UFH control

[Munchincocopops]

Hi – I’m after some advice.

I have a large garden room 60m2 with water underfloor heating, 7 circuits, all on or off as system demands.

Due to increased energy costs I’m wondering if there is a better way of controlling boiler running.

 

The boiler is a System boiler, 32kW, running at 60 deg C. It heats a hot water tank, house central heating and garden room underfloor heating each with separate timer control and stat. We have the bathroom radiators piped so they come on whenever the boiler runs. Mostly we only use the garden room and so only the underfloor heating is used and rarely do we have the house heating on these days. This time of year the ufh is on all the time though the stat drops back to 17 deg C at night then back up to 20 deg C during the day, usually we adjust this daytime temperature depending how cold it is.

 

So I’m thinking the boiler (about 5m from the underfloor heating manifold) will not be on long before the underfloor heating manifold mixer valve (set at 35 deg C) turns off which in turn will eventually cause the boiler to shut down once bathroom radiator pipes reach 60 deg C or so causing the boiler to shut down on its own temperature stat. Though it may fire up again later due to its own stat even if the manifold mixer valve remains closed, though garden room stat is still calling for heat as the room is not yet up to temperature even though the ufh pipes are at 35 deg C.

 

I have thought of a buffer tank but I’m wondering if there is something that can measure the underfloor heating manifold temperature so when it reaches 35 deg C it can be used to turn off the boiler as the room stat is still not up to temperature so asking for boiler heat. I’m thinking when the manifold temperature drops below 35 deg C, then the boiler is enabled again and controlled by the room thermostat.

 

Ideally, I wondered if there was a manifold mixing valve that had a means of allowing a control system to know when it was open to accept hot water and also when it was closed and not accepting hot water.

 

At the moment I’m thinking of fitting an electronic temperature sensor to the manifold and setting it to the same temperature as the mixer valve, though I realise the mixer valve is not exact so there will be some trial and error. Hence ideally an intelligent mixer valve would be best I think.

 

The other idea is to use a PLC (maybe a raspberry pi) as the controller to monitor manifold temperature and when it records a maximum temperature, i.e. not increasing anymore, I assume the mixer valve is closed and shut off the boiler. Once the temperature, say, drops 3 deg below this maximum the plc enables the boiler to be controlled by the room stat once again.

 

Anyhow, been googling but so far cannot find such a solution.

 

Any help or comments appreciated.

[Temp]

The mixer on the manifold doesnt doesn't normally just switch from all boiler to all recycling, it blends the two. So even when the flow temperature is 35C it is mixing 60C from the boiler with the return from the UFH (25C ?). At those temperatures the ratio would be about 10/35ths Boiler flow and 25/35ths UFH Return....

 

(10/35)*60 + (25/35)*25 = 35C

 

If you turn off the boiler the UFH flow temperature will fall quite quickly to the UFH return temperature.

 

What happens if you just turn the boiler temperature down? Does it modulate down or start short cycling?  I know the DHW should be kept at 60C to avoid legionnaires disease. 

 

 

 

 

Edited October 19, 2022 by Temp

[Munchincocopops]

Thanks Temp for the reply and explaining the draw of heat mixing with the return to maintain 35 deg c. Stupidly I overlooked this obvious (now) effect of colder return water.....I was thinking water was easily raised to 35 deg and valve closed until pipework dissipated the heat when the floor is up to temperature ...thinking heat is not easily dissipated into already heated floor....but your explanation is making me think again.

 

Its a new Baxi Megaflow and I do not know if its short cycling or regulating. I assume regulating as its heating the bathroom circuits not only the UFH.

 

I'm just wondering if there is a way to use less gas....it may not even be possible....what I have might be as good as it gets and only a buffer tank would help?

 

 

[HughF]

6 minutes ago, Munchincocopops said:

Thanks Temp for the reply and explaining the draw of heat mixing with the return to maintain 35 deg c. Stupidly I overlooked this obvious (now) effect of colder return water.....I was thinking water was easily raised to 35 deg and valve closed until pipework dissipated the heat when the floor is up to temperature ...thinking heat is not easily dissipated into already heated floor....but your explanation is making me think again.

 

Its a new Baxi Megaflow and I do not know if its short cycling or regulating. I assume regulating as its heating the bathroom circuits not only the UFH.

 

I'm just wondering if there is a way to use less gas....it may not even be possible....what I have might be as good as it gets and only a buffer tank would help?

 

 

To use less gas you need this setup as DHW priority, with full weather comp when on heating. The boiler is also probably far too big so needs range rating to the heating requirement.

 

Urban Plumbers has a few videos on priority DHW: https://www.youtube.com/c/UrbanPlumbers/

 

[Munchincocopops]

Thanks for that HughF. The house is a big house before building the garden room and my old 24kW boiler struggled to heat the house in mid winter....its all 15mm insulated copper and oversize radiators...hence upgrade to 32kW. What I did not realise is that we would tend to only use garden room as it is always nice and warm with underfloor heating though there are times when we use the rest of the house with visitors and family and the 32kW cooks the house well.

 

Will look into DHW priority but we only have this on each morning for 1.5 hours before we get up and it gives us stored hot water for the day. I'm assuming you are suggesting DHW priority idea for the bathroom radiator circuit?

 

I've also wondered if a second small boiler just for underfloor heating circuit would be a better idea?

 

Thanks again.

Edited October 20, 2022 by Munchincocopops

[JohnMo]

DHW priority let's the boiler run at two different temperatures, hot for the cylinder heating, warm for weather compensation.

 

I am currently putting 0.5kW into the floor of our house with 36kW boiler.  Your demand will never be that low with an older house.

 

Your boiler will turn down to about 10kW, but if you only want to heat part of the house you may need a buffer.  This will soak up excess heat, the boiler eventually trip on excess return temp, the heating will continue extracting heat from the buffer, a period of time later the boiler will kick in.  You may only need a 25-50l buffer, pipe as a 2-port, you don't need a thermostat on the buffer if you use WC.

 

You don't need a second boiler, that could be a world of pain depending on your gas meter and pipe sizes.

 

 

[HughF]

Have a look on the heat geek map, find a local heat geek and get them in. Tell them you want pdhw and weather comp, let them get on with it. There will be some re-piping and controls adjustment to do.

 

Have you had a heat loss calculation done? Either your house is massive, or it leaks like a sieve if a 24kW couldn’t heat it properly.

Edited October 20, 2022 by HughF

[Munchincocopops]

Never had a heat loss calc done. House has no draught's but also no cavity wall insulation as house is built of stone and advised not to have cavity wall insulation in case of cavity bridging with stone. Not much loft insulation either as its boarded out mostly.  Though extension has insulation. as per current building regs.

 

So thanks for the advice - it looks like a buffer tank will do what I'm looking for.

Kind Regards