Condensing boilers and stored DHW.

[chrishutt]

Condensing boilers require return temps of below 55C to condense and achieve higher efficiencies. With regard to CH this can be achieved on a 20C temp drop with a flow temp of up to 75C (say 70C to allow a margin). Return temp will then be around 50C. In warmer weather the flow temp (and therefore return temp) can be reduced.

However it's not so simple when heating a hot water store (cylinder) where the object is to bring the store to say 60C within a reasonable time. When the hot store is relatively cold, a flow temp of say 70C will result in very low return temps but as the store approaches 60C the return temp will also approach 60C and the boiler will not be able to condense.

I have an idea for resolving this problem which I would welcome comments on. My suggestion is that instead of having the HW thermostat fixed to the cylinder wall it is fixed to the return pipe from the cylinder coil and then set to switch at say 50C so that the boiler return would never exceed that temp and it would always condense.

The HW circuit would then work for any boiler flow temp down to about 55C. Boiler thermostat can therefore be adjusted over a wide range (80C - 55C) in accordance with external temps (this function could easily be automated with an external sensor). During the summer the boiler thermostat could be left on any setting within the range since return temps would never be above 50C.

The actual HW temp would vary with the flow temp, although I don't think the return stat would allow HW temps to exceed 60C (if so it might be necessary to retain the cylinder mounted stat as well). The result would be slightly higher HW temps in cold weather and vice versa, which would be appropriate to demand. During the summer a higher flow temp will give faster reheat times.

Does anyone think this makes sense, or can much the same be achieved with the HW stat on the cylinder wall?

 

[htgeng]

When the hot store is relatively cold, a flow temp of say 70C will result in very low return temps but as the store approaches 60C the return temp will also approach 60C and the boiler will not be able to condense.

I have an idea for resolving this problem which I would welcome comments on. My suggestion is that instead of having the HW thermostat fixed to the cylinder wall it is fixed to the return pipe from the cylinder coil and then set to switch at say 50C so that the boiler return would never exceed that temp and it would always condense.

It may work if the cylinder is large enough but would be tricky to ensure it gave consistent results over many different installations. If the cylinder is small and the coil has a low output rating then you may find the hot water only reaches 50c + a few c.

Another reason I don't like the idea of the stat on the pipe is that it will cause the boiler to cycle a lot - giving uneccessary wear and tear. When heat is called you want the boiler to fire up to say 15kw, then as the hot water cylinder reaches the desired temp the boiler is just down to a few kw before it switches off.

Some boilers will have a sensor on the return and you will program into the boiler what temp you want the hot water cylinder to be at. As the return temp approaches the desired cylinder temp the boiler will be running at a very low output anyway.

 

[croydoncorgi]

The Keston / Ideal Imax / ?? solution covers both bases with two stats on the boiler or two different setpoints depending on whether the call is for CH or HW or both (in which case you get the HW setting). If you also have weather compensation, that only affects the CH setpoint. Job done!

See also 'Opentherm', coming soon (I hope) to a boiler near you!

You've disregarded part of the function of the cylinder stat. The water in the cylinder SHOULD stratify (convect so that the hot water stays at the top). As more cold comes in at the bottom, eventually the 'cold layer' reaches the thermostat (which is why it's a third of the way up the cylinder) and only then will the boiler will be called for heat. The heat should stay on until the WHOLE cylinder is hot again. Ideally, this should not take too long and to achieve this the boiler setpoint needs to be raised (in the case of Imax, the HW offset is 20 degrees increase on the CH setpoint, ex-factory).

 

[noonespecial]

Chris - the heating return temp from the cylinder needs to be somewhat in excess of 60 degC if you are controlling the stored water at 60 Deg C.

 

[Agile]

If you wanted to do it properly you could have a sensor on the flow, the return and the cylinder 1/3 way up.

Then use a little analogue electronics to do a simple assessment to determine when boiler operation was desirable or required.

Better, if you have a few days, is to write a simple computer program to compare those three readings and control boiler as you think best.

Tony