Perfect boiler temperature re ufh?

[DH202020]

Good day everyone!

 

Trying to set the 'sweet spot' temperature on the combination boiler for ufh in my newly finished build.

 

My ufh mixer is set on 35.5° (lowest point)

Worcester Greenstar combi set at 46°

Room thermostat at 19° (open plan g/f)

 

Is there such a thing as a truly eco setting?

Should I raise the ufh setting on boiler knob to  'E' (think that's some kind of economy setting around 58°?)

 

Confused

Thanks

David.

[SteamyTea]

As I understand it it is a balance between the amount of energy in the gas and the amount of energy recovered when the boiler is condensing.

You will probably have to play about with the settings a bit to work out what is best for your set up.

 

[Nickfromwales]

Not going to be that easy sorry. 

As the UFH will never ask for more than 35oC, the return temp will never get high enough for long enough as the boiler will undoubtedly short cycle or stay in full modulation. 

This will be particularly problematic when the UFH is up to temp and requiring very little heat input. Fitting a buffer tank in between the boiler and the UFH would solve this, where the boiler would fire infrequently at higher temp to maintain the buffer stat set temp, whilst the UFH was able to draw as much or as little heat as it required. 

As your install stands, it's likely your boiler will just sit at a low modulating temp for the rest of its days, away from the range required for condensing. . 

Can you retrofit a buffer tank, say in an airing cupboard, where the heat loss would be useful?

( The E setting iirc is the sweet spot for condensing to occur ). 

Hope that helps 

[Jeremy Harris]

At our old house I spent a fair bit of time faffing about measuring flow and return temperatures and the flue gas temp at the terminal, to try and determine if there was a "sweet spot" in terms of efficiency.  It turned out that if the return temperature was above 55 deg C  the boiler flue gas temp was sky high, and it was definitely not condensing.  As the return temp dropped there was a big jump in efficiency at around 50 deg C, and a corresponding big drop in flue gas temperature.  From then on, reducing the return (not flow, but they are clearly related) temperature further gave a fairly linear improvement in efficiency, until I reached the point where there wasn't enough heat output to keep the house warm.  Our boiler was over-sized, because I didn't size it, the installers did, and they refused point-blank to take account of the added insulation I'd fitted (350mm in the loft, plus ~50mm bonded graphite bead EPS in the walls) or the fairly decent (for the time) 28mm DG.   It means that all the radiators are too big now, and the boiler has too high a capacity, but the one advantage is that I can run at a 48 deg C flow and about 43 to 44 deg C return with the boiler in fully condensing mode.  It makes a worthwhile difference; probably knocks about 10 to 15% off the gas bill.

 

As Nick says, for low temperature UFH you need something for the boiler (or heat pump) to work into, so that the boiler (or heat pump) can run efficiently and without going into short-cycle protection mode.   A buffer is pretty much essential with a boiler, I think, as otherwise it's going to try and modulate down as low as it can, still chuck out too much heat and then it'll turn off and wait of the short-cycle delay before firing up again and doing the same.  This won't be efficient, as it takes a few minutes of running for a boiler to stabilise and start running efficiently, so the greater the number of times it fires in a day the worse the efficiency.  Ideally you want it to fire and stay on for a fair time, then turn off and stay off for a long time, whilst the heating draws stored heat from the buffer.  @TerryE is going for a system of using the slab as a buffer, which should work with the low flow temperature from a heat pump, but won't really work with the higher flow temp from a boiler, I think.

Edited February 23, 2017 by JSHarris

[TerryE]

6 hours ago, DH202020 said:

Is there such a thing as a truly eco setting?  Should I raise the ufh setting on boiler knob to  'E' (think that's some kind of economy setting around 58°?)

 

Simple Qs, but the answer is complicated.  The first thing to establish is what your rough average heating requirement is.  This depend of a buck of losses: through the walls, slab, roof, windows, air exchange losses.  In a house build corrrectly  to the 2013 Bregs, this should work out at maybe 5 kW.  However houses as built by most of the major builders are far below this one average and clearly the earlier your house was built then in general the less energy effcient it will be and  the more heat losses that you will need to balance.  At the other extreme you've got super insulated houses like Jeremy's and mine which need less than 1kW input in the depths of winter.  So my Q1 back to you is: how much heat do you need to supply as a base input to keep your house in balance.

 

So if you need 5 kW, say, and you have 200m² of UFH area then the floor will need to deliver ~25 W/m² of heat into your lining space which requires the floor surface to be at roughly ambient + 4°C.   For your house this might be more or less, but if it is much more than 10°C then you will have problems with hot spots.   So you need to work out roughly what the ideal floor temperature should be for your demands.  Clearly this will drop as the weather gets warmer since your heating demands drop. 

 

So now this is the circle that you've got to square.  On the one hand if you pump water into the slab a lot hotter than this, then your controls system will end up on a roller coaster of heating  the floor and waiting for it to dump heat and cool down again,  but if you have the water demand temperature too low then the boiler will continually short-cycle, which is very inefficient and this really kills the life of the boiler.   As  Jeremy mentioned in passing, the only sensible way out of this is to have a decent lagged buffer tank.  Your boiler reheats this buffer on on/off temperature thresholds so that each heating chunk is a sensible amount - - say 30-40 mins of continuous output at its optimum output setting to lift the buffer to 40°C say.  Your UFH manifold / pump / mixer then blends this buffered hot water into the slab at  ambient + 6-8°C if you want to sustain +4°C, say.  The boiler then run for 30mins every 3 or so hours, and the UFH pump circulates pretty continuously, trickling just enough heat to keep the rooms comfortable..

[DH202020]

11 hours ago, Nickfromwales said:

 

Can you retrofit a buffer tank, say in an airing cupboard, where the heat loss would be useful?

( The E setting iirc is the sweet spot for condensing to occur ). 

Hope that helps 

And that there may be the biggest problem....right space...mmmmmm ...thanks Nick

[DH202020]

Jeremy/TerryE

 

I used Jeremy's calculator for possible heating requirement...works out a little less than 1kWh a day in mid winter (we are in southeast so probably outside temperature is a little higher these days)

 

House is fairly insulated...MBC insulated base/floor.

Walls 16

Roof 14

Windows triple passive

 

House warms nicely within 1hr to around 19.5° (sometimes doesn't come on for 48hrs)

Have set room thermostat 19°

 

Was wondering... could set room thermostat to just fire up for 1hr or less and try altering the boiler for best results??

 

I have noticed that setting boiler at 48°

Floor mixer to min 35° the return is always cooler (not sure temp) than flow!)

Will keep a close eye on it for a time

[Jeremy Harris]

I'm logging some data at the moment to try and see what's really going on in terms of heat output into the house. 

 

Anecdotally, I can say that our heating often goes for a day or two in mid-winter without coming on at all, then maybe fires up for an hour or so and shuts down for another day or so.  I have our room stat set for 20.5 deg C and the ground floor seems to be pretty stable, at between 20.5 and 21 deg C, with the unheated bedrooms upstairs being about 1 deg cooler as a rule.

 

I'd be inclined to set the boiler to as low as is reasonable within its modulation range, which depends very much on how much heat the UFH is pulling from it when its on.  48 deg C flow should be pretty good in terms of boiler efficiency, as the return will be cooler and the boiler should be fully condensing.  With the floor mixer right up at 35 deg C there's a risk you might get big room temperature overshoots.  This happened when I had our mixer set to 28 deg C; the room temperature would continue to increase for a couple of hours or more after the heating had turned off.  I run our UFH mixer at around 23 to 24 deg C now, just about as low as it will go, and that helps a great deal in terms of keeping the temperature more stable. 

 

I did hunt around for thermostatic mixer valves that would operate reliably at low flow temperatures, as this was something that had come up a few years ago on another forum, where someone with a similar spec house had problems getting the UFH to control properly.  It was where I learned about the significance of having a low flow temperature and that few mixer valves allow the sort of low temperature flow adjustment a low energy house seems to need.  The Wunda manifold I used uses an external temperature probe, fitted inside a pocket in the centre of the flow manifold, to control what looks very like a large thermostatic radiator valve on the hot inlet side.  Sadly it seems that Wunda have now changed the design, and I've no idea if the three way mixer they now fit will adjust down to such low flow temperatures.

[TerryE]

18 hours ago, DH202020 said:

I used Jeremy's calculator for possible heating requirement...works out a little less than 1kWh a day in mid winter

 

I do have to wonder how you are going to use your boiler.  Sounds like you need a buffer tank to me.  If you want your UFH to output roughly 1kW then you'll only want the slab a few degree's above target if you are heating over extended periods but don't want to cook.  Have a read of both Jemery's and my blogs where we discuss the design issues.

[PeterW]

11 hours ago, JSHarris said:

Sadly it seems that Wunda have now changed the design, and I've no idea if the three way mixer they now fit will adjust down to such low flow temperatures.

 

Wunda now use Esbe valves which are pretty bullet proof when I've used them as load valves. I asked the question a month or two back as my target temp is 31c and got this response :

 

The temperature range for the ESBE valve on the manifold in question is 30 degrees to 70 degrees.

 

HTH

[Jeremy Harris]

Health warning -  I've not tried this!

 

When I was first trying to model how our UFH would work, I had a great deal of help and advice, from several sources.  I found that some had found ways to modify TMVs to run at lower temperatures.  As it happens I didn't need to do this, as the Wunda manifold I used had a TMV that work very well down to about 24 deg, then loses control at around 22 deg C.

 

Somewhere I have some links as to how to adapt some TMVs to work at lower temperatures as there's a very worthwhile benefit for a low energy home in having a low flow temperature, as it minimise the room temperature overshoot problem.  It's a relatively easy mod, as I recall, so if I can find it then it may well be useful and I'll try and remember to post the details here.

[DH202020]

4 hours ago, TerryE said:

 

I do have to wonder how you are going to use your boiler.  Sounds like you need a buffer tank to me.  If you want your UFH to output roughly 1kW then you'll only want the slab a few degree's above target if you are heating over extended periods but don't want to cook.  Have a read of both Jemery's and my blogs where we discuss the design issues.

Thanks TerryE, will have  a look round.

Just a note on over heating... we have not had that problem at all via the boiler, seems to heat 1° above target, floor feels warm but not hot and remains like that for most of the day (depending upon weather conditions).

We have stone tiles and bamboo floorings.

[DH202020]

14 hours ago, JSHarris said:

I'm logging some data at the moment to try and see what's really going on in terms of heat output into the house. 

 

Anecdotally, I can say that our heating often goes for a day or two in mid-winter without coming on at all, then maybe fires up for an hour or so and shuts down for another day or so.  I have our room stat set for 20.5 deg C and the ground floor seems to be pretty stable, at between 20.5 and 21 deg C, with the unheated bedrooms upstairs being about 1 deg cooler as a rule.

I'd be inclined to set the boiler to as low as is reasonable within its modulation range, which depends very much on how much heat the UFH is pulling from it when its on.  48 deg C flow should be pretty good in terms of boiler efficiency, as the return will be cooler and the boiler should be fully condensing.  With the floor mixer right up at 35 deg C there's a risk you might get big room temperature overshoots. 

Thanks Jeremy

Now for the really ignorant bit!

How do I know when it's fully condensing? (Or not?)

Sorry to be a real thick head but I don't actually know what that should look (sound) like?

Any pointers would be appreciated

[TerryE]

23 minutes ago, DH202020 said:

How do I know when it's fully condensing? (Or not?)

stand outside an look at the vent.  If its condensing, it will come out as steam rather than turning to steam 10-30cm away from the vent. 

[DH202020]

Guess what I will be doing this weekend then?

[SteamyTea]

6 hours ago, DH202020 said:

Guess what I will be doing this weekend then?

Log down your gas usage at the same time.  It is easy to get external temperature data afterwards to correlate it with.

[Jeremy Harris]

12 hours ago, DH202020 said:

Thanks Jeremy

Now for the really ignorant bit!

How do I know when it's fully condensing? (Or not?)

Sorry to be a real thick head but I don't actually know what that should look (sound) like?

Any pointers would be appreciated

 

 

As Terry says, or you can just measure the return temperature to the boiler.  If the return is over about 50 deg C the chances are it's not condensing much, if at all.  Or you can measure the flue gas temperature at the outlet and see how hot it is.  The flue gas temperature drops sharply as the boiler starts to condense.  If doing this, then use a small temperature probe so as to not restrict the flue outlet.