UFH v DHW = cold shower...

[Diablo]

Gents,

 

I live in a 240sqm bungalow that we built last year.  This is our first period of cold weather, so we are using the heating system properly for the first time.

 

We have a 21KW oil boiler feeding a UFH systems that runs throughout the house, and a 300L unvented cylinder. We try to maintain about 20deg in the house which means that one stat, or other, will be firing the boiler at some point in the day.  

 

The issue I have found is that the when the heating is on, the pipes going to the DHW tank become tepid.  If I go around and switch off the stats, the same pipes warm up considerably.   

 

Is this just bad design? Any thoughts welcomed.

 

Best regards,Rog

 

 

[PeterW]

Is it on zone valves or a 3 way valve ...??

 

Tank will only be heated when the valve is open to the tank coil so you would expect that when heating is on then the tank pipes would be cool. 

[Diablo]

Hi, We have two UFH circuits and DHW circuit, each with their own zone valves.

 

The issue occurs when the DHW and UFH valves are open.  DHW takes much longer to heat up.  It's as though the UFH system takes the majority of the HW coming from the boiler.

 

 

[ProDave]

That is exactly what I have,  oil boiler, 3 circuits on their own 2 way valves, UFH downstairs, UFH upstairs, and DHW (via 250L unvented tank)

 

I have no problems at all.

 

If the UFH appears to be taking too much demand from the boiler, I would be looking at the UFH manifolds. These have a thermal mixing valve to regulate the UFH temperature, and a lot of the time most of the HW from the boiler is going straight back down the return pipe as it doesn't need much heat to top up the water flowing round the UFH system.

 

Check your UFH is not set to run at a way too high temperature for a start and if you have a thermometer measure the flow and return temperature to each manifold and the UFH flow temperature.

 

Because UFH takes a while to warm up, we have the programmer set to turn on the heating earlier than the DHW. So by the time the DHW turns on, the heating has been on an hour or more, plenty to warm up the system so it's then running at a low demand just to keep it up to temperature.

 

[Diablo]

The boiler, Manifold 1 and the DHW tank are all in the laundry room, so in close proximity. In the middle of the house, we have Manifold 2, which feeds the right side of the house.

 

Both manifold are set at 45deg.  We have attempted to 'balance' the demand a little by setting the pump on Manifold 1 at setting two and Manifold 2 on setting 3 seeing as it is further away.

 

I will get some temperature readings. Do you see any issue with what I have described so far?

 

(PS: I should add that the UFH manifolds have not been balanced yet, as the UFH pipes were put in well before the boiler was installed - not sure if this would have any impact?)

[Nickfromwales]

Is it a system boiler ( with integral pump ) ? And are there any gate valves on the flow and return to the cylinder ?

[Diablo]

Hi,

 

It is a System boiler with integral pump.  I cannot see any gate valves in line with the cylinder, just a bleed valve and two pressure relief valves.

[Diablo]

IIRC, the plumber made a modification as follows:

 

He Tee'd off from the boiler output pipe (prior to the boiler's pump) and supplied the two UFH manifolds.

 

The boiler's 'pumped' supply goes to the cylinder only.

 

The thinking behind it was that there would be sufficient pressure to get the hot water to the manifolds and their respective pumps would take over thereafter.

[PeterW]

That's a bit of an odd modification to a system boiler ..! 

 

Are there any zone valves ie two port valves, or is it all open circuit ..?

[Temp]

+1

 

Presumably there are pumps on the manifold that draw water through the boiler and these are over powering the pump supplying the DHW tank. 

 

If this part of the system can't be balanced you could consider giving the DHW tank priority over the UFH as it won't take long to heat/top up the DHW tank. The UFH probably has lot of thermal mass so the temperature of the house won't fall much while the DHW is calling for heat. I've lived in houses where the rads and DHW worked like this.

 

 

[PeterW]

Priority to DHW is a W-plan system and needs a motorised valve, not a 3 way valve (although I think I worked out how to make it work with a pair of zone valves ..! ) and then it will only release the heat to the floor when the tank is satisfied. 

[ProDave]

Priority to DHW could just be achieved with the programmer, turn on the DHW 1 hour before the heating.

 

Turn the boiler pump up to 3, and the manifold pumps down to 1 to try and ballance the system a bit.

 

 

 

How is the boiler cycling? If everything really is running tepid, it sounds like the boiler cannot cope with the demand? in which case it will be burning continuously rather than cycling on and off to maintain it's flow temperature. Could it be the boiler is too small for the size of house and area it is heating?  Also, what is the floor make up? how much and what type of insulation under the UFH?  For that matter how well insulated is the house? near to passive house standard or just scraped through the minimum requirements of building regs?  And where are you? what is your climate like?
 

[riv990]

The main boiler pump should supply hot water to the manifolds, zone valve and cylinder zone valve.  The pumps on the manifolds recirculate the water through the uf pipes via the thermo mixing valve which just keep topping up with hot water unless you have a low temp supply eg heatpump when you can run direct.

 

There should be an auto bypass after main pump - see boiler manufacturer's brochure for info 

[Diablo]

Our floor build up is: Concrete slab, 175mm of EPS70 and a  70mm screed.  UFH is based in 16m  pipes at 100mm centres. Floor finishes are either porcelain tiles or carpet (low tog)

 

I have noticed that the boiler runs for long bursts, rather than cycling frequently.  It only runs tepid when there is demand coming from both manifolds and DHW. UFH or DHW, in isolation, seem to run fine.

 

A plumber who was doing some other work on the property mentioned that we might need a low los header?

[Nickfromwales]

11 hours ago, Diablo said:

A plumber who was doing some other work on the property mentioned that we might need a low loss header?

He's right. There is no way an oil boiler should be servicing the UFH directly, even more so with it connected directly to the heat exchanger and no means of bypass. In the boiler manufacturers installation instructions ( MI's ) it'll specifically mention designing the system to reduce / eliminate short cycling. This can be done with a LLH or small buffer tank, I'd go for the buffer myself as a LLH will typically have much smaller volume which would give less result vs cost to do.

The heating all comes off the buffer, 3 sets of tappings ( boiler in and 2 x UFH out ) and the dhw still comes direct from the boiler via the motorised zone valve.