Setting for condensing gas boiler on solar buffer tank

BigGeorge · 21-01-2012 07:33PM #1

By way of background, I've a small condensing gas boiler on a large buffer tank which is heated by solar from the bottom by solar ( 1-3 hours these days mid winter fairly consistently). Its a large tank & there are no sudden surges in demand really- its extremely steady usage

its a stratified tank which feeds UFH about half way down & DHW (via a stainless internal coil ) out at the top. Had the TI at the top ( 2/3 of the way up) set to call up in the the boiler when it dropped to 50C and to cut out then when it hit 56, this heating cycle typically takes about 3 hours and the boiler return temp was set at 65 ( as according to the 'expert' it need to give the incoming feed a 10C boost). So...the top of the tank is consistently around 50C and the bottom around 25, which means I think the 'expert' may have missed the fact that the boiler feed comes off half way down so would be around 40C. This 40C water is drawn off and returned 3/4 of the way up at about 65C by the boiler

So , my questions:

What is the optimal & minimal gas boiler burn time? Is an hour or 2 sufficient?
What is the optimal boiler return temp?
At what boiler return temp does condensing start to happen?
What range for the TI at the top of the tank make sense, currently 50-56? Obviously trying to keep this as low as possible
In a stratified tank, how low a bottom temperature am I looking for..or is the answer ' as low as possible to maximise solar'?
If so, what can I do to lower that bottom temperature or check if stratification is being disrupted?

heinbloed · 21-01-2012 09:00PM

Since you have build a passive house - what type of small modulating gas boiler is installed (modulating range)? Combi boiler? What flow temperature of the CH system is demanded according to the planning?

BigGeorge · 22-01-2012 12:49AM

Its a fully modulating ( 20-100%) 11kw nat gas unit, CH demand is low on quantity & temp demand is 30C

JohnnieK · 22-01-2012 10:47AM

Most condensing boilers would condense from 55oC return temp or lower. They normally love a differential from the flow to the return of 20oC. Is there a high set point you need to achieve for your system. Did anyone give you this?

heinbloed · 22-01-2012 11:55AM

The OP states:

Its a fully modulating ( 20-100%) 11kw nat gas unit, CH demand is low on quantity & temp demand is 30C

So the min. output is 2.2 kW ? At what efficiency? (see boiler manual, manufacturer's data sheet)

How high is the minimum thermal demand of the structure?
Was a the heating load calculated (EN12831) and was the boiler choosen accordingly? For a PH it seems far overdimensioned.

The optimum return temperature for a gas condensing boiler is <0 degrees Celsius.

However this can hardly achieved in a practical set up/installation, the most next-to-practical return temperaturewould be at around 30 degrees Celsius.Check the manufacturer's manual.

What JohnnieK states is the maximum allowed return temperature, not the ideal return temperature.

With a correctly chosen return temperature and a correctly sized boiler/thermal source there is no need for a buffer tank. So it seems to me that your engineer/heating system designer ripped you off. But answer the questions asked, it might be just a question of an optimising of the system.
Is the system maintained, the flow rate correctly adjusted?

Min. output (kW)?
Minimum and maximum demand?
Method of distribution (surface and/or radiator)?
Calculated flow rate (liters/minute)?

Etc.....

What is a " TI " ? (

Had the TI at the top ( 2/3 of the way up) set to call up in the the boiler when it dropped to 50C and to cut ....

)

heinbloed · 22-01-2012 12:18PM

The efficiency of a condensing boiler can be meassured at home. Write down the fuel usage and compare this with the amount of condensate.
A 100% usage of the gas fuel would result in

1.63 kg of water per m3 of H-gas (11.46 kWh/m3)
and
1.53 kg of water per m3 of L-gas (9.78 kWh/m3)

BigGeorge · 22-01-2012 02:59PM

Thanks Johnnie K, no top temp indicated at any stage, with about 52C at the top I can get DHW of almost 50 when it running; can obviously bring this down as I dont need 50C DHW. Brought the return temp down to 54 & will monitor the gas use

With all due respect, I'm looking for constructive comment on a number of set points for the system I have based in people practical experience - not a thesis / PhD or if I've been shafted by a plumbers, overspecified etc....That not going to help me fine tuning the install I have. Many thanks

heinbloed · 22-01-2012 05:38PM

With all due respect, I'm looking for constructive comment on a number of set points for the system I have based in people practical experience - not a thesis / PhD or if I've been shafted by a plumbers, overspecified etc....That not going to help

me fine tuning

the install I have. Many thanks

It's not being you who should do the "fine tuning ". It should be a competent person, someone who is able to read and to understand the basic informations provided by the boiler manufacturer. The vital informations.
A fine tuning can't be done if the first initial installation is faulty, criminal.

All condensing boilers operate only safely in the condensing modus.

From your statement

.... no top temp indicated at any stage, with about 52C at the top I can get DHW of almost 50 when it running ....

any practitioner can read that the max. allowed return temperature (guaranteing the creation of condensate) is not kept. That the actual return temperature is higher than the max. allowed temperature.

Get a heating engineer in, soon. Turn off the apliance until it's installation and correct setting is done by a competent person.

JohnnieK · 22-01-2012 08:21PM

BigGeorge wrote: »

Thanks Johnnie K, no top temp indicated at any stage, with about 52C at the top I can get DHW of almost 50 when it running; can obviously bring this down as I dont need 50C DHW.

You should bring your tank up to 60oC at least once a week to eliminate any chance of developing legionnaires. It would be best practice to do this as the point that it is completely killed is 55oC or higher.

If your set point is 58 I would try 60 on the boiler. get a couple of stats and monitor the flow and return. If you find that's its hard getting the differential there are valves you can get to try and achieve this.

Also remember that it will condense at first because of the load on the cylinder but as that nears set point the boiler wont condense as much because the temps will be stabilizing.

Hope this helps.

BigGeorge · 22-01-2012 08:41PM

Many thanks - youre definitely onto something. The DHW is going through a stainless coil and coming out the top with storage

when I queried the 60C cycle once a week with the plumber he said that becasue there is no storage & the DHW is used directly in the loop into the house. Was he in error in saying this or is it just good practice to heat the tank anyway? If so is there any industry standard doc I should use to demonstrate that its best practice

JohnnieK · 22-01-2012 10:32PM

Your kind of confusing me here a bit. Is your DHW stored or not? Dose the cold water go through the coil and come out hot the other end to the taps when you turn it on. Could you clarify that for me.:)

BigGeorge · 22-01-2012 10:49PM

Sorry - fat fingers - no storage at all, comes in the bottom as cold water & out the coil at the top as DHW to feed into the house, hence the query on the necessity for the 60C cycle - many thanks

DoneDL · 22-01-2012 11:00PM

If the water is mains fed through a buffer tank then the water does not need to be heated to 65C as it would if it were stored in a hot water tank.

BigGeorge · 24-01-2012 12:11PM

Ok, have been playing with the system setting and set the call temp for the gas boiler for the temp probe on the buffer tank to be 47C and the off temp to 52; also set the return temp on the gas boiler at 54. Looking at the gas use it seemed less efficient and spent a long time try to get to the final 52C set point; gas usage seemed higher

have upped the gas return temp to 60C so will see how that performs, its drawing liquid from the tank off over half way down so its feeding the boiler at about 40.

Setting for condensing gas boiler on solar buffer tank

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