Setting up boiler for optimum efficiency

[Andy Burns]

Does the store really want to stay constant? I'd have thought with a non-modulating boiler, firing in frequent short bursts would not be the most efficient while the UFH is drawing out heat.

[John Rumm]

I think there is a danger of over thinking this one! Beyond basic requirements like making sure the flow is hot enough to get the store to its set point, there are only a few variable to play with, and each has pros and cons - so pick whichever is most important to you. None are necessarily right or wrong.

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.

You will do if the store is nearly depleted, but obviously as it warms up that will get harder. All that means is you will get a reduction in boiler efficiency for the last phase of the reheat.

[news]

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.

AJH

[Tim+]

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?

Tim

[news]

Why it's just a matter of changing where the thermostats that trigger the burner are placed except on the underfloor system a different stat triggers on from the one that turns off the boiler?

At over £700 each?

AJH

[John Rumm]

The problem there is that with a domestic supply you are going to be limited to about 50kW which is only about 17 lpm at a 40 degree temperature rise. The thermal store with a large plate HX will be able to heat the water on demand at well over double that rate.

Below 54 is ideal...

You can tweak these things by selecting where you sense the temperature of the store. You can also use two stats some distance apart to increase the hysteresis and hence boiler run time to get larger more efficient recharges.

What was your suggested solution?

[John Rumm]

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

An alternative would be to drive the UFH from a modern boiler directly with blending valves. The devote the store to just giving you DHW.

No real advantages over a unvented hot water cylinder though it has to be said.

[Tim+]

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

[news]

Return the cold water tangentially to prevent the mixing, the colder water then inducing a slight vortex and falling to the bottom, any warmer water at the same level would be less dense so would rise and not tend to move past the denser colder layer from centrifugal effects as long as there was little turbulence. Hot take offs were already from the centre of the 3000litre cylindrical tanks.

AJH

[John Rumm]

Yup fair point, It was intended more of a "in comparison to what the thermal store can deliver", rather than in absolute terms.

Many multipoints are only about 24kW though - so 9 lpm of hot water.

[Tim+]

Okay, not a multipoint then, just a big combi. ;-)

Tim

[rick]

Certianly not in may case

Certainly not in may case as single boiler already installed... In general the store is not particularly expensive .. and in my case I also dump all spare electrically generated power form my Solar PV array ... very easy to mix any heat source as the store is just a storage battery. It also provides a very large buffer - so if tehre is demand for a lot of heat for heating or DHW .... don't have to wait ... it's there and immediate.

[rick]

The issue is the stratification is there with the horizontal baffle in the store .. but once you start running DHW the pump on the primaries is operated to 'stir the store' ondoing any stratification. I have 'improved' things with a delay .. DHW has to be running for >45sec before pump will start ...... which prevent lots of small runs.