Megaflo- optimising temperature to get maximum efficiency

[VillageIdiot]

Just wondering if anyone with experience of Megaflo systems could give any advice.
Have a Megaflo system, indirectly heated via the boiler. Have Megaflo tank temperature set at the lowest recommended temperature which is 60 degrees (lower is possible but legionella risk rises).
The question is what the optimum boiler flow temperature should be. Surely there is a competing balance of 1) ensuring sufficient temperature gradient across the heat exchanger coil to ensure the tank heats up in a reasonable length of time eg. 30-60 mins 2) keeping the boiler flow temperature low enough so it is doing some condensing.

In other words, is there a minimum temperature gradient that is recommended ? is it, say, 5-10 degrees centigrade?

it seems to me that the problem with the Megaflo system is that as the hot water tank works via a heat exchanger, you need the boiler outflow temperature to be higher to the point that the boiler probably can't be run at a condensing temperature.

Megaflo blurb that came with the installed system seems to recommend pretty much setting the boiler temperature at 78-80 I think but that strikes me as sensible only in an era when gas was cheap.

Any thoughts would be appreciated- thanks

 

[ShaunCorbs]

65c you could run it at 60 but this doesn’t allow for any heat lost due to un insulated pipes etc

Return should be below 50 for max condensing

 

[Chuck]

I'd also recommend 65°C flow as the minimum. Below this figure the boiler will short cycle as the tank approaches 60°C because the heat transfer rate will fall below the boiler's minimum.

As far as higher flow temperatures are concerned, in practice, the return temperature should still stay relatively low for almost all the time the tank is heating because the return end of the coil is at the bottom of the tank which stays cool until the whole tank is up to temperature due to stratification. To some extent, what you loose during the period of higher return temperature is offset by what you gain from less short-cycling.

Stratification makes it complicated to 'guess' accurately how these systems behave in detail. If you really want to know how a specific system performs, you need to attach some thermocouples and a data-logger to the system.

In practice, there is not much to be gained in energy savings from tweaking this part of a system. Fitting an eco head to your shower(s) and learning to shower quickly is the way to go; this will make a relatively huge difference.

 

[VillageIdiot]

I'd also recommend 65°C flow as the minimum. Below this figure the boiler will short cycle as the tank approaches 60°C because the heat transfer rate will fall below the boiler's minimum.

As far as higher flow temperatures are concerned, in practice, the return temperature should still stay relatively low for almost all the time the tank is heating because the return end of the coil is at the bottom of the tank which stays cool until the whole tank is up to temperature due to stratification. To some extent, what you loose during the period of higher return temperature is offset by what you gain from less short-cycling.

Stratification makes it complicated to 'guess' accurately how these systems behave in detail. If you really want to know how a specific system performs, you need to attach some thermocouples and a data-logger to the system.

In practice, there is not much to be gained in energy savings from tweaking this part of a system. Fitting an eco head to your shower(s) and learning to shower quickly is the way to go; this will make a relatively huge difference.

thanks both. That hadn't occurred to me. I will try this and see if it makes any difference to the gas usage.
good advice