John Rumm wrote:
On 12/11/2016 15:15, Tim+ wrote:
wrote:
On Sat, 12 Nov 2016 13:00:32 +0000, John Rumm
wrote:

You have a setup that was really conceived as a way of gaining
efficiency on systems with a demand for low flow temps (i.e. UFH) and a
fixed output non condensing boiler.

As addition benefits it allows high delivery rates of mains pressure DHW
without being limited to the instantaneous power available from your
gas/oil supply, and makes it easy to incorporate multiple sources of
heat into a system.


Not being much good at plumbing but having some experience of thermal
stores:

My first point is that it's far better to store the energy as natural
gas and only convert it to heat as you need it.

So the OP's system has a heat store for the two reasons John has
discussed

1) to deliver more instantaneous hot water than the boiler can supply
from the top part of the thermal store that, I assume this goes
through a plate heat exchanger so returns to the bottom of the store
at near ambient? Water from the main tends to be around 10C here.

2) To allow a blending valve to take water from the top of the thermal
store but under the part reserved for DHW, mix it with water going
around the under floor circuit and then return is at around 25C to
the bottom of the tank.

The need is always to return water below about 40C to the boiler to
keep it in condensing mode?

My thoughts are on how well the thermal store remains stratified or is
the flow too great such that it induces a lot of turbulence?

If it remains stratified then why can the boiler not be set to burn if
the DHW reserve falls below a high set temperature, whilst still
drawing cool water from the bottom of the tank?


The demand for underfloor heating then being sensed when the blender
valve input from the thermal store falls below the set underfloor
temperature it can maintain?


The boiler then heats the whole of the tank until it starts drawing
water above 40 C from the bottom of the tank?

The system I used to service a pellet boiler on in a block of flats in
Brixton failed to maintain stratification because the flows where so
high they constantly mixed the store's contents. I advocated a
possible solution but the german makers of the thermal store would not
countenance it.


It seems an awfully complicated way of meeting two very different demands.

Wouldn't two boilers be simpler/cheaper? Condensing multipoint for HW and
standard condensing boiler for CH?

Only if you like hot water dribbling from the taps ;-)

A bit unfair on instantaneous water heaters. For sure not going to match
the best output of a stored water solution but more than adequate for many
folk who don't run a bath too often.

Tim


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