Re: Boiler control update and query

["Neil Fuller" <neil@xxxxxxx>]

Dave

I think that you may have a poorly balanced system. Looking at the trace I
see . . . .

System starts, slug of 'cold' water circulates from the house.
The boiler heats up it's jacket fairly quickly and the circulating water is
also heated to setpoint - Boiler turns off - Pump remains on.
A few minutes later, water has cooled sufficiently to demand the boiler
back
on again. Again, the water is heated quickly and boiler turns off.

Meanwhile, there is still a demand from the space so the pump continues to
run and the boiler carries on cycling on and off.

In the evening, the space eventually (at about 19:30) is satisfied and the
demand is turned off - Pump stops and the jacket temperature peaks at about
85DegC.

However, as there is no 'hot' circulating water, the space cools (as does
the water in the rads) and after about 1/2 an hour, the space again demands
the system to start.

The pump turns on and the 'cold' (well, I suppose its only cool) slug of
water hits the boiler. This results in the downward swing to about 50 DegC
on the flow - Water heats up quickly, satisfies the space and the cycle
repeats.

The problem with this is that 'dry cycling' the boiler is both very energy
inefficient and stressful to the boiler hardware, especially if it's a
condensing boiler or a low water content heat exchanger.

All you are doing is heating the jacket and a bit of the water. The boiler
turns off and cools down, throwing a lot of the heat to atmosphere,
straight
up the flue!

The solution is to get the correct flow through the boiler (check the specs
and the specs of the pump and to correctly balance the rads.

Balancing is, essentially, ensuring that you have a good differential
temperature across all the rads and hence a suitable temperature drop
across
the boiler. If it's too small, it'll dry cycle.

However, check the specs on the boiler coz it may require a bypass to
ensure
that the back end doesn't get too cold and result in condensing on the
HtExch resulting in corrosion.

I just googled and found this which tells you almost everything you need to
know

http://www.diydata.com/projects/centralheating/balancing/balancing.htm

If you don't have a differential temp sensor (believe me, by *far* the
easiest way to 'balance'), Email me off list and I'll sort one out for you
over Xmas

HTH

Regards

Neil
----- Original Message -----
From: "Dave McLaughlin" <dave@xxxxxxx>
To: "Ukha" <ukha_d@xxxxxxx>
Sent: Tuesday, December 16, 2003 11:36 PM
Subject: [ukha_d] Boiler control update and query


> Hi All,
>
> My new AVR Microcontroller based boiler control is now operational and
has
> been for about 3 weeks now. It works a treat and is high tech enough
for
me
> as a gadget freak!!! :o)
>
> One thing puzzles me though and that is the temperature cycling of the
> boiler. This link will take you to the website with a page showing the
> boiler chart.
>
>
http://www.embeddedcomputer.co.uk/Projects/Controller/BoilerCharts/boilercha
> rts.html
>
> http://tinyurl.com/zjzo
>
> At the bottom I give a description of the heating cycle and I am
puzzled
to
> the larger temperature cycling in the latter part of the evening. This
is
> the same each day. The chart only shows the last 24 hours of each day
on a
> rolling cycle.
>
> The temperature of the boiler is shown as the green trace. Has anyone
any
> idea why I am seeing a much larger cycle at night. It always starts
off
the
> same as the morning and midday cycles, but gradually increases. The
strange
> part is that it goes much lower too.
>
> I thought it was something to do with the outside temp as the boiler
is
> located in the garage, but tonight (the chart on the website was
uploaded
> tonight) has been a few degrees warmer than the last few nights which
where
> down to and below zero.
>
> Any ideas?
>
> Regards
> Dave...
> ---
> Very funny Scotty, now beam down my clothes!!!
> ---
> http://www.embeddedcomputer.co.uk
> http://www.v8carlton.com
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