Underfloor Problem - Water temp too low

[pmt]

Hi folks,

first time poster & great forum. Have a problem with my UFH and hope I can find some help. To begin, here's my set up. 2600 sq ft bungalow. 10 rooms with 16 loops in total. Primary heat source is a wood pellet boiler. Also have a multi-fuel stove which we use primarily for boosting heat in living area though it also has a back boiler. A "System-Link" box is used to take the flow & return from the boiler & stove & from the system link are three independent outputs to UFH, towel rails & DHW coil. My problem relates to the flow temp going through UFH loops. Essentially when a room stat calls for heat, the boiler is fired, sends hot water (Circa 60 deg C) across to the system link & hot water is pulled from the system link through a TMV (to reduce temp) and pumped through a manifold to the UFH loops. I notice when I have 7/8 loops all calling for water or open, the flow temp through these loops drops to an unsatisfactory 30 deg C meaning rooms are not heating. As I progressively reduce the number of open loops, flow temp increases until such time as I only have one or two loops open & the flow temp approaches 40 - 45 deg C. For some reason, the water at acceptable temp (50-60) is reaching the system link but not being pulled through it at sufficiently high temps when there are multiple loops open. Are there any theories as to how I can pull more hot water when I have multiple loops open or proposed solutions? I know I could just have one/two loops open at a time but this would lead to the boiler running for very extended periods of time & consequent fuel consumption which is undesirable. Thanks in advance, pmt

 

[chris watkins]

How long are you running the U/F heating system for, is it on a timer??
Is the TMV not adjustable / what is it set to?

 

[Cds]

A few possibilities:

1. Pipework from the heat source to the underfloor heating (UFH) is undersized. If this pipework is 22 mm or larger then this is unlikely but should be considered especially if the manifold is a long way from the source.
2. The thermostatic mixing valve is not working correctly in that it is not letting in enough hot water to the underfloor heating. Make sure the valve is turned up to maximum to see if this improves the situation then try it at various settings to see if there is any change in its function. It might be necessary to replace the valve but look at 4) first.
3. The UFH pump is faulty or not working correctly so there is no enough flow around the system. This seems unlikely as it would give rise to hot water entering the UFH but significantly colder returns.
4. The two most common problems with UFH are a) Air locks in the UFH pipework and b) Incorrectly wired thermostats/actuators.

To remove any doubt that the thermostats/actuators are incorrectly wired remove all of the actuators.

To remove any airlocks then it will be necessary to purge the UFH pipework.

I would do the following:

1. Check the pipework from the heat source to the UFH if it is 22 mm or greater, then it should be OK.
2. Remove all of the actuators and make sure the TMV is set to maximum.
3. Purge the UFH loops to remove any air (follow your suppliers instructions).
4. If the above does not work/find the problem come back with more information (pictures of set up etc.)

Hope this helps

 

[pmt]

Hi Chris, running UFH for 3 consecutive hours daily. TMV is adjustable and is set between 45-50 deg C. Thanks, pmt

 

[pmt]

Hi Chris, running UFH for 3 consecutive hours daily. TMV is adjustable and is set between 45-50 deg C. Thanks, pmt
Hi Cds, pipework is min 22mm. All actuators working - when stats kick in, water is sent through corresponding loops almost instantaneously. Toyed with TMV and setting to minimum almost immediately sends cool water through loops though setting to max doesn't make appreciable difference when multiple loops open. Haven't tried purging system. Retailer out of business and haven't contacted mfr'er. Thanks, pmt

 

[chris watkins]

I know you are asking specifically about the flow temp but running an U/F system for just 3 hours a day may not be allowing it or the structure to get up to a working temperature have you tried leaving the system on for longer ? U/F systems are typically left on for longer periods but at lower temperatures due to the time taken to lift the room temps.

 

[Cds]

As Chris says 3 hours is not long enough. To check if an UFH system is working OK run it for 24 hrs for at least 3 days then check if temperatures are as expected. Then if the system is working OK reset the heating times.

 

[peteheat]

UFH should not be controlled using a timer, they defeat the purpose of a low temperature system by as in your set up only allowing a 2,600 sq ft concrete radiator three hours to reach the desired temperature.

Do you have a buffer tank fitted?

The way we set them up is all calls for heat are to the buffer, when the buffer temp drops to 50c it calls the boiler on, when the buffer reaches 80c the boiler goes off, this reduces boiler cycling and allows for an efficient burn while the boiler is on.

Every room in the house is fitted with a stat so it never drops 2c below set point, this means each room gets the heat it needs when it needs it, for example south facing rooms will have solar gain during the day and will require more heat at night, north facing rooms may need heat during the day (no solar gain and higher heat losses) so the need for each zone should be satisfied when it needs the heat not when you decide the timer to activate.

If you set it up this way you can drop your flow temperature as low as 30 > 35c which means you save money and have a comfortable home 24 / 7 / 365.

 

[pmt]

thanks again for replies all & agree that ideally I would have system running constant to attain desired room temps & then allow stats to "do their thing" thereafter. My concern with that method is that if each stat happens to call for hot water at different times which is quite possible, I could have a situation where the boiler is on virtually constant with the consequent impact on fuel consumption. My boiler is either on or regardless whether one zone (loop) calls for hot water or all zones call for hot water. Notwithstanding that (though I will bite the bullet & run non-stop for a few days), is the problem I have with respect to only lukewarm water being pulled to the flow manifold not independent of how long the system runs for? Specifically with respect to a buffer tank, I don't have one fitted pete.

 

[chris watkins]

Who designed your system pmt ? it sounds like U/F was not the best option, if you want intermittent heating but you may be able to use a buffer tank as pete suggested to iron out the boiler running.

It sounds like you need some professorial advice from a design engineer in this field.

 

[peteheat]

thanks again for replies all & agree that ideally I would have system running constant to attain desired room temps & then allow stats to "do their thing" thereafter. My concern with that method is that if each stat happens to call for hot water at different times which is quite possible, I could have a situation where the boiler is on virtually constant with the consequent impact on fuel consumption. My boiler is either on or regardless whether one zone (loop) calls for hot water or all zones call for hot water. Notwithstanding that (though I will bite the bullet & run non-stop for a few days), is the problem I have with respect to only lukewarm water being pulled to the flow manifold not independent of how long the system runs for? Specifically with respect to a buffer tank, I don't have one fitted pete.

If your boiler modulates then it should not be expensive to run 24 / 7 as the heat load could be as low as 25%, what you could try while conducting your experiment is to increase the temperature at the boiler to say 70 > 75c that should give you a high flow temperature to divide between the zones.

What is the boiler output?

 

[chris watkins]

If your boiler modulates then it should not be expensive to run 24 / 7 as the heat load could be as low as 25%, what you could try while conducting your experiment is to increase the temperature at the boiler to say 70 > 75c that should give you a high flow temperature to divide between the zones.

What is the boiler output?

Pete, do these wood pellet boiler modulate ? never fitted one so not sure I know they have control by speeding up or slowing the air fan but not sure how low they will go ? is it a low as 25% ???

 

[peteheat]

Pete, do these wood pellet boiler modulate ? never fitted one so not sure I know they have control by speeding up or slowing the air fan but not sure how low they will go ? is it a low as 25% ???

Most good ones modulate, one type we use will go to 20% another will drop from 30 Kw to a little over 3Kw but I usually program that type to shut down at 12kw.

Basically as the system heats up the boiler does what I call "over shoots" the set temp by between 5 & 10% as it's doing that the fans, pellet feed etc are slowing down to maintain the set temperature, when it over shoots by 10% the boiler goes into pause mode or shuts down until the temperature drops about 10% below set point and re-fires itself.

I designed our own controller for the buffer to call the boiler on at whatever temperature I set it at, a simple but very effective control as it prevents boiler cycling.

There are some good products available, there's not so good but can be made to work well and of course there's plenty of rubbish, the technology is the key as many systems can be fitted to the boiler you prefer, Grant have a very nice pellet boiler, modulates and condenses, click on the link for the Grant Spira and take a look at the manual

 

[RGII79]

hi folks,

first time poster & great forum. Have a problem with my ufh and hope i can find some help. To begin, here's my set up. 2600 sq ft bungalow. 10 rooms with 16 loops in total. Primary heat source is a wood pellet boiler. Also have a multi-fuel stove which we use primarily for boosting heat in living area though it also has a back boiler. A "system-link" box is used to take the flow & return from the boiler & stove & from the system link are three independent outputs to ufh, towel rails & dhw coil. My problem relates to the flow temp going through ufh loops. Essentially when a room stat calls for heat, the boiler is fired, sends hot water (circa 60 deg c) across to the system link & hot water is pulled from the system link through a tmv (to reduce temp) and pumped through a manifold to the ufh loops. I notice when i have 7/8 loops all calling for water or open, the flow temp through these loops drops to an unsatisfactory 30 deg c meaning rooms are not heating. As i progressively reduce the number of open loops, flow temp increases until such time as i only have one or two loops open & the flow temp approaches 40 - 45 deg c. For some reason, the water at acceptable temp (50-60) is reaching the system link but not being pulled through it at sufficiently high temps when there are multiple loops open. Are there any theories as to how i can pull more hot water when i have multiple loops open or proposed solutions? I know i could just have one/two loops open at a time but this would lead to the boiler running for very extended periods of time & consequent fuel consumption which is undesirable. Thanks in advance, pmt

underfloor heating is not for instant heat like a rad system only performs well over longer periods of time

 

[RGII79]

70 degrees min at system like on you size system

 

[pmt]

thanks pete. Boiler is 25KW & not modulating variety to the best of my knowledge. Has a small 200 ltr internal store of water maintained at 75 to 80c. When UFH calls, this store empties and for the remainder of the cycle or time there is demand on the boiler, it reheats the return water to approx 55C and send across to the house. There is a bypass on the hot water flow behind the boiler that I can control via a valve to redirect hot water to the cold return before it enters the boiler but as I divert more hot water through this bypass, I seem to lose it at the house. My logic though was that even if I send hot water across to house at 55C which should be sufficient for UFH, why is the temp on the flow manifold at 30C when multiple loops are open? Thanks again, pmt

 

[pmt]

thanks for contributing RG. What exactly do you mean by your last post though....."70 degress min at system like on you size system".

 

[RGII79]

thanks for contributing RG. What exactly do you mean by your last post though....."70 degress min at system like on you size system".

You need 70 degrees at the system link min feeding the UF heat manifolds the blending valves will mix f + R pipes to achieve 55 degrees through your zones. when up to temp will divert back to boiler anyway
your not putting enough heat into the system for itss size thats why you losing the tempture when more zones come on

 

[peteheat]

thanks pete. Boiler is 25KW & not modulating variety to the best of my knowledge. Has a small 200 ltr internal store of water maintained at 75 to 80c. When UFH calls, this store empties and for the remainder of the cycle or time there is demand on the boiler, it reheats the return water to approx 55C and send across to the house. There is a bypass on the hot water flow behind the boiler that I can control via a valve to redirect hot water to the cold return before it enters the boiler but as I divert more hot water through this bypass, I seem to lose it at the house. My logic though was that even if I send hot water across to house at 55C which should be sufficient for UFH, why is the temp on the flow manifold at 30C when multiple loops are open? Thanks again, pmt

You need the return temperature to enter the boiler at 55c or expect to replace it soon because less than that and you are causing condensation and possibly thermal shock in the boiler, that is why I fit a load valve on the return that can not be adjusted, some manufacturers want 55c others want 65c one gasifier I know wants 70c on the return.

From what you describe I think you have a very old fashioned by-pass or shunt instead of a load valve / mixing valve, the modern valve will open and close as needed to satisfy the return temperature into the boiler so the higher your flow temperature means you have more hot water going to the house and your boiler is being protected against low return temperatures.

I think you need a buffer about 1,000 litres should be sufficient a rule of thumb is 50 litres of water in the buffer per Kw output of the boiler, most are well insulated so heat loss should not be a problem.

As RGI179 has explained the mixing valve at the manifold will not waste any heat, it must have the heat required for the zones when they call for it.

With three hours on time and a low temperature flow to the system I would expect all or most zones are open when the boiler fires because it is very unlikely that all zones are up to temperature at the end of the 3 hour period so the whole system will rarely if ever be satisfied, the buffer tank allows you to iron out the the difference between the boiler trying to satisfy itself (at set temperature) and to satisfy the demand for heat from the house.

All biomass boilers (pellet, gasifiers etc) are selfish by design to protect themselves from condensation issues caused by low temperature returns, if you take a look at the physical size of a modern 25Kw gas or oil boiler and the minimum flow temperatures of 70c it will give you an idea of the differences in the technology.

 

[pmt]

Thanks Pete. You're correct about the bypass. Its just a branch off the flow with a gate valve controlling the rate. With the return coming back at around 20 I cannot see how i'll be able to blend in enough hot water to bring it to 55 before it enters the boiler? Do you think a buffer will solve this issue and any idea how much i could expect to spend? Thanks

 

[peteheat]

Thanks Pete. You're correct about the bypass. Its just a branch off the flow with a gate valve controlling the rate. With the return coming back at around 20 I cannot see how i'll be able to blend in enough hot water to bring it to 55 before it enters the boiler? Do you think a buffer will solve this issue and any idea how much i could expect to spend? Thanks

On boilers my preferred load valve is made by ESBE depending on the boiler and built in controls you may need to control the On / Off temperatures for your pump.

A buffer should make a big difference to both the performance and efficiency of your system, as to cost I am hesitant to give a guide as I could be very far out either way (too expensive / too cheap) as there are a lot of variables to try to take into account that I normally take note of at a glance when looking at a system, also my location would play a big part as the material and installation costs do vary between Ireland and the UK.