Tank choice for Combi, UFH and futureproofing

[Olf]

Current situation: combi ( Atag iC28 (23kW heating output, 30kW hot water output @ 11.5 l/s flow) ) rads, 1 bathroom - easy

 

In progress: UFH and 2nd bathroom

 

In plans: solar panels

 

So at minimum I'd need a buffer tank to marry up combi with UFH, but DHW really needs something better than sharing 1 pipe across 1 bath, 2 showers and 4 taps so let's say UVC.

However secondary return circuit is high on wish list and that starts tipping scale towards thermal store, that could satisfy all 3 requirements.

Having immersion heater (or 2, bootom and middle) should be possible in both cases so a draw here.

I'd be happy to ditch the boiler for ASHP the moment it breaks, but with less heating load (effort in insulation) and less cycling (buffering by cylinder(s)) it will probably run well enough to see hydrogen era, where both cost and green calculations may look different than today. Anyway, if either of choices will make the switch easier, bonus points awarded.

 

So I ended up with:

1.       Buffer tank for UFH, UVC for water

+ heating circuit can be turned off when not needed

+ smaller DHW tank

~ maybe easier to find place, but then 2 tanks instead of 1

- G3 installation and maintenance

 

2.      Thermal store

+ simplified plumbing as combi turned into ‘system’ (apart from the cylinder itself)

+ easier to accommodate secondary return

 

I tend to be biased towards thermal store option, but:

- is secondary return option with UVC much more complex?

- I made a draw in immersion heater department - am I right to assume both can be easily specced with 2?

- I made a draw in heat loss - here probably a lot is down to the set temp of thermal store and that a function of heating and DHW demand, so I'm in control to some extend

- I assume UVC will cost me fortune in servicing (annual inspection, at let say £50 a visit) - is it really more complex than checking for leaks and pumping the expansion tank? Or it doesn't matter, as  even cowboy's stamp would ultimately (in insurer's eyes) be worth more than proper, but uncertified job?

 

 

 

Edited April 20, 2021 by Olf
sausage fingers hit wrong button half way through

[PeterW]

53 minutes ago, Olf said:

However secondary return circuit is high on wish list

Telford UVC has Secondary return as standard. 
 

54 minutes ago, Olf said:

In plans: solar panels

PV..??

 

54 minutes ago, Olf said:

is secondary return option with UVC much more complex

No it is simpler. Returns to the top of the tank, TS with a coil cools the whole tank to keep it going. 
 

55 minutes ago, Olf said:

made a draw in heat loss - here probably a lot is down to the set temp of thermal store and that a function of heating and DHW demand, so I'm in control to some extend

TS heat loss is higher due to higher tank temperature. 
 

56 minutes ago, Olf said:

assume UVC will cost me fortune in servicing (annual inspection, at let say £50 a visit

£20 on your boiler service if he’s there anyway

 

56 minutes ago, Olf said:

simplified plumbing as combi turned into ‘system’ (apart from the cylinder itself

Combi won’t care either way - same with a UVC. 

[JohnMo]

It may be a bit late, but just commissioned a system similar to what you are looking at, here is what I have, after a few changes.

 

Atag combi boiler,

Telford thermal store (acting as a buffer with benefits) with boiler heating coil, DHW heating coil, mixing valve at outlet.  Thermostat (middle of tank) set at 45 degrees, to reduce heat loss.

UFH running at 30 degrees

Solar PV

 

Hook up, cold water to DHW coil, this passes through TS, and into combi, if it is hot or cold, the Atag boiler is designed to take pre heated water, so boiler doesn't care, the boiler only puts the energy it needs to the correct outlet temp.  Upside, if preheated, you get great hot water flow.  In the summer solar PV should do a lot of preheat.  Have a secondary circuit, pump with simple timer at the out DHW outlet, so it can pick up hot water from TS coil or pass through the combi.

 

Boiler was short cycling connected direct to UFH, so the UFH manifold now has mixer valve and pump added, this is connected direct to thermal store.  Boiler connected direct to TS heating coil, boiler temp set at 50 degree.

 

Advantages, boiler is always in condensing mode.  Simple hook up, low standing losses as the tank is no hotter than 45 to 50 degrees.

 

 

 

 

[Olf]

Thank you, that is an interesting concept!

How exactly is the secondary circuit connected, if your DHW goes through combi?

And can combi really modulate so low, that it even with TS at 40-ish deg it does not complain when only low flow requested? I guess that below some cut-off point it will just pass lukewarm water from TS?

 

[JohnMo]

The cold water enters the combi via the TS DHW coil.  The recirculation pump is on the cold side of the coil.  Check valves force the recirculation return flow though the DHW coil and to the combi.  The pump is the bit with the shiny plastic top, next to the immersion.

 

Have tried lots of running curves and the 50 degree one is about as low as I can go.  The pump runs for about 10mins every hour or so.  The combi basically runs until the return flow is just about the same as delivery flow.

 

[Olf]

6 hours ago, JohnMo said:

The recirculation pump is on the cold side of the coil.

I get it now

 

6 hours ago, JohnMo said:

The pump runs for about 10mins every hour or so

So in effect tops up the TS via DHW side of the combi in the process

[JohnMo]

The pump runs for about 10mins every hour or so

So in effect tops up the TS via DHW side of the combi in the process

 

I have written that incorrectly.  Should have said boiler runs for about 10mins per hour.  That is the central heating runs for about 10 mins per hour.

 

The secondary return, will pick up heat from the store if it's available, the boiler will fire the DHW as required.  I only run the secondary return for an hour in the morning and the same at night.