer ) - The burner is turned off, the fan will continue to run for 30 seconds and the pump will stop immediately.

 

[Johntheo5]

So it does a pump overrun.

I don't even have a gas fired boiler but I think its fairly standard practice not to have pump overrun when DHW demand ceases.

See below. If this is how it operates then it can't run as a modulating boiler as the SP temperature and the burner cut temperature cannot be the same, burner cut out temperature is generally SP+5C, if the burner cuts out at SP+5C because the boiler output is > than the heat demand, then the circ pump continues to run and the burner will refire when the anticycle time has elapsed, provided the flow temperature is SP-5C.

"When the set temperature is reached, the burner is turned off. The fan continues to run for 30
seconds, after 3 minutes the burner can relight if required."

Central Heating Mode
If there is a call for heat, the fan will run and the premix burner will light. The burner output then automatically adjusts to suit the
system demand; as the temperature of the water in the boiler approaches that set by the adjustable temperature thermostat,
the burner output is reduced. When the set temperature is reached, the burner is turned off. The fan continues to run for 30
seconds, after 3 minutes the burner can relight if required. If the primary sensor has not registered the preset temperature but
the room thermostat is satisfied the burner is turned off. The fan continues to run for 30 seconds. In this instance there is no
delay before the burner will relight. If the pump is connected to the boiler pump terminals a 10 second pump overrun is activated

 

[MNW67]

So it does a pump overrun.

I was thinking about the situation where the boiler is turned off either by the room thermostat or the programmer, and all the zone valves close. In that situation it only has a 10 second overrun. Other manufacturers have an overrun of several minutes when that happens.

If the primary sensor has not registered the pre-set temperature, but the room thermostat is satisfied, the burner is turned off. The fan continues to run for 30 seconds. In this instance there is no delay before the burner will relight. If the pump is connected to the boiler pump terminals a 10 second pump overrun is activated

 

[Johntheo5]

The whole statement is very ambiguous to me anyway. It still seems to imply by saying that "If the primary sensor has not registered the preset temperature" that the burner will trip once the primary sensor HAS registered the preset temperature, it simply can't operate in a modulating mode if so.
It does then say allright though that if the roomstat is satisfied that the burner will trip (correctly) that it just does a 30 sec fan overrun + a 10sec pump overrun, fan overrun will cool down a HEX, so maybe they have calculated that 30 secs is sufficient.

"If the primary sensor has not registered the preset temperature but
the room thermostat is satisfied the burner is turned off. The fan continues to run for 30 seconds. In this instance there is no
delay before the burner will relight. If the pump is connected to the boiler pump terminals a 10 second pump overrun is activated

 

[MNW67]

The whole statement is very ambiguous to me anyway.

Yes, I agree.

I am probably going to get a heat only Vaillant in the Spring. It's the safe option! But I am also intrigued by this Alpha heat only boiler as it has several interesting features. Alpha tech support are hopeless, though, at answering queries.