Anti cycling boiler with low return temp.

[Pete]

Having done some reading into boiler fitted with weather compensation I can sort of see the logic of reducing the flow temp as outside temp increases.

I also see that a boiler should only be fired where it can acheive a 10deg increase in temp to prevent short cycling (rapid on and off to keep the flow temp around the desired temp)

This is find when the flow is say 80degC, to fire the boiler once the return gets to below 70degC and heat until 80deg.

The problem occurs at lower temps. If the boiler requires a flow of just

25deg, (say outside is 20deg), you cannot acheive the 10deg differencial so the boiler would not fire. How do the commercial weather compensators cope with this? or do they increase the ammount of short cycling?

>Example of Micromat: > >On the Micromat, the CH flow temperature can be set to one of three preset

levels - 50, 70 and 85 degrees. At low temperatures, say -1,

the boiler will run at its preset temperature. However, at higher

temperatures outside, less heat will be needed, until at 20 degrees

outside, nothing is required.. The end points of the graph can be adjusted

as appropriate. In practice, there are various effects -

the main ones being that at switch on, the boiler doesn't automatically go

to full power, and that the power is starting to increase as it's getting colder outside

[Andy Hall]

Yes and no. If you are talking about a boiler with no modulation, but simple on/off control, then this may be true.

There are two factors though:

- If the outside temperature is approaching the inside temperature then the heat loss is reduced anyway. Therefore a simple on/off boiler will only need to fire much less regularly to produce enough heat to compensate for the loss. If anything, the boiler will fire for a short on period and then be off for a long time. This isn't short cycling in the conventional sense where the system is unable to get the heat away fast enough at high flow temperatures and cycles more rapidly on and off. THat represents a mismatch between load and supply.

- With a modulating boiler, the power level goes down as the temperature increases, so at reasonable temperature differentials to outside it will fire continuously during the heating period but at low level. THe Micromat, for example, will go down to 3kW, which in the context of the whole house is fairly small. Below that, the boiler will tend to cycle. Again, bear in mind that emission from the radiators is proportional to the temperature difference to the air, so the rate of heat delivery will be less at lower operating temperatures. This in turn again will reduce the rate of cycling.

At the low end, this boiler will achieve a flow temperature peaking at under 40 degrees and average somewhat less.

In the case of the Micromat, the pump is modulated as well which also helps with the matching of heat supply to load. The manufacturers. MAN, also supply a control system whereby several boilers can be ganged together using a manifold and the output controlled from all of them.

You are assuming a conventional boiler with 10-12 degree differential on the heat exchanger. Condensing models achieve 20 degrees or more if needed. However, there is nothing to force that situation. The power level and flow rate can be adjusted down for lower levels if needed.

That depends on whether the boiler is a modulating type.

.andy

To email, substitute .nospam with .gl

[IMM]

Commercial systems tend to have a pumped primary loop from boiler flow to return. Off this the CH and DHW loops are taken. The weather compensator modulates a 3-way valve to obtain the optimum flow temperature.

Normally a direct acting (acting on the boiler burner) compensator will have anti-cycling incorporated, based on time and/or temperature depending on the model. Some boilers also have anti-cycling devices too, so a belt and braces approach. Even programmer/stats, such as the CM67, have anti-cycling functions too.

When a weather compensator is a part of the boilers controls, as said below the boiler comes on at its lowest modulated temperature and maybe a time factor of an off period is included.