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Thread: Mixed Radiator & Wet Underfloor Heating

  1. #1
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    Default Mixed Radiator & Wet Underfloor Heating

    This is really a question about designing a mixed system and how to control it.

    I am planning to replace the existing DHW & CH boiler with a condensing combi. Existing radiators will be replaced on the ground floor except for the kitchen, ground floor toilet and utility room where a wet underfloor system is desirable. First floor radiators will also be replaced except for the bathroom where a towel rail will be installed together with wet underfloor heating.

    What are the problems associated with such a mixed design?

    In particular, how do I manage the earlier warm-up requirement of rooms using underfloor heating to compensate for the thermal lag of such systems especially when it it is common practice to use the cooler CH return to the boiler to drive the underfloor system?

    Thanks,
    ric

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    Here is an answer from a website I found. When I was looking into doing something similar. Ended up just addding more rads to the system.

    "A. 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). Providing the temperature of the return water is adequate, this is adequate to heat your floor/s. Of course though, as water passes through or under your floor it will continue to give up heat to the floor and therefore cool. At this time, a sensor on the manifold detects this temperature drop and if necessary automatically adds water from the boiler flow (water that has just been heated) to ensure the required temperature is maintained in the underfloor loop."

    The site is here http://www.gasapplianceguide.co.uk just click on the frequently asked questions tab.

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    Quote Originally Posted by toscal
    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)...
    Thanks for your reply. This is the crux of the problem. Because of the thermal lag, underfloor heating will have to come on before conventional rads are heated. If underfloor heating uses the cool return it seems to me the underfloor heating is dependent on hot water to the rads and therefore cannot be heated before the conventional rads are heated.

    I'm thinking others must have been here before and found a solution.
    ric

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    You could always fit some sort of bypass valve, that would by pass the rads and just go straight to the underfloor.
    Another option is to use electric underfloor heating. Warm up times are in minutes.
    I'm having an extension built and will use electric UFH. When used with the marmox insulation, the cable is laid on top of this then you just tile straight over the cable the warm up time is about 25 minutes. The company is Floor Heating Systems Ltd, www.floorheatingsystems.com , They also do a thin film system for wooden laminate floors, and they do the cable on a mat for easy laying if you want.

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    Quote Originally Posted by toscal
    You could always fit some sort of bypass valve, that would by pass the rads and just go straight to the underfloor...
    I have been doing some more reearch and have found a number of UFH systems that take water direct from the boiler supply and mix it with cooler water from the boiler return to obtain a more optimised temperature of around 50 deg C. There are also systems that incorporate over-temperature sensors to prevent floor temps in excess of 29 deg C as well as systems that incorporate under-temperature sensors to maintain adequate floor temps in bathrooms - especially where tiles are used.

    Not yet sure about combi systems where the pressure is higher - 6 bar seems to be the upper limit which is way below my mains water pressure.

    What's clear is that the UK has a number of suppliers of UFH systems, each slightly different, so there is much to consider (floor distribution plate or no floor plate / loading on joists when using dry mix screed, notching sizes for joists, etc.) Seems like a structural engineer is needed.

    Retro-fitting UFH is a big job and, since it involves major upheaval, is not for the faint-hearted.

    Thanks for the links by the way - very useful reading.
    ric

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    I have a combi boiler and the water pressure for the heating of the rads is about 1.5 to 2.5 Bar. So I don't think you will have a problem. My guage goes up to 6bar but the water pressure shouldn't go above 3.5, if it does you get a wet floor, as the pressure relief vavle goes off.
    But yes you are right, many companies doing similar products all saying that theirs is the best etc. If you go for a mixed system I would definately try and find someone who has done this sort of installation before.
    One company that springs to mind is Wirsbo at www.wirsbo.com, their pro panel series might be what your looking for.

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    Thanks everyone for your replies.

    I have done the calculations for the heat losses from the proposed rooms. In every case the the heat that can be supplied by UFH from the net area availale for heating is way below the heat losses from the room itself. I think this is a common problem in older properties where the U-values are way below current building regulations. I used an ouput of 70W/sq m for the UFH output - a typical value for suspended timber floors. I also used a nominal outside temperature of -3C rather than the more conventional -1C.

    Even with an upgrade to full loft insulation, low-e double glazing and insulation fitted underneath the suspended timber floor (ground-level), the figures do not stack up. The only advantage is that where tiles are to be fitted, the UFH would provide a warm floor, nothing more. However, the disadvanteges are considerable: complexity of marrying UFH and conventional rads; capital cost of installing UFH; more equipment to go wrong, etc...

    Because each room where UFH could be fitted would need supplemental heating from conventional rads, the most obvious and cost-effective solution is to maximise insulation where possible and rely solely on conventional radiators.

    The exercise has been useful - even if it didn't supply the results I would have hoped for. Anyone else reading this post should most definitely consider UFH only where the building itself is sufficiently insulated. In my case, the existing vented cavity walls are the main source of heat losses and, being vented, cavity wall insulation is not an option.

    Thanks again,

    Ric

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    Hello Gentlemen

    I read your posts with intrest recently.

    I am a tech support engineer for Speedfit who are the gloabal leaders in UFH systems & Plastic plumbing products.

    There is no need for Rads when using UFH, more than enough heat is pushed out to keep you warm.
    Total loading on a system is tiny, I have done systems over 300 m2 that have single figure loadings on any boiler.
    At Speedfit we offer a completly free quotation service & will provide you or your installer with detailed plans with full CAD drawings, again I must point out this is all completly FREE. If you are a first time installer I will even come back tot he job & assist you on the day of installation & show all the tricks of the trade. After this, when the screed (if applicable) has gone off I will return & set the system up for you.

    Our products are available from every plumbing, building, DIY or catalogue company out there so supply is never an issue.

    Regards

    Speedfit Tech Support

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    In particular, how do I manage the earlier warm-up requirement of rooms using underfloor heating to compensate for the thermal lag of such systems

    If you use Hometronic to control the underfloor heating manifold one of the set up options is optimum start. The self learning optimum start will enable the underfloor heating circuit based on the current room temperature, ensuring the room is at the right temp 21 deg Cwhen you want occupancy. When un-occupied the temperature setpoint is reduced to a preset say 16.

    A demand for underfloor heating will enable the boiler / circ pump, all you r other radiator zones will be disabled until required by their individual time programs. Again the radiators can be set to optimum start.

    If you would like any further info on Hometronic I would be pleased to offer advice.

    Jason

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    Default Re: Mixed Radiator & Wet Underfloor Heating

    ABout the underfloor heating, you can read the article at http://dezinebathrooms.com The name of the product is vysal. Have a look, i hope its help to get more information.

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