Advice on how water heating

[flanagaj]

Can anyone advise as to the best option for hot water when we are having an ASHP for underfloor heating.  I like a good flow rate shower, so was thinking that we should have an oil boiler for hot water.   What have others done when you have to have an ASHP?

[JohnMo]

You just need a unvented cylinder - nothing more nothing less. Make sure it's sized for the house and has a big heating coil 3m² or bigger.

 

The heat pump will switch from UFH to cylinder heating via a 3 port change over valve controlled by heat pump. At the same it will bump up the flow temperature.

[flanagaj]

17 minutes ago, JohnMo said:

You just need a unvented cylinder - nothing more nothing less. Make sure it's sized for the house and has a big heating coil 3m² or bigger.

 

The heat pump will switch from UFH to cylinder heating via a 3 port change over valve controlled by heat pump. At the same it will bump up the flow temperature.

Does it basically switch to using an immersion heater to heat the water to 70c?  I always thought an ASHP was only efficient at heating water to low temperatures, eg underfloor heating?

[JohnMo]

13 minutes ago, flanagaj said:

Does it basically switch to using an immersion heater to heat the water to 70c?  I always thought an ASHP was only efficient at heating water to low temperatures, eg underfloor heating?

No immersion should not be involved.

 

Your cylinder is heated slowly over about 40 minutes, heat pump output temperature slowly increases over the heating period, you should be getting a CoP of about 2.5 to 3.5 depending on heat pump and outside temperature.

 

Your UFH should be getting a CoP of 4 to 5, maybe 6 at times, that's the difference in high temperature flow and low temperature.

 

This is mine from the other week. The big peak is DHW heating, the other sides is UFH. The red is flow temperature and green return temperature.

 

 

[BotusBuild]

With an unvented cylinder (UVC) the flow/pressure is related to the pressure of the mains and size of the pipework.

 

Also with a UVC you don't need water above 50 for showering, washing etc. (otherwise you'll just scald yourself, or use lots of cold to mix it down). Using an ASHP to go any higher than 50ish and you'll just be running it VERY inefficiently. 

[MikeSharp01]

11 hours ago, JohnMo said:

Make sure it's sized for the house and has a big heating coil 3m² or bigger.

I have been looking for a 200l cylinder, we only need about 150l, and I have not found on that has a coil that big, I have found one with 1.9m2 if you connect both the main and secondary coils in series. 

[JohnMo]

1 hour ago, MikeSharp01 said:

I have been looking for a 200l cylinder, we only need about 150l, and I have not found on that has a coil that big, I have found one with 1.9m2 if you connect both the main and secondary coils in series. 

Unless you are a 1 bedroom house 150L is too small really.

Just on to Cylinder2Go @Nickfromwales go to place they have 3.3m² I believe.

 

I have an Ideal slimline 210L that is 3m².

[Nickfromwales]

1 hour ago, MikeSharp01 said:

I have been looking for a 200l cylinder, we only need about 150l, and I have not found on that has a coil that big, I have found one with 1.9m2 if you connect both the main and secondary coils in series. 

You need to be looking for a heat pump specific cylinder? 3.3 with the Telford ones I go for, plus you’ll get a good price off Trevor at Cylinders2go. 

[Nickfromwales]

9 hours ago, MikeSharp01 said:

I have been looking for a 200l cylinder, we only need about 150l, and I have not found on that has a coil that big, I have found one with 1.9m2 if you connect both the main and secondary coils in series. 

I wouldn’t go less than 300, as you’re robbing yourself of an opportunity to store cooler and also some headroom to batch higher temp heat in via the immersion (future proofing), taking into consideration how the electricity market has ‘recently’ favoured the buyer.

 

For one client (retired couple) I didn’t even bother connecting the ASHP to DHW, and they’re super happy with the daily costs / operational methodology of only using the immersion for off peak and solar PV input.

 

A service agent recently flagged up an ‘error’ when checking the history via the controller, so the client rang me to report this finding…. He stated that the unit (I fitted it nearly 3 years ago now iirc) had never done a defrost cycle.

 

“Yup, 👍“.
 

The guy said he’d never seen one which had never done a defrost, and after servicing the unit he asked the client for my number so he could give me a ring to pick my brains over the M&E design / cooling / DHW etc.

 

If you’re intending to cool then the HP shouldn’t be doing DHW afaic, as you’re just beating the shit out of it, asking it to go forward at max then backwards at half wallop, and then back again, and repeat, all summer. No ta. 

[MikeSharp01]

8 minutes ago, Nickfromwales said:

I wouldn’t go less than 300, as you’re robbing yourself of an opportunity to store cooler and also some headroom to batch higher temp heat in via the immersion (future proofing), taking into consideration how the electricity market has ‘recently’ favoured the buyer.

Had a look at the Telford Tempest, 200l we are only a two bedroom home so that should be OK and I am confident we cannot fit 300l into the space we have. Although if I could do away with 100mm of straight pipe from the outlet we could go 100mm higher but max Diameter I 600mm.

[Nickfromwales]

2 minutes ago, MikeSharp01 said:

Had a look at the Telford Tempest, 200l we are only a two bedroom home so that should be OK and I am confident we cannot fit 300l into the space we have. Although if I could do away with 100mm of straight pipe from the outlet we could go 100mm higher but max Diameter I 600mm.

You can use an angled 1” x 22mm union to leave the cylinder top immediately horizontally.

 

[JohnMo]

8 minutes ago, Nickfromwales said:

forward at max then backwards at half wallop

Not exactly going backwards or forwards, always runs the same direction that never changes, the 4 way valve just moves the refrigerant to different places, so hot or cold goes to condenser or evaporator. Compressor doesn't care it's doing the same thing all the time.

[Nickfromwales]

I would make every effort to go for 300L. 

[BotusBuild]

We wanted  300, but the installer would only put in a 250. May change it sometime down the line after we've lived with it a couple of years and had a couple of family gatherings.

All to do with the fxxking MCS calcs

Edited Monday at 16:02 by BotusBuild
Add MCS

[Nickfromwales]

Just now, JohnMo said:

Not exactly going backwards or forwards, always runs the same direction that never changes, the 4 way valve just moves the refrigerant to different places, so hot or cold goes to condenser or evaporator. Compressor doesn't care it's doing the same thing all the time.

Entire wet hydraulic arrangement;  distribution pipework, manifolds, all that volume of water, buffer if there is one etc etc. 

[Nickfromwales]

6 minutes ago, BotusBuild said:

We wanted  300, but the installer would only put in a 250. May change it sometime down the line after we've lived with it a couple of years and had a couple of family gatherings.

 

Assuming they MCS’d it? They just look at full reheat times imo, vs looking at partial reheat for when 50-60% capacity has been used up. 250 is much better than 200/210. 😉

 

The one I’ve done most recently, I installed a ‘guest’ switch to bring on the immersion for times of duress, so if there was an occasion where you feared (or already knew) that the HP would struggle then the immersion could be used to temporarily bolster DHW production. Bingo. 

[JohnMo]

35 minutes ago, Nickfromwales said:

Entire wet hydraulic arrangement;  distribution pipework, manifolds, all that volume of water, buffer if there is one etc etc. 

That doesn't change direction either, the temp just changes. You will have a volume of warm or cooler water for the heat pump to process. But nothing out of the ordinary. Really not seeing why DHW and cooling is anything of a deal to worry about.

 

Thermal gradient will be way higher mid winter, for people that run on thermostats or zone to death. Cooling lowest temp is 12ish and that only for a few minutes an hour, the rest of the time it's in recovery mode so anything up to 19.5. degrees.

[Nickfromwales]

Ok ok. I now wish I hadn’t said “change direction”. 
 

My proclamation goes out, throughout my land…”I meant regarding temperature” and I now renounce my throne.

 

The king is dead, long live the king. 🙏

[MikeSharp01]

5 hours ago, Nickfromwales said:

Although if I could do away with 100mm of straight pipe from the outlet

Is that not a G3 / MCS requirement - 100mm clear run of pipe straight out of the top! (I was told this was a requirement by one of the possible suppliers we investigated for our system.)

[ProDave]

13 minutes ago, MikeSharp01 said:

Is that not a G3 / MCS requirement - 100mm clear run of pipe straight out of the top! (I was told this was a requirement by one of the possible suppliers we investigated for our system.)

Where are you mounting a cylinder with such limited height?

[MikeSharp01]

1 minute ago, ProDave said:

Where are you mounting a cylinder with such limited height?

Its a long story, we have 1600mm x 600mm x 600 and this is directly below the MVHR unit. At this point the house is single story and it is close to the outer wall so everything is tight. The original plan was a Sunamp unit where we are now thinking we will put the UVC the Sunamp being much lower. However given the tribulations with the Sunamp units we have chosen to go for a UVC instead run from the heat pump but also chargeable via cheap electricity (Octopus agile / solar PV). So the space is limited by the original plan. We can get a Telford tempest (200l) in there, just but only if we can turn directly out of the top of the unit. Their 170 & 150 units fit with plenty of head room. There is only two of us and the house is just two bedrooms, two bathrooms, albeit spread of 153m². 

[Nickfromwales]

35 minutes ago, MikeSharp01 said:

Is that not a G3 / MCS requirement - 100mm clear run of pipe straight out of the top! (I was told this was a requirement by one of the possible suppliers we investigated for our system.)

I doubt this is a stipulation, as you can have side exit DHW outlet with a 'dip' pipe.

 

[Nickfromwales]

AFAIC as long as the pipework has a slight rise it is A1 to exit via that fitting. There is no rhyme or reason I can think of why that 100mm of pipe makes a single jot of difference tbh.

 

Zero mention of it in the Telford MI's....

 

"STEP6(all cylinders) Connect hot water draw off to connection labelled “Hot Water Draw Off”. NB: If the secondary circulation system (where used) contains more than 15 litres of water a separate expansion vessel must be provided to compensate for the larger stored volume.

 

We recommend a double check valve should be fitted to the hot water draw off to prevent any back pressure."

[Gus Potter]

@Nickfromwales

 

Would your idea be equivalent to the traditional "Surrey Flange"

 

The dip pipe concept.. it invites problems?

[Nickfromwales]

4 minutes ago, Gus Potter said:

@Nickfromwales

 

Would your idea be equivalent to the traditional "Surrey Flange"

 

The dip pipe concept.. it invites problems?

I don't think so, as the SF's were designed to not suck aerated water (very hot water plus microbubbles) into shower pumps.

 

The dip pipe solution is kosher, as it's mains pressurised water so any air that accumulates just gets pulled through in normal use, plus modern G3 installations all avoid / prevent the extremely high water temps that cause these microbubbles.