# Underfloor heating questions



## David_Dublin (4 Feb 2016)

Hi - just a few questions regarding underfloor heating, I'd appreciate if someone could provide some input.

We're building a kitchen extension at the gable end of the house. It will be approx 10 metres long (the depth of the house), 5 metres wide. It will connect to the existing house through two large openings without doors to a room 4m x 4m

does it "make sense" to have effectively an open plan of two spaces, one which is underfloor heated, the other which is not
would we need to heat the adjacent room with a rad, or "should" the UFH be sufficient to heat both spaces. Maybe we'd need to just see how it goes?
the intention would be to have a double sided stove either side of the two large openings between the spaces (this is a chimney), does that make sense when there is UFH already in place?
how do I know if my current boiler will be able to run underfloor in one room, what I have now is a Baxi Megaflo 24 (I think that's what it is called)
Anything else I need to consider?
Thanks!


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## Branz (4 Feb 2016)

U/F in one wont heat the other room.
I would not mix the two systems, can be done but very messy.
How big is the rest of the house vis a vis the mega flo  24: if the 24  is kW, then may be too small for the extra 50 sq meters


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## David_Dublin (5 Feb 2016)

Thanks for the reply. The current house is about 1600sq ft. There are 9 rads, all doubles, 3 pretty big ones.


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## David_Dublin (5 Feb 2016)

@Branz - I just read this article about mixing UFCH with Rads, seems like a fairly simple solution https://www.underfloorheatingsystem...raditional-radiators-with-underfloor-heating/

Have you any more info about how it can be very messy, maybe some pointed on what issues that it is likely to cause, or complications in putting it in place.


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## Branz (5 Feb 2016)

from an energy perspective, heating water to 65 or 70 and then blending it back down for the UFH is just bad science.
UFH is usually available 24 7 which means that the boiler must be available 24 7 and the blending process will result in repeated cycling of the boiler, reducing efficiency and boiler life.
the return flow may also be too high to allow the boiler condense which means that it is less efficient.

From a practical standpoint, the additional manifold and pump and blending valve will all have to be accommodated in the new extension, especially is existing floors are concrete

50 sq meters is a big addition to 1,600 sq feet so I would get some math done on existing heat demand, including HW


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## David_Dublin (5 Feb 2016)

Thanks again branz, I appreciate you taking the time to post such a detailed reply.


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## Buddyboy (5 Feb 2016)

That is exactly the system we have, oil condensing boiler, underfloor in the living room and kitchen, and rads in the hall, utility, and bedrooms. It works extremely well (for us).  We don't have the underfloor on a 24/7 basis. I replaced the originally installed thermostats with programmable (heatmiser) thermostats, as also referenced in the article. This makes a big difference, and as we are both out during the day, I can step back the underfloor during the day and at night.

The underfloor takes around 1 hour to change 1 degree, so it never drops too low when not on.  In fact, as the room is facing due south with a lot of glazing, we have in effect a massive heat sink in the floor so the underfloor doesn't even turn on for half the year.

As it is an oil boiler, it isn't variable, so it's 70 degrees or nothing (gas boilers can modulate).  The "blending" is in fact  a valve on the underfloor manifold, which mixes the 70 degree water with the returning underfloor water and pumps it back (using the pump on the manifold) to the underfloor pipes at around 45 degrees. As the underfloor isn't on all day, the water in the underfloor pipes is initially around 20 degrees, so the water returning to the boiler has enough delta for the boiler to be in condensing mode. At least until the temperature rises. But then the same can be said about radiators in rooms. Once the room is warm enough, the rad isn't losing enough heat to allow condensing mode.

Is the the most efficient?  couldn't tell you, but it is the best and most realistic solution for us, given what we had to work with.   And has proven pretty cheap to run. And the underfloor heat is joy instead of rads.  Rads would not have worked in our living room, which is about 65 sq meters with two walls of glass and not a reasonable chance to hang rads on the remaining 1 & 1/2 walls.

The way I see it is that I have lovely radiant heat from the underfloor all the time I'm there in the living areas, and I have immediate heat from the rads for the half hour in the morning and half hour in the evening when I am in those rooms.

The manifold is in the boiler house with the boiler and water tanks (hot and cold).

The house is around 2000 sq ft bungalow, and we use around 2400ltr tanks of oil a year.  It is pretty well insulated with 800mm of insulation in the attic and standard block wall construction with pumped insulation between the leaves. House is around 45 years old, but with upgrades. Two occupants, both out during the day.

I'm not a plumber/boiler technician/builder and don't play one on TV, but did a lot of research before we upgraded.


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## David_Dublin (5 Feb 2016)

Thanks for the detailed post Buddyboy. 



Buddyboy said:


> We don't have the underfloor on a 24/7 basis. I replaced the originally installed thermostats with programmable (heatmiser) thermostats, as also referenced in the article. This makes a big difference, and as we are both out during the day, I can step back the underfloor during the day and at night.



Regarding the above comments, what times would you typically have it one for? Presumably you'd end up running the boiler for longer periods because of the length of time that the UFH takes to warm up, i.e. it would need to be on earlier than if you had rads only. We're thinking about solar for water, but I'm not convinced about this. We have a large roof south facing.

@Branz : Existing floors are wood, old wood floors (with gaps) nailed to joists. We're planning on sealing/insulating them from below as part of the renovation.


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## Buddyboy (5 Feb 2016)

IIRC - I've set the times for the UF for 16 degrees - from 10pm onwards (It normally never gets this low).  Don't need to turn it on in the morning - up and out for work.  20 degrees from 4pm - it is normally around 17 or 18 degrees at that point anyway which means it is 20 degrees at 6 pm when we need it.  So in effect it is 20 degrees from 4 to 10 pm. 

One added benefit of having the whole floor prepared as an underfloor, i.e. insulated all round, is that we created a massive heat sink so it rarely deviates more than a few degrees, and with the south and east glazing, stores a hugh amount of solar (free) energy.

We also looked at solar for water, but could never make the figures add up.  I run the boiler for a half an hour in the morning and half an hour at night during the summer to heat the water as I don't need the rads as the bedrooms are warm enough with ambient temps during the summer. and run it in conjunction with the rads for the same time during the winter, as the boiler is more than capable of running both.  So the extra cost of heating the water is minimal, and keeps the boiler ticking over during the summer when it wouldn't otherwise be used. The couple of thousand to install solar wasn't worth if for us, but may be if there are a few teenagers in the house.


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## Leo (5 Feb 2016)

Buddyboy said:


> One added benefit of having the whole floor prepared as an underfloor, i.e. insulated all round, is that we created a massive heat sink so it rarely deviates more than a few degrees, and with the south and east glazing, stores a hugh amount of solar (free) energy.



That'll be the real key to the success of such a system, how well you can insulate that mass so it doesn't lose too much heat, and then require massive energy and hours to bring back up to temperature.


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## David_Dublin (5 Feb 2016)

@Leo for that input. As this will be a new extension, with new foundations/poured floor/insulation, it should be possible to achieve good insulation so that there is as little heat loss as possible. With that in mind, and I know there are data sheets available for minimum suggestions, but if anyone had any suggestions for what thickness/type of materials to use in the floor I'd appreciate it. Always good to have an idea for the builder/QS.

@Buddyboy thanks for the response. When you say "I've set the times for the UF for 16 degrees" does this mean that it kicks in when it gets to 16 degrees, regardless of what time of the day/night it is. And then you have it set to heat to 20 degrees between 4 to 10pm. Sorry if a stupid question.

We will be running 4 showers each morning before long so might look at solar with a large HW tank. I need to move the HW tank (and cold actually) as part of the renovations anyhow.

We'll probably need to be running it in the morning, and have it warm from lunch time I guess as the kids are around from 2pm. I suppose then I would have to have it running from early morning through until mid evening time. 

I suppose it would make sense to turn down the boiler temperature when we are there and only the UFH is needed, albeit we'd inevitably end up forgetting to turn it up again for HW/Rads.


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## Buddyboy (5 Feb 2016)

" When you say "I've set the times for the UF for 16 degrees" does this mean that it kicks in when it gets to 16 degrees, regardless of what time of the day/night it is. And then you have it set to heat to 20 degrees between 4 to 10pm. Sorry if a stupid question.

We will be running 4 showers each morning before long so might look at solar with a large HW tank. I need to move the HW tank (and cold actually) as part of the renovations anyhow.

I suppose it would make sense to turn down the boiler temperature when we are there and only the UFH is needed, albeit we'd inevitably end up forgetting to turn it up again for HW/Rads."

The advantage of the programmable thermostat is that you can set multiple time and temperature combinations. (8 weekday/8 weekend iiirc).  I only need two for the underfloor.  I've set it to go no lower than 16 degrees from 10pm (you're right, it would kick in if the temperature dropped below that - but it never has), and to go to 20 degrees at 4pm - which means the heat turns on then and it takes until about 6pm to rise to that. It then maintains that temp (by running the boiler if needed) until 10pm, at which point it gets set back to 16 degrees (and the boiler would fire if the room dropped below that during the night).

Obviously you can also override any of the above - but remembering that UF takes a long time to change temperature. In fact, I have the tempreture set to 19 degrees at 4pm for Saturday and Sunday, assuming we will be out, but if we are staying in, I up it to 20, and it reverts back to it's programming the following 10pm.

That is a long-winded way of saying you're right in your statement above.

I don't know much about gas/adjustable boilers, but I assume it must be automatically set to fire to 70 degrees when it is asking for hot water or rads, and 45 degrees when it is set to heat the underfloor.

I'd also suggest, if you are building an extension, looking at the various options for glazing etc.  Our windows are argon filled wooden ones, and you can see condensation on the outside, which is a good indication of how efficient they are.

I'd also give consideration to the boiler, a correctly sized boiler should be running flat out when possible, it is far more efficient than an oversized one that isn't being operated to full capacity.

And we moved all the water tanks to the boiler house - its nice to know that they are not overhead in the attic, and it meant that we could have 250ltr tanks.



Research done now will really pay for itself later on.


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