On 11/01/2018 10:04, Tim Lamb wrote:

I see the plumbers are active so....

I need a plan for piping and operating the cottage heating/DHW system!

Progress so far... gas supply and meter installed. Largest thermal store
I can fit in the space, installed. Upstairs and downstairs underfloor
heating pipework laid and coupled to separate manifolds.

Decisions needed.

This store, is it intended to run the CH and the DHW?

Buffer tank? It has been suggested that running the boiler to satisfy a
thermostat calling for heat for say a bathroom circuit will lead to
boiler cycling and inefficient operation. How does one include this in
the control system and how large does it need to be.

If the store is running the CH, then this is a non issue. If you are
running (small) CH zones in isolation, then going for a boiler with a
good modulation range will help it load match under a wider variety of
situations.

The thermal store is probably undersized based on the usual
shower/bathroom number calculations but space limited. With the expected
slow response from underfloor heating, it seems best to prioritise the DHW.

Chalet bungalow with limited headroom. Pressurised system? System
boiler? Serious lack of knowledge area:-)

Anything gravity fed will be poor in this circumstance[1], with the
possible exception of a gravity fed heat bank where its heat is used but
not its water pressure (i.e. it can feed a plate heat exchanger or
internal coil for mains pressure DHW).

Do you have decent cold mains flow rate and pressure?

[1] We had exactly that when we moved in here. DHW header tank was about
5' off the floor of the upstairs living space. That meant even lifting
the shower head too high while rinsing the bath was enough to stop the
flow of water.

I spent some time looking at alternatives (keep in mind I did not have
UFH, and the heatloss rate is far from modern standards), and concluded
that splitting the CH into two zones each with its own stat, and adding
weather compensation would probably get the heating about as efficient
as it was realistically possible to do. That just left DHW to worry
about, and it came down to heat bank or unvented cylinder. Although I
quite liked the idea of the heat bank, I could not do it for anything
close to the price of a decent unvented cylinder.

Went for a system boiler since that would integrate with all the
controls I wanted, and allow split temperature operation (i.e. running
low weather compensated flow temps for the heating when required, but
switching up to high flow temps for cylinder recharging). (I did look at
doing it with a combi - having it heat a cylinder like a system boiler,
but then also using its internal DHW for a kitchen tap to eliminate the
longer dead leg out there, but I could not find a solution that quite
did what I wanted). With hindsight the dead leg to the kitchen seems far
less noticeable now there is copious hot water flow and pressure.

Apart from picking the brains in here, is there a useful on line source?

There is a reasonable amount in the wiki.

http://wiki.diyfaq.org.uk/index.php/...heating_design
http://wiki.diyfaq.org.uk/index.php/..._Water_Systems
http://wiki.diyfaq.org.uk/index.php/...and_Heat_Banks
http://wiki.diyfaq.org.uk/index.php/DIY_Heat_Bank
http://wiki.diyfaq.org.uk/index.php/Unvented_DHW


--
Cheers,

John.

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