Not enough hot water if UF heating is running at same time time as HW

[Graham W]

Hi,
Please can anybody help with why my CH system does not produce enough hot water when the boiler is required at the same time to provide heat to the underfloor heating? More details here :
1. If the CH is programmed to just provide HW then running it for an hour in the morning produces plenty of HW for the two usual occupants of the house to have showers etc in the morning and leaves plenty of hot water for use throughout the rest of the day.
2. But if the CH is programmed to run for say two hours in the morning but to provide for both HW AND the UFH then once the two of us have had showers there is no hot water left in the tank for the rest of the day.
3. It’s been like this since we moved into the house in April 2021.
I’ve given some details of the house and system below.
Thanks.
Graham
• 2011 build house – modern, very well insulated.
• UH heating to ground floor. Rads and electric UF heating upstairs.
• Vaillant ecoTec Plus Gas boiler- serviced January 2022 and everything fine.
• Main unvented 300L cylinder. Cylinder stat set at 60C. Motorised valve seems to be working fine. Expansion tank checked Jan 2022 and repressurised – was well down before that.
• There is a secondary return pump which seems to be working fine and is timed to run in AM when CH working and when there is demand for showers.
• Mains pressure is excellent.
• Flow out of the taps is strong – H and C.

 

[ShaunCorbs]

Just stagger the times or leave hot water on all the time between 6 and 10 at night

 

[exedon2]

How are you running your underfloor system? Are you running it like a radiator system (on and off)
Or running it correctly with back set temperatures?

 

[Graham W]

Should maybe have said, the boiler is an ecoTec Plus 637.

UFH is set up to run with back set temperatures on the 5 individual RF Polypipe area stats. So UFH Honeywell programmed to be on from 6 AM to 10 PM subject to demand / no demand intsructions from stats. Hope that makes sense and answer your question exedon?

FWIW the demand for the UFH during the day is minimal due to very high insulation and big solar gain though very large double glazed high efficiency windows during daylight hours. So UFH is not generally on at all except for initial period of an hour or so from say 7 AM. Apologies if all that is irrelevant but don't want to leave out something that might be significant.

 

[Graham W]

Staggering the times seems like a good idea so I will give it a try. Thanks Shaun. But I have never had to do that in any previous house? Though not had UFH in any of those.

I presume running the DHW at a different time to the UFH will add to gas consumption. Is that right?

 

[exedon2]

What back set temperature you using? Should typically be 4 C below your target temperature.
As has been said could try staggering timings.
My own cylinder is same size as yours I just leave it permanently on .

 

[Graham W]

Set backs are 4 C below targets.

I've set the DHW to come on for an hour before the UFH kicks in tomorrow so will see how that works out. Many thanks for everybody's help. Best wishes.

 

[SJB060685]

If the slab drops in temperature considerably overnight then come the morning when there’s a demand it will starve any radiator or cylinder circuits.

 

[EvilDrPorkChop]

Run the UFH 24/7, like it should be at a set point. It isn't designed to be timed, you're just wasting heat, heating up a slab that won't give off the heat for another 2/3 hours after you've heated it.

 

[gpbeck]

Resistance of your underfloor piping is very low so boiler flow preferentially goes to the underfloor, not the cylinder. What flow temperature is your boiler delivering?

 

[SJB060685]

If you use mass flow rate triangle you can calculate what the UFH will pull in lps (litres per second) from the boiler at whatever flow temperature the system is designed for and the return temperature from the UFH. As myself and @EvilDrPorkChop have mentioned above UFH heating should be kept on constantly to produce a nice even temperature constantly. If you let the slab go cold over night then when the UFH comes on in the morning it will sap up all the flow from the boiler, possibly even back siphon through other circuits. In this situation you will always get the problem you are encountering. Without more details I couldn’t give exact figures in lps but what we’ve both said is 100% true.

 

[Timmy D]

Just leave underfloor on all the time and adjust/program the required temps for when occupied/unoccupied.

If you turn it off, every time it comes on, it requires all/most of the boiler output to heat the slab before the room. If it’s on 24/7, the underfloor only requires a constant trickle of the boilers power rather than all of it for a few hours. This will prevent it starving other uses (rads/hot water).

You’re not wasting a great deal, if any energy by leaving it on. Just changing how/when the energy is used (big amount/short time period versus small amount/long time period).

The property still requires x amount of heat energy to keep it at y temp.

The hot water is similar. New hot water tanks typically have a standing loss of 1.5kw worth of energy over a 24 hour period. It will say on the data badge.

If you heat the tank, use the water and leave the tank warm (rather than heat it back to stat temp). When the next hot water cycle kicks in (next morning), the boiler has to heat the whole tank in one go. If the hot water was left on for the whole day, it can trickle heat it as it’s used.

Both of these changes, help the boiler to fire for longer at a lower output, rather than full flame. Low and slow outputs are overall better for the components and efficiency.

 

[gpbeck]

As boiler flow temperature increases, less will be allowed through underfloor blenders, resistance of that circuit will rise and more will pass to the hot water coil. So you could adjust boiler temperature to speed up hot water generation.

 

[SJB060685]

Increasing boiler flow set temperature will do as you say yes but you’re then using more fuel to solve a problem that should have been designed out at the design stage, plus the boiler is having to burn hotter to achieve the same result. A common misconception is UFH can be on/off. Whilst it’s true it can but it will cause problems if not timed right.

 

[Timmy D]

Increasing boiler flow set temperature will do as you say yes but you’re then using more fuel to solve a problem that should have been designed out at the design stage, plus the boiler is having to burn hotter to achieve the same result. A common misconception is UFH can be on/off. Whilst it’s true it can but it will cause problems if not timed right.

To add to this, you’re then changing the system temps. Once everything is up to temp as it should be, the return temp will likely be to high for the boiler to condensate...

 

[John.g]

Just wondering what kind of hysteresis blending valves operate to, presume not as tight control as say shower thermostatic valves etc. This will have a big effect on how much water is recirculated by the UFH TMV, presumably none as the TMV is coming up to temperature from cold so all the UFH water is supplied and then boosted by the manifold pump until almost up to the required UFH temperature, has anyone watched this temperature from cold and noticed any over or under shooting or any changes in the manifold flow rates?.

 

[Graham W]

Thanks to everybody for the input. Some of the comments may be a bit above my pay grade but I get the general drift.

Not sure what is meant y the boiler flow temperature but the dial for the heating on the boiler is at 68 C?

Running the DHW for an hour before the UFH came on this morning has made a big difference. Shower water was much hotter and there is still hot left over afterwards.

The UFH has been set to run from 0700 to 2100 up to now, but I'm happy to switch it to 24 hours because I get the point about the inefficiency of the slab cycling up and down.

But will that make much difference to my situation where because of the high insulation and solar gain during the daylight there is no need for the UFH during the day and almost no call for it in the evening unless it is particularly cold?

We have a flueless gas fire in the living area which we use to add a bit of extra heat if needed in the evening / to get the visual benefit of the flames.

 

[EvilDrPorkChop]

Thanks to everybody for the input. Some of the comments may be a bit above my pay grade but I get the general drift.

Not sure what is meant y the boiler flow temperature but the dial for the heating on the boiler is at 68 C?

Running the DHW for an hour before the UFH came on this morning has made a big difference. Shower water was much hotter and there is still hot left over afterwards.

The UFH has been set to run from 0700 to 2100 up to now, but I'm happy to switch it to 24 hours because I get the point about the inefficiency of the slab cycling up and down.

But will that make much difference to my situation where because of the high insulation and solar gain during the daylight there is no need for the UFH during the day and almost no call for it in the evening unless it is particularly cold?

We have a flueless gas fire in the living area which we use to add a bit of extra heat if needed in the evening / to get the visual benefit of the flames.

Providing your room is warm enough anyway, the stat won’t kick in unless there’s a demand. Even if you set it to 24/7.

 

[SJB060685]

Just wondering what kind of hysteresis blending valves operate to, presume not as tight control as say shower thermostatic valves etc. This will have a big effect on how much water is recirculated by the UFH TMV, presumably none as the TMV is coming up to temperature from cold so all the UFH water is supplied and then boosted by the manifold pump until almost up to the required UFH temperature, has anyone watched this temperature from cold and noticed any over or under shooting or any changes in the manifold flow rates?.

The accuracy of manual and electronic mixing valves is pretty good. There may be a slight hysteresis but I can’t imagine much at all. Remember UFH manifolds should have a thermal cut off switch whereby if the flow temperature to the manifold is excessive (due to a failed valve etc) then power is cut to the pump and no flow is permitted through the loops. This is to protect any screeds, slabs etc and any bare foot homeowners.

Let’s take a working example. Say we have an exact 20kw heat loss at design max load. Say 10kw for UFH and 10kw for space heating. Quite large loads but I’m using it as an example.
Using mad flow rate you will see a 20kw boiler aiming for a ΔT of 20°c will have a flow rate of roughly 14.285 Lpm. Now the UFH which requires roughly three times the flow of a boiler or emitter targeting a ΔT of 20°c has a flow rate of 20.408 Lpm at a ΔT of 7°c across the loops. From those figures you should already see a potential problem arising.
Now let’s say the UFH was off overnight and the slab cooled to 10°c. When the UFH first comes on in the morning the return water to the boiler will be at, or very near to 10°c. Any pipe work above ground might have some negligible temperature difference. If you know the volume of the boiler heat exchanger and the flow rate through the boiler you can calculate how long water entering the boiler takes to leave with a temperature increase of 20°c. Say a 5 litre exchanger and a flow rate of 14.285 Lpm converted to alps = 0.238 Lps. 5/0.238 = 21.008 seconds for water entering the boiler to leave at 20°c higher.
So after 21 seconds we have a new flow temperature of roughly 30°c. Again if you know the volume of pipe work and flow rates you can calculate how long that water takes to reach the manifold. So the water arrives at the blending valve at 30°c but the target temperature leaving the valve will be around 45°c. From here you can see all the flow from the boiler will be pulled by the UFH pump. Now as the slab temperature increases over time so shall the return temperature and in time the boiler flow temperature will rise and the blending valve will need less and less water from the boiler flow. Say the boiler flow is at 65°c and the UFH return 37°c after some time. Using mass flow you can calculate that the UFH will pull around 5.1 Lpm from the boiler, leaving roughly 9.18 Lpm spare if you will. This 9.18 Lpm will be circulating around the space heating, or cylinder, or both, which if designed properly is enough to make the system function.
From the above you should clearly be able to see that if you let a slab go cold overtime then when you do bring the UFH on again it will initially steal all the flow until the slab and return temperature start to rise.
This is exactly why myself and EvilDrPorkChop have stated that UFH should be on constantly. They are designed to be left running to provide a nice even room temperature throughout the day, not switch on and off when you feel like it.

 

[John.g]

Agree that UFH works best on 24/7, it's not clear (to me) if Graham W's CH consists of UFH only, if so then even from a cold 10C start one might expect the boiler, firing at 100%, to only take minutes to reach a required UFH manifold temp of 45C, the TMV should then start throttling the boiler flow more and more as the boiler carries on to its setpoint temp of 70C or whatever. Calling for DHW at the same time will obviously slow down the process but one might still expect the cylinder to be fully heated to 60C in 2 hours, especially with a 37KW (not range rated) boiler. If there is a temperature gauge installed in the manifold it would be very interesting to see how long this takes to reach 45C from a cold start with both systems calling.

 

[SJB060685]

If a slab is allowed to cool to say 10°c that’s an awful lot of mass that needs to be heated again before the return temperature starts to rise.
From a cold start a temperature gauge and timer would give a good indication to the time it takes for water to reach 45°c but remember even when the boiler flow is at 45°c the manifold will still be pulling all the boiler flow. Only when the boiler flow increases above UFH target flow temperature will the mixer start to throttle in. Use the heat balance equation and you will see.

 

[John.g]

It requires 15.7kwh to heat 300 litres of water from 15C to 60C, in two hours the boiler (if not range rated) would/could have produced 74kwh, so if the cylinder had reached 60C then the manifold loops would have emitted 58.3kwh or a heat output of > 29kw, the cylinder, apparently wasn't reaching this 60C so the loops output even greater or the cylinder coil is very modest or the boiler is range rated IMO, but we'll never know but the OP is happy now so doesn't really matter.

 

[SJB060685]

I don’t know the boiler output or UFH output but the problem with UFH is the amount of flow it requires due to smaller ΔT. If the slab or screed is cold or you start introducing some form of modulating controls etc then the UFH will pull more and more from the boiler. In a situation like this keeping the UFH constantly will solve the problem and ideally have the design flow temperature for the rads at 20°c higher than UFH return will keep the system flow nice and balanced.
If the UFH demand was irregular then realistically you would consider some sort of separation with a common flow sensor signaling the boiler to increase the flow temperature if needed.
Separation is not something I’m really familiar with though. I understand it and the problems it can cause if used when not needed but there are for more qualified people to discuss that out there than myself.

 

[exedon2]

Just wondering what kind of hysteresis blending valves operate to, presume not as tight control as say shower thermostatic valves etc. This will have a big effect on how much water is recirculated by the UFH TMV, presumably none as the TMV is coming up to temperature from cold so all the UFH water is supplied and then boosted by the manifold pump until almost up to the required UFH temperature, has anyone watched this temperature from cold and noticed any over or under shooting or any changes in the manifold flow rates?.

John over shooting temperatures when system not operated properly is very common.
We're polypipe registered underfloor contractors,regularly called to look at systems we haven't installed.
Most common complaint is temperature over shooting virtually alway caused by customer miss use/ poor set up by installer.
Run it 24/7 with max of 4 c back set and it will heat rooms perfectly (assuming enough heat output)
A lot of real problem ones are either diy or installed by screed team.
Pipe distances too great screed too thin pipework not pressurised or even tested before screed going down seen it all.

 

[SJB060685]

It’s also worth noting that the controls strategy for any hydronic heating system is important to get right at the design stage. If a dwelling has high occupancy and/or high internal gains like solar gain etc then you should consider a closed loop system with some sort of room influence. I’m an oil burner technician by trade so I’m not overly familiar with heating systems like an installer should be but I’m lucky enough to have been taken under the wing of some very knowledgeable people who do this day in day out. I have heard a lot of temperature overshoots in situations like UFH in conservatories with high solar gain and simple on/off room stats. In a situation like that you would require some sort of room influence which like an on/off stat is constantly aware of the internal temperature but also learns and calculates and adjust the boiler flow temperature to prevent any undesirable overshoots.
Although on/off controls work and have done for decades they really are primitive compared to modulating controls.

 

[John.g]

Edit: Rads returns should be 60C, with 70C boiler temp, (I copied wrong cell) but calcs are basically quite correct.

DHW sheet attached

Actually I see that there are rads in the above system so I just inputted a few numbers in my spreadsheet which may (or may not) be interesting.
Assuming UFH demand only of 10kw only requires a boiler circ pump flowrate of 4.48LPM@70C, this means the boiler will run with a dT of 32C, maybe problematic for some makes?. the only other remarkable is if the boiler temp is either reduced to 45C or the point that you are all making, that it has fired up from cold and hasn't become > 45C so the boiler pump circ rate would have to be 44.4LPM v/vs 28.4LPM with boiler temp at 70C (all 3 systems calling), hence the slow cyl warm up time or whatever?.