Help! Different temp outputs for UFH and rads

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vincechiew

We are looking to install UFH in an extension (roughly 60m2) in a Victorian house and the rest of the house will have radiators.

I have a concern that it won't be very efficient system as we will have to reduce the temperature for the UFH and reheat it for the radiators. That is my understanding anyway running on a combi system which is our preferred heating system although as it is a renovation we are open to other ideas.

Do you know if this cost is negligible or whether there is a way to have different outputs for radiators and UFH?
 
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I thought on that area the running costs for electric would be quite high.

Would people recommend an electric system in this case?
 
vincechiew said:
We are looking to install UFH in an extension (roughly 60m2) in a Victorian house and the rest of the house will have radiators.

I have a concern that it won't be very efficient system as we will have to reduce the temperature for the UFH and reheat it for the radiators. That is my understanding anyway running on a combi system which is our preferred heating system although as it is a renovation we are open to other ideas.

Do you know if this cost is negligible or whether there is a way to have different outputs for radiators and UFH?
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First thing to do is check that a combi boiler (instant hot water) is suitable. This will depend on:

a)the number of hot and cold water outlets which could be running at the same time.
b)the flow rate and pressure of the cold water entering the house.

Don't forget that the water coming in to the house will be shared by all outlets, hot and cold.

As for your mixed rad/underfloor question, this might help:

[DLMURL="http://www.gasapplianceguide.co.uk/UFH%20questions.htm#Q._How_can_water_in_and_underfloor_heating_system_work_at_a_cooler_temperature_than_the_radiators_in_the_rest_of_the_house_in_a_mixed_wet_system"]Underfloor Heating and Frequently Asked Questions (FAQ)[/DLMURL]
 
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Thanks 'doitmyself', believe the flow rate is sufficient for our needs.

In regards to the FAQ:
Wet underfloor heating systems actually use the boiler's return water. In a heating system, return water (water returning to the boiler having been around the system) is cooler than the flow (water from the boiler that has just been heated).


My understanding was that UFH operates at around 40 degrees whilst radiators at 70 degrees, are we saying that the return loses 30 degrees? that seems a rather large loss.

My concern is also that if I were to just run the UFH in isolation then I would still need to add cold water to reduce it from the 70 degrees which sounds like a waste of money.

Any ideas what the running costs are for UFH and radiators?
 
vincechiew said:
My understanding was that UFH operates at around 40 degrees whilst radiators at 70 degrees, are we saying that the return loses 30 degrees? that seems a rather large loss.

My concern is also that if I were to just run the UFH in isolation then I would still need to add cold water to reduce it from the 70 degrees which sounds like a waste of money.
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A boiler can run at virtually any temperature between 40C and 80C. The power output from a boiler is proportional to the temperature differential between flow and return. So a boiler running at 40C/30C will give the same output as one running at 80C/70C. Though, having said that, condensing boilers are more efficient when the return temperature is below 55C, so the output will actually be slightly higher in the first case.

Although boiler outputs are the same, radiator outputs are related to the flow, return and room temperatures. So a 1kW boiler only gives that output under specified conditions (laid down by BS EN442)' It is not like a 1kW electric heater which always gives off 1kW.

That means that you can vary the output of a radiator by varying the flow temperature. This feature is made use of in systems with weather compensation.

You could, therefore, run your entire system with flow and return temperatures the same for the radiator and underfloor systems, provided the radiators have been properly sized to take account of the lower flow and return temperatures.

You say that the rest of the house will have radiators. So I assume they have not been installed yet. Have you purchased or calculated the size of the radiators? If not, then assuming you want to run the rads at temperatures of 60c flow and 50c return, radiators will have to be 60% oversized. So, if you need a 1kW rad you will have to install a 1.6kW rad to give the same amount of heat.

The underfloor heating can then run with a 50C flow and 40C return. This means the total drop across the boiler is 20C (60C-50C-40C). Most modern condensing boilers are quite happy running with a 20C differential.

The underfloor system will have its own regulator, e.g a mixer valve, to make sure the temperature is correct.

Also see [DLMURL="http://www.warmafloor.co.uk/knowledge-center/system-design.asp"]Underfloor heating system design[/DLMURL]
 
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Thanks again, the property doesn't have central heating at the moment so that sounds like a good idea about oversizing the radiators to allow the system to operate as a constant temperature.

On another matter can a combi supply adequate pressure for 2 bathrooms in a property? What sort of mains pressure do I need and how do the flow rates and outputs KW translate?
 
generally 2 circuits are installed from the boiler.
the ct circuit which is the operating temperature of the boiler and the temperature of the water in the radiators.
the vt circuit (variable temperature) which can be varied to suit the design of the underfloor heating circuit. the vt circuit is just "tee'd" off the ct circuit, and a 3 way valve linked in - so when the required temperature is met the excess heat is returned directly to the boiler. the components for this set up are generally included 'all in' the ufh manifold.
you never want to exceed 55 degress on an ufh set up, and for lower temps you could have it fixed, or use an optimizer to make it warmer in cold weather and cooler in warmer weather.
have fun.
 
Thanks Shaun.

Are the running costs the same in the following three situations:
1. Radiators only
2. UFH only
3. UFH and Radiators

Am I right in thinking that with a combi you get greater efficiency with a lower return flow temperature so UFH only would mean reduced running costs.

Would the vt circuit for the UFH would be in isolation from the rest of the radiators?
 
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running costs.. good question. you need to define running costs!
any house requires a certain amount of heat to warm it up, and keep it at that temperature, an input of more heat is required to keep it at that temperature - covering losses.
one way of looking at it is:- you put 50kw of heat into the house to keep it comfortable and nice with associated losses.
another way is 50kw of heat into a building split between ufh and radiators some bits lose more heat than others.
the only way to test exact effiecency is by having an experienced energy effiecency engineer come into your house and paying him a lot of money to do that !!

i like 1st idea best. and thats what i use. everybody will have a different point of view !!

my next house (new build) will be a mix of ufh and rads with a gas boiler and air source heat pump, and a nice & complicated control system. if the control system breaks down then effiecency will go out the window!
 
Good point Shaun. Think I will suck it and see.

You don't happen to know about insulation and ventilation do you? We are thoroughly insulated the extension but the only ventilation will be via trickle vents on the doors and I am wondering what effect this will have on my u-values which I have tried so hard to keep low.

Don't think I can afford a heat exchange system just for the one room.

And good luck with the new build!
 
and for one room not worth it. mine is an ongoing project, if im lucky completed within 10 years.

now your looking for difficult answers !!
every instalation is different and generally not transferable calculations. i didnt make mine, paid somebody to help as it is beyond my scope
what i realised afterwards is "it really dont matter that much"
 
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