Juggling different temperatures with one boiler

[hendriQ]

An issue came up today though is that not sure how to set up a circuit for the two towel rads we're having. Here are the different factors we're juggling:

 

1. The UFH will need to be kept fairly low temp, because our engineered floor cannot be exposed to a subfloor that is hotter than 27 degrees. So I'm guessing we will run the UFH at 30C - though not sure how to calculate what temp to give 27? And presumably different rules apply for heating screed on a ground floor than for heating a non-screed system on the first and second floors.
2. The hot water cylinder I understand needs to be at 60 degrees minimum to avoid legionella. Our plumber thinks this is a waste of energy, but I don't see a way around it, although I know some people just heat it once every so often to sterilse, but either way the temp of the UVC will be much higher than the temp sent to the manifolds won't it?
3. Then what to do about the two towel rads. Presumably these are fine at 40 to 45 degrees, but that is higher than the UFH and lower than the UVC. Or can each UFH manifold lower the temperature of the water it is getting from the boiler?

 

We are not having buffer tanks for the UFH because we've gone for a broad range modulating boiler and a low loss header instead.

 

[dpmiller]

Each UFH manifold has it's own mixing valve, use one for the towel rails... that's exactly how I'm running my towel rails.

[JohnMo]

Will do the easy bits.

 

UFH, will be controlled by a temperature control valve, on the manifold, so set the the heating temperature at what you want for the towel radiators, the UFH will let that temperature down by mixing with the return flow.  So you can could manage that as a single circuit.

 

Have you considered electric towel heater's, so you can use them in the summer, or when the heating is off?  We have them with 150W elements, time them to come on for 3 hours in the morning same at night.

 

Assume your UFH is in a screed or similar, if so the flow temperature, will be dispersed through the screed, if your floors are near 27 you will feel it very hot under foot.  We have 300mm spacing on our pipes for the UFH and a flow temp of around 33 deg, our floor temp is about 21/22 deg.  

 

I was sure I didn't need a buffer, or a temperature control valve on the UFH manifold, as I had good modulation on the boiler, it could also do the flow temps I wanted etc, but when put into service we had lots of short cycling, may of been ok when we are designed temps, (-4 deg) but not so good at 10 degs outside temperature.

 

Not up to speed on the different operating plans for a cylinder, but someone will help with that.  I am using a thermal store and combi.  But that's a different story.

 

[hendriQ]

10 minutes ago, JohnMo said:

Will do the easy bits.

 

UFH, will be controlled by a temperature control valve, on the manifold, so set the the heating temperature at what you want for the towel radiators, the UFH will let that temperature down by mixing with the return flow.  So you can could manage that as a single circuit.

 

Have you considered electric towel heater's, so you can use them in the summer, or when the heating is off?  We have them with 150W elements, time them to come on for 3 hours in the morning same at night.

 

Assume your UFH is in a screed or similar, if so the flow temperature, will be dispersed through the screed, if your floors are near 27 you will feel it very hot under foot.  We have 300mm spacing on our pipes for the UFH and a flow temp of around 33 deg, our floor temp is about 21/22 deg.  

 

I was sure I didn't need a buffer, or a temperature control valve on the UFH manifold, as I had good modulation on the boiler, it could also do the flow temps I wanted etc, but when put into service we had lots of short cycling, may of been ok when we are designed temps, (-4 deg) but not so good at 10 degs outside temperature.

 

Not up to speed on the different operating plans for a cylinder, but someone will help with that.  I am using a thermal store and combi.  But that's a different story.

 

Thanks, very helpful. What combi boiler do you have and what is its modulation range. Do you also have a low loss header?

[JohnMo]

Atag A325ECX - 32kW, modulation 6.1 to 30.9kW in heating mode and 34.3 kW in hot water mode.

[JohnMo]

No I don't have a low loss header.

[hendriQ]

2 hours ago, JohnMo said:

No I don't have a low loss header.

Get one. I’m told it will sort any short cycling.

[JohnMo]

I sorted mine by connecting the boiler central heating to the thermal store boiler coil.  Boiler heats the store to 40 deg. UFH is connected to thermal store, via pump on UFH manifold and 2 port valve at thermal store.  If the house thermostat calls for heat, the 2 port valve opens flow from thermal store, 2 port valve sends a signal to boiler via the thermal store thermostat for heat.  Boiler fires only if store is below 35 deg and house calls for more heat.  Boiler central heating temperature is set to 60 degs and remains in condensing mode throughout, boiler fires for approx 20 mins a couple of times a day.

 

UFH manifold pump runs 24/7, to distribute heat better through floor even when hot is not being supplied.

 

DHW is pre heated as it flows through DHW coil on its way to the combi, to improve performance.

[Nickfromwales]

15 hours ago, JohnMo said:

DHW is pre heated as it flows through DHW coil on its way to the combi, to improve performance.

Not so sure about that. 
The TS would have 22mm cold in / DHW out, which you then reduce to 15mm to feed to the whole of house via the combi. The showers and baths would get much better flow rates if connected directly. 
A combi is very efficient at heating DHW itself, and the economics of pre-heat via the gas device it’s trying to save, are spurious IMO. 
Generating high grade heat in the boiler to heat a TS, and also to also suffer losses from the connecting pipework plus the heat losses from the TS itself is a very elongated and inefficient path.

Do you have solar thermal or PV?

Do you just shut off the TS during summer and allow cold to just flow through it?

[JohnMo]

Main reason for the the thermal store is for the summer to divert solar PV to immersion.  Hopefully will not use any or very little gas in the summer.

 

No real economy being looked for, I wanted a combi, my wife wanted to be able to run 3 showers at the same time (and we can) we also have PV and wanted to self consume as much as possible; so have an immersion diverter.  

 

In the past, the combis I have had, always flowed less in winter compared to summer, so some preheat in winter gives me summer (or better) performance all year round, we are using a similar principles to the Superflow, see attached.

 

The water is quite a bit warmer coming of the thermal store DHW coil than when it goes in, not hot but the stores only at 40 degs.  In the summer the water will come out DHW coil and temperature controlled by the tcv and pass through the combi without further heating; as the boiler is designed to take preheated solar water.

 

The water from stop cock is 22mm to the thermal store, with a reducer at the outlet of the temperature control valve from the DHW coil to the combi.

Combi-SuperFlow-White-Paper-v1-2-4.pdf

[markocosic]

You will lose efficiency with this approach

 

The 325ECX has flue gas heat recovery (preheats incoming dcw with a heat exchanger on the flue before heating it further via the main heat exchanger)

 

Running for flue gas heat recovery THEN preheat THEN main heat exchanger would eek out a little more water for your gas but means fkucsing with the (water side) of the boiler internals.