Condensing Oil with UFH and Solar - Tank Question

Wolfhound14

Hi,

After much research I think a condensing oil boiler with solar and UFH is the system for my new build.
My question is around Thermal Stores and DHW.

Should I use one large buffer tank (>500L) for both Heating and DHW. What are the advantages/disadvantages of this setup?

Is the alternative a seperate tank for water and heating. If so, can you run 2 feeds from an oil boiler?

Thanks

heinbloed

You would have to contact someone who will calculate the demand. And this demand then will have to be met.
Nowadays from most major boiler manufacturers packages are available which consist of modulating (combi-) condensing boiler, thermal storage tank and ST collectors.
But one has to know how high the energy demand is to avoid a wrong buy.
Therefore a signed and guaranteed report on the situation has to be done.
If the heating system is under- or oversized you could claim your money back from the professional who advised.

Carlow52

Wolfhound14 wrote: »

Hi,

After much research I think a condensing oil boiler with solar and UFH is the system for my new build.
My question is around Thermal Stores and DHW.

Should I use one large buffer tank (>500L) for both Heating and DHW. What are the advantages/disadvantages of this setup?

Is the alternative a seperate tank for water and heating. If so, can you run 2 feeds from an oil boiler?

Thanks

If u have done the 'much research' then u must have the answers to 'What are the advantages/disadvantages of this setup?'

What do you mean by DHW in this context?

My concern here is that UFH generally runs at a much lower temp than the 60 min required for HW so how will you 'step down' the temp from the ST or the oil for the UFH.

You can have have as many feeds as u like, they are called zones:) I have 5 on mine setup.

On a // tangent: if going for UHF what impact will this decision have on the floor insulation requirements?

HighlyCooL

Wolfhound14 wrote: »

Hi,

After much research I think a condensing oil boiler with solar and UFH is the system for my new build.
My question is around Thermal Stores and DHW.

Should I use one large buffer tank (>500L) for both Heating and DHW. What are the advantages/disadvantages of this setup?

Is the alternative a seperate tank for water and heating. If so, can you run 2 feeds from an oil boiler?

Thanks

i can only ask why you want to put a buffer tank in this set up, usualy a buffer tank is used in geothermal i believe, or with wood gasses boilers.
if i was doing what i think you are doing i would get a dual coil Hot water cylinder, a oil condencing boiler, a few zone valves and clock, and underfloor heating manifold.

there is no need for a buffer tank to "step down" to step down the temp form the boiler, this is done with the blending valve set up in the under floor heating manifold.

as for using solar panels to heat your under floor heating........... how big is your roof? and when the panels are getting all that heat you will need wont need the heating on but a cool shower!:D

Wolfhound14

Thanks for the replies:

heinbold: Can you advise who the advisor should be. i.e. what type of professional is best to do this type of calculation? I agree with the approach, just not sure what profession to work with.

Carlow52: I do have my own ideas on what the advantages etc are. I just wanted other peoples opinions. Preferable from people who have installed systems.
DHW, I mean enough for a family of 4. Currently 2 adults and 2<3yrs, but ultimately 4 adults.
I share the concern about the UFH lower temp and that is exactly the purpose of the buffer tank.
By feeds I mean outputs from the boiler. Zones are outputs from the manifold, but ultimately all derive from a single boiler feed.
Going with UFH definately impacts floor insulation. I think u value of 0.15 is required by regs if UFH is used.

HighlyCool: I never mentioned solar for space heating, just interested on how it all hooks up together.
The idea of the buffer tank is to reduce boiler cycling. i.e. turning on and off multiple times per hour. This is the least efficient way to operate a boiler...from what I have read...but to each their own.

Thanks

heinbloed

Wolfhound wrote;

heinbold: Can you advise who the advisor should be. i.e. what type of professional is best to do this type of calculation? I agree with the approach, just not sure what profession to work with.

Well, there are several professions which could be available for the energy demand calculation.
The first aproach should be the designer of the new build. According to my knowledge such an energy DEMAND calculation is a legal requirement, part of the planning permission.
However how to cover the demand when using various methods which are still to be decided on is propably out of the designer's capeabilities.

A heating engineer, a civil engineer, an energy advisor (not all of them!) would be capeable.
The professionals which usually get the more precise answers are the heating contractors. They not only offer the energy supply for a fixed price (regardless where from and how) but also are able to listen to specific demands. Like the inclusion of ST energy, a CO2 neutral supply, cheap and available, long or short term. Whatever the client's demand are, they usually can help with guaranteed advice using international ( IN) , European (EN) and/or Irish standards (IS), whichever is preferred.

So check the www. or the golden pages. Not all energy contractors are willing to work for the small client, some only work on larger projects. But they know each other and you will be directed towards one.
Tell them straight away what you want ( the energy demand calculation according to EN and advise on the first five or ten or twenty 'best' methods to cover the demand) not that they think you're looking for a cheap electricity supplier.

The 'best' method must be defined, are you looking for a low initial investment as a priority, for a long lasting and low maintenance initial investment, a high re-sale value, 'green' issues ect..

For a standard household a ST system producing only DHW will be put down by most advisors. For economical and ecological reasons. To pay for itself a ST system must be cheap to buy (per kwh potential output) and cheap to maintain. Both options are not ready available in Ireland, ST installers here are where continental installers were 30 years ago (price per delivered kWh over 10-30 years equipment's life time). They simply can't deliver energy-saving ST energy with expensive toy installations.
Ask one for a guarantee covering this issue.....
As an alternative a simple ST system installed as a DIY exercise without subsidies could be far more energy efficient. Thermosyphon method, vakuum flasks, no anti-freeze or anti-corrosive chemicals. These complete systems (incl. the tank)are available for less than €1,000 and are aproved with a solar keymark and 5 years manufacturer's guarantee. But to cheap for an Irish installer to be touched, no profits. And no understanding of the system.

About the boiler: A good boiler, one that suits the demand, has absolutly no need for a storage tank. One doesn't load/compress a spring when driving a car to avoid the cycling of the motor (smiley).

The idea of the buffer tank is to reduce boiler cycling. i.e. turning on and off multiple times per hour. This is the least efficient way to operate a boiler...from what I have read...but to each their own

Again the advice of a heating engineer is necessary: figure out what the minimum demand is, this would be then the minimum output of your modulating boiler. Figure out what the maximum demand would be. This would be the max. output of your boiler.

As a very simplyfied example: min. demand 2kw (one radiator) , max. demand 20 kw ( ten radiators). Boiler output : modulating from 2-20 kw.
With such an installation there won't be a storage tank necessary. The boiler would supply all radiators with as much energy as they demand. All together or any number.

Storage tanks can't be justified with modulating boilers, esp. if a combi boiler is used, one that produces DHW independantly from the space heating modus.


If you are willing to spend more money because you can't find a ST installer who guarantees the ammortisation of his work - if you have to buy what the SEI propagates that is- then you have to install a thermal storage tank within the house.

In this case it makes sense to run the ST DHW system independently from the central heating system as well.

Reason: A central heating system should be run at a temperature as low as possible. Depending on construction method and energy demand this would be at ca. 50 degrees Celsius flow temperature at maximum with radiators and at about 35 degrees Celsius with UFH. If you have to use higher flow temperatures the design is wrong. Lack of thermal insulation, undersized heat emitting surface.

So the heat produced by the boiler won't be enough to heat the DHW. Every heatexchanger (the hot water pipe running through the cylinder) has a certain inefficiency in thermal transmission, a loss of transmission capacity is build-in with every heat exchanger. The very best designs still have a build-in loss of about 5-10 degrees Celsius. Meaning the boiler set on 50 dgrees flow will only deliver 40 degrees in the cylinder/DHW tank. Then some of this energy will be lost through it's shell, insulation. So to get 40 degrees Celsius warm domestic water from 50 degrees flow will be expensive. Since it would take a long (boiler-running)time to heat up the tank.

As an alternative you could use a ST system with it's own tank and tap the water -at whatever temperature there is available from it- to an electric shower. Where it will be warmed up to the necessary temperature - if necessary. Modern electric showers are nowadays available esp. for warm water connections, be it ST or geothermal or district heating, whatever.

But you have to ask for this feature.

Some el. shower manufacturers do not state SOLAR on their product. Then check the max. allowed feed-in temperature. As long as this is 50 degrees Celsius the electric shower would be fine for a solar thermal feed-in. To avoid scalding an automatic mixing valve should installed, so the shower's heating element as well as the person under the shower won't be exposed to more then 50 degrees Celsius in case the sun shines...
Electric showers coming with a " ST o.k ". are frequently already equipped with such a mixing valve, check it out.

The combination of the cheapest and most energy efficient methods will do away with a thermal storage/buffer tank in the central heating system.
The safed costs on the tank will buy two or three electric showers ready for ST. And the gained space is for free.

So my advise on the proposed lay-out by Wolfhound (and check this with a professional!) is to use the cheapest ST collector with an included tank and an electric shower suitable for ST connection.
And a modulating condensing boiler covering the demand (min. and max.), calculated by the professional.

Note that it should be envisaged to reduce the flow-temperature of the central heating system as far as possible. Maybe an outside thermostat at the (shaded) north side of the building in combination with a light sensor at the south side of the building would be an option. These sensors could be included into the stearing of an intelligent heating system. But the boilers which come with such an 'intelligence' are usually more costly.
In my opinion a surplus feature, but it depends on the local situation, the planned usage of the structure. Two cable ducts in the plaster won't cost the world and will give greater freedom for future changes of the installations.

Good luck !

Carlow52

When I wrote:
What do you mean by DHW in this context?

The question was what the D stood for.

Some folk have it as Domestic

Others have it as Direct as opposed to Indirect, particularly in the context of (stratified) buffer tanks, heat exchanger manifolds and the like.

I was not sure which option you had in mind

HighlyCooL

heinbloed is right a properly sized modulating boiler will but out the heat that is needed and will only start and stop once when called for. its a easier and cheaper option than the buffer tank.
the only reason i thought you wanted to use the solar heating to heat the underfloor was when you said that you were planing on using a buffer tank.

Wolfhound14

Thank you all for your input.

That is certainly a different approach for the Domestic Hot Water.

I assume the way you reduce the flow temp for UFH is to increase the surface area of the UF pipes. I guess this would be achieved by keeping the pipes closer together.

heinbloed

Wolfhound wrote:

I assume the way you reduce the flow temp for UFH is to increase the surface area of the UF pipes. I guess this would be achieved by keeping the pipes closer together.

Yes, indeed. Not only the flow and return temperature could be reduced by this method but the time it takes to heat up the floor's surface would be reduced as well. And the time it takes to lose the thermal energy from the slab (once the boiler is switched off) will take longer, water has a higher thermal capacity per volume than concrete. 2 to 2.5 times as much.
On the other hand plastic (for the pipes) would have less, copper about the the same as water.
So if going for plastic it might make sense to use a 15mm bore or even larger instead of the sometimes offered 10-12mm.

Most UFH suppliers have calculation programs for their clients available where this difference can be calculated, usually free of charge.

Go for more than the legal min. of insulation under the slab. Heat is not only transmitted upwards but downwards in the slab as well.

Paragon Energy

I have installed a few of these systems. They work very well if they are correctly designed and correctly installed.
A buffer tank is the correct way to go with underfloor heating. For DHW, you can either install a second smaller HW cylinder fed with it's circulating pump from the buffer tank. The buffer tank must have 2 coils, one for the solar circuit and another for a back up heat source, such as a condensing oil boiler, etc. The solar circuit coil should be located at the bottom of the tank, whilst the back up coil should be at the top.
Another way of achieving this, is a tank-in-tank set up. This is where there is a smaller tank located inside the upper level of the buffer tank which feeds the DHW. This also works well, but I find they are extremely expensive. If you are not stuck for space, then I would go with a dual coil buffer tank with a smaller second cylinder for DHW.
The size of the buffer tank will be determined by size of the UFH circuits and the DHW cylinder size will be determined by the amount of HW that you require.
Hope this is of some help.
Shane

heinbloed

Paragon Energy wrote:

A buffer tank is the correct way to go with underfloor heating.

Read the posts above again, Paragon Energy.

The slab of an under floor heating IS the thermal buffer, must be calculated as such. If a calculation is done by the installer... or engineer. Correctly sized boilers do not need a buffer tank. Buffer tanks cause waste of thermal energy and financial capacities.
They are only used in cases where the calculation of the the supply and demand shows that the two sides don't match, where an over-supply or an under-demand is the result of a bad installation. A walking stick for the lame so to say.

Paragon Energy

Wolfhound14 wants to combine solar with UFH. The only way you can achieve this with solar & UHF is with the use of a buffer tank as an energy store.
If he wishes to heat the UFH without solar, then yes of course, he will not require a buffer tank.
Apologies if I am not reading correctly!

heinbloed

@ Paragon Energy:

Well, the "buffer tank" is used as a buffer, to avoid fluctuation of the return temperature.
The "thermal storage tank" is used to store produced heat/thermal energy to be used at a later point.
For a ST system designed to capture ST energy and making it available at a later point the thermal storage tank is needed, but not the buffer tank.

I mix these terms up frequently myself.

About sizing the thermal storage tank if intending to supply the under floor heating:
I'm using myself UFH and ST energy. A 5,700 liter water tank (no glycol to avoid making the collectors inefficient !) charged in winter on a sunny day and used immediatly for 96m2 space heating will deliver enough thermal energy for about 2-3 days, including the sunny 'charging day'.

So a 500 liter tank, as planned by Wolfhound14, would deliver only enough thermal energy for a few hours, maybe 1-2 hours. After running the pump for a day (100Watt per hour?) it would have been cheaper to use direct electric heating for this 1 or 2 hours and save the entire ST installation.
Size matters, the surface-volume ratio as well as the thermal storage capacity of a 500l tank is simply to bad to store energy for more than a few more hours.

The commonly seen ST installations are toy installations, they don't have the capacity to save on primary energy or direct costs.

If no larger tank and solar collecting capacity can be installed ( to bring down the price per thermal kWh) then it would be more economical and ecological to charge a smaller storage tank to a higher temperature for DHW and to send the rest of the collected thermal energy via a thermal mixing valve straight through the floor. Where it will be usefull without delay and the associated storage losses.

Slabs are great for storing thermal energy, so make use of what is there anyhow.

Wolfhound14

Thanks again.
I think I'm going to stick with the ST in a 2~300 Litre tank for DHW and use the Condensing boiler directly for the UFH.

Next research topic is Condensing Oil Boiler. I am expecting my first plumber quote back at the weekend so that will give me some ideas on costs for the various components.

heinbloed

Don't give up that easily!
Get a heating engineer to calculate the costs versus the benefits, the solar harvest versus the thermal energy demand.
Using vacuum tubes (the evacuated double glass type) in combination with a vacuum insulated storage tank would reduce thermal losses considerable.

This would give a much better thermal coverage than the bog standard ST system.