boiler with two water temperature for heating

[evgeny]

Does anybody know a boiler that is able to supply hot water for heating with two different temperatures.
We need to heat megaflow cylinder and under floor heating. For Megafloow we need water of 80-90 degrees and for UFH( under floor heating) 60 degrees.

We can of course heat all water to 80 degrees and then mix with cold water to get to 60 degrees suitable for UFH, but this is much less efficient.

Any help or reference would be great.

Thanks.

 

[dontknowitall]

I've not done underfloor heating, but have you got those the right way round? Domestic hot water should be at 60 degrees.

Possible confusion here though. One or two details might be wrong but essentially ...

Inside the cylinder is a coil which keeps the central heating water (underfloor in your case) seperate from the hot water. The boiler heats water through a primary pipe system (flow from the boiler, through the cylinder then return to the boiler). Water will be sent from the boiler at around 80-90 degrees.

There are one or two valves near the cylinder for hot water and central heating. The programmer will turn on one or two valves (depending on system) and supply hot water to either the hot water only or the heating only or both (depending on your system).

If supplying hot water only, hot water will travel from the boiler via the flow to the coil then return to the boiler. The water in the cylinder will pick up heat from this coil through convection and will warm up to the thermostat level (60 degrees).

If supplying heating only water will travel from the boiler via the flow to the pipework near the cylinder then be diverted to the heating, then return again to the boiler.

There are various setups depending on your house, the design of the heating system and so on but the above is a summary of what happens in most cases.

 

[evgeny]

Right, is it possible that when boiler indirectly heat cylinder it heats it with water 80 degrees, but when heat underfloor heating it heats it with 60 degrees?

 

[dontknowitall]

Am I missing a trick here?

Hot water from the boiler goes to the cylinder coil at, say 80 degrees and returns at, say 70 degrees. (Primary circuit Flow and Return as explained above.)

The hot water cylinder's thermostat will be set to 60 degrees. If calling for heat at say 40 degrees, boiler turns on and heats up the primary circuit chucking out water at 80 degrees. When cylinder reaches 60 degrees, it sends a satisfied signal to boiler which then turns off again. The boiler is capable to heating water to 80 degrees but the thermostat is preventing this happen.

As an aside, the hot water cylinder's water should not exceed 60 degrees due to risk of scalding and any responsible plumber will turn it down if he sees it at the wrong setting.

I've just had a (very) quick look at some instructions for underfloor heating and they seem to be supplied with safety mechanisms designed to keep temperature down to below 60 degrees and also with zone valves (as mentioned above) which will stop excess heat entering the manifold. The instructions I looked at seemed to indicated an operating temperature of 45-50 degrees.

So in the same way that you cannot heat a hot water cylinder to more than 60 degrees (or whatever the thermostat is set to) you cannot overheat an underfloor heating system (again due to its thermostat).

The radiators will require more heat (up to say 70 degrees).

So as long as a boiler can chuck out 80 degrees it can heat the radiators, underfloor heating and hot water at 70 degrees, 45 degrees and 60 degrees respectively. The various thermostats prevent excess heat reaching those elements.

If you think in car terms, a car can be driven at 80mph (boiler chucking out heat at 80 degrees) but we prefer to drive them at 60mph on main roads (thermostat set to 60 degrees hot water cylinder) or 40mph in town outskirts (thermostat set to 40 degrees for underfloor heating). Just because a car is capable of being driven at 80mph all the time does not mean it's always being driven at those speeds.

 

[evgeny]

Correct, what is happening for UFH when it is calling for heat the boiler turns on and supply a water of 80 degrees. This water is then mixed by the manifold valve with the return to the 50 or 60 degrees and sent into pipes under floor.
What we would like to achieve is: instead of heating water up to 80 degrees and then mixing with cold water in manifold mixing valve to get water of 60 degrees simply to try to heat it by boiler up to only 60 degrees. This should be much more energy efficient.
The problem is if we set boiler output water temperature to 60 degees the same 60 degrees water will be trying to indirectly heat megaflow cylinder. This will not work since 60 is too low.

( I have tried to discuss possible setup with Worcester 30CDI bu it seems impossible http://www.ukplumbersforums.co.uk/c...nt-temperatures-fo-ufh-megaflow-cylinder.html )

Any help will be great

 

[doitmyself]

The Avanta System boiler with weather compensation will do what you want, but you will still need a mixer valve. However this does not mix cold water with hot, it mixes part of the returning water with the flow.

See Option 19 in the Schematics Manual.

To explain how it works.

The boiler can be in either central heating mode or water heating mode; you can't have both at the same time. This is controlled by the diverter valve just below the boiler, which is installed the "wrong" way round. Normally this valve has port B open, so water circulates through the heating circuit. The temperature of this water is controlled by the weather compensator.

When the hot water needs to be heated up, the diverter valve, controlled by the cylinder thermostat, switches over so the water now travels through the HW cylinder. At the same time, the boiler temperature automatically rises to maximum.

Although the diagram shows an internal clock, the system can be used with external clocks, Opentherm devices, and any programmer/thermostat.

 

[evgeny]

Thanks do you know if the same can be done to Worchester 30CDI. It also have diverter valve and different temperature for DHW ?

 

[WHPES]

We can of course heat all water to 80 degrees and then mix with cold water to get to 60 degrees suitable for UFH, but this is much less efficient.

No. It makes little difference - you still put in the same amount of heat and get out ths same amount of heat. If it is hotter and it is being mixed, it requires less hot water to mix with the return water so same amount of energy used. In fact it is usually less efficient with similar temperatures as you have less thermal potential and so heat transfer is slower than between two surfaces at greater temperature difference.

Like the old conundrum - does my tea cool quicker if I leave it for a couple of minutes and then put the milk in, or should I put the milk in and then leave a couple of minutes. The answer is of course the former as the energy is more readily transferred from the tea to the air when it is hottest.

Likewise if say you wanted your cylinder at 60 degrees so you set your boiler output at 60 degrees. The result would be that it would take ages due to the low potential between the two liquids, the boiler would be more or less constantly running and your latent heat losses would be great.

In industry, multi tubular high pressure superheated steam boilers are often used as the heat transfer is most rapid so losses are reduced.

 

[migoplumber]

many cylinders store water at 80 degrees, sat with a blending valve on top set to 60 degrees, gives you an extra 25% stored hot water.
most ufh manifolds also have a blending valve at input reducing temperature to 50degrees. at 55degrees most screed flooring is going to start cracking.

 

[Blackcatgas]

you could use a Low loss header, (but whats wrong with using a mixing valve integral with the ufh?)

 

[doitmyself]

Thanks do you know if the same can be done to Worchester 30CDI. It also have diverter valve and different temperature for DHW ?

The 30CDi system boiler can have an optional diverter valve, which is used with either the FR110 or FW100 programmable thermostat to provide heating or hot water. The FW100 is a weather compensated stat, so the flow temperature varies with heating requirement.

Thinking logically, if the weather compensator has set the flow temperature to, say 50C, and the HW side now calls for the water to be heated to 60C the boiler's temperature must rise above 60C or the water in the cylinder would never reach 60C.

You said, in your other post that you have a flat. This begs the question: why do you need a 30kW boiler? How did you size the boiler?

 

[evgeny]

Thanks, indeed I think FW100 or FR110 will do exactly what we need. The reason we chose 30kW boiler was our plumber recommendation. Now looking at spec I think this is the only boiler that can have different temperature for cylinder heating and CH.

 

[doitmyself]

The reason we chose 30kW boiler was our plumber recommendation.

A 30kW boiler to heat a flat is overkill. Use the Boiler Size Calculator to find out what size boiler you really need.

 

[F_P_H]

Im fairly new to this game (only been working with heating for 2ish years) so this is probably going to be a really stupid suggestion but hey ho, Ill give it a go anyway!
How about using a combi boiler, send the CH flow out to the UFH (that's a bit more controllable that the DHW), then instead of cold mains water going in at the input set it on a loop linking it to the DHW outlet and through the coil of the cylinder. Add a pump into the mixture to keep it moving as if the cold mains was running through it. Set the hot water temp very low as not as much heat is going to be needed as if it was heating cold water, fiddle about with the temperature of the DHW until you get the right amount of heat going to the cylinder to heat the hot water.
Please forgive me if this is a ridiculous idea, and actually I would be interested to know of a reason why this cant be done as Im always keen to learn more!