Gas boiler vs ASHP & short cycling in low heat loss property

[marshian]

Slighly off topic @JohnMo because I can’t find the thread in question

 

when installing a london loop - does it matter if you have to go up 15cm to then go horizontally for min 15 cm then down for 15cm, across horizontally for 15cm and then up for 15cm?

 

I can’t fit a london loop on my HW tank return without doing this as I’d have to go down into the floor and through a floor joist - if I go up first I can package a london loop easier - it won’t look pretty but it can be done

[JohnMo]

Think the important bit is to drop down, that kills any thermal gradient which leach away heat from the cylinder.

 

If it's pipes at the bottom of the cylinder I wouldn't bother, as that area takes in cold water when you open the tap so is cold quite quickly anyway.

[marshian]

11 minutes ago, JohnMo said:

Think the important bit is to drop down, that kills any thermal gradient which leach away heat from the cylinder.

 

If it's pipes at the bottom of the cylinder I wouldn't bother, as that area takes in cold water when you open the tap so is cold quite quickly anyway.

Thank you @JohnMo I only really notice the “leaching” when I’ve done the weekly legionaires cycle (an hour or two before some HW is consumed) and when I heat the water much earlier than I need it (like at a weekend when it could be a couple of hours before any water is used)

 

The return side of the cylinder is a bit horrible right now so I’d like to tidy it up and this would be the ideal time to stick a london loop in - even if it has only a small benefit window I might as well put one in

[marshian]

Oh and below is the response from Viessmann technical regarding range rating the boiler (I have another email confirming the same for flow temp max, also adjustable for CH only (not when using HWD box)

[marshian]

22 hours ago, marshian said:

Thank you @JohnMo I only really notice the “leaching” when I’ve done the weekly legionaires cycle (an hour or two before some HW is consumed) and when I heat the water much earlier than I need it (like at a weekend when it could be a couple of hours before any water is used)

 

The return side of the cylinder is a bit horrible right now so I’d like to tidy it up and this would be the ideal time to stick a london loop in - even if it has only a small benefit window I might as well put one in

to round this slight off topic element off - london loop fitted to return from coil - was a right pain to make and fit because of space constraints in the airing cupboard but I normally see 0.66 deg tank loss per hour (it used to be 1 deg per hour before I reconfigured the zone valves) 

 

1900 hrs yesterday - heated the tank to 50 deg - at 8am this morning it had dropped to 46 deg (no major consumption of HW during that time) 0.3 deg loss per hour is an improvement 

 

The biggest obvious sign was that flow and return pipes at the boiler were identical - normally I see a higher return pipe temp before I even start space heating

 

I don’t need one on the top of the coil because the zone valve is right next to the coil inlet.

 

I might have to put one on the HW tank outlet now as that’s probably another driver for the losses.

[MrPotts]

How can you get a return temp that’s higher than the flow? Unless the heat in the HW cylinder is being sucked out by the boiler flow. My return temps are normally about 10c lower than flow when heating the HW cylinder and about 20c when space heating.

[marshian]

3 hours ago, MrPotts said:

How can you get a return temp that’s higher than the flow? Unless the heat in the HW cylinder is being sucked out by the boiler flow. My return temps are normally about 10c lower than flow when heating the HW cylinder and about 20c when space heating.

Clarification if I wasn’t clear (I thought I was but apparently not) return pipe hotter than flow when the boiler is “off”

 

I have probes on flow and return pipes after several hours of no boiler activity the temp in the return pipe above the boiler is higher thsn the flow - I believed this was down to thermo circulation - hot moving to cold

 

the addition of s london loop has stopped it “dead”

[marshian]

On 26/10/2024 at 08:38, JohnMo said:

Can't you just change the max flow temp setting down from the default 82 to something lower. Condensing mode stops when return temp exceeds about 54 degs, it has no care what the flow temperature is, so if you are getting a 10 Deg dT across your coil you could just the max flow temp to 64, have condensing, good efficiency all year.

 

So now your boiler runs WC in heating mode, and up to a max defined flow temp (by you) in cylinder heating mode.

 

 

Gone old school and found a way forward to cap the boilers output - can't stop it going to 80 deg or trying to shove 20kW at the Hot Water circuit with the Demand Box parameters but I'll update that in my Viessmann Boiler thread.

[ruggers]

@marshian thanks for the info. I understand what you mean and realise in PDHW is looks to minimise the down time of the CH so gives it max power to reheat the water quicker, but I thought there'd be an option like John suggested to change this. 

What flow temperature does it provide in PDHW mode, is it 70-80? What do you set the cylinder hysteresis to so that it doesn't cycle too much when showering, or do you operate it based on time only? 

 

 

[marshian]

8 hours ago, ruggers said:

@marshian thanks for the info. I understand what you mean and realise in PDHW is looks to minimise the down time of the CH so gives it max power to reheat the water quicker, but I thought there'd be an option like John suggested to change this. 

What flow temperature does it provide in PDHW mode, is it 70-80? What do you set the cylinder hysteresis to so that it doesn't cycle too much when showering, or do you operate it based on time only? 

 

 

 

First question - OK - there probably is an option to change temp or power on a Viessmann 4 pipe boiler but I don't have one of those - I have a two pipe "dumb" boiler with Viessmann "Extras" (actually calling it "dumb" is unfair it's really surprised me with just how clever the program in it is.......

 

I think I've explained elsewhere reasons why I chose the 100-W heat only

 

I couldn't fit a larger boiler in the space I have and hide it in a cupboard

 

I didn't want to run another set of flow and return pipes for the system boiler

 

Only the 100-W has flow and return exiting the boiler at the top (same as my old boiler)

 

So I was pretty much picking what worked best for location and minimal changeover costs

 

I've found a way to rein in the boiler modulation when doing HW, it's a bit old school but it's almost done the job (like everything it's a swings an roundabouts situation - or you could say actions have consequences but I've got a back up plan)

 

Second Question - Temp wise it's 80 deg that's what the boiler targets

 

I have the cyl stat set to 60 deg (it ends up at 63 on a friday legionaires cycle) and that's the only time the cyl stat actually gets to be in control - rest of the time - I have HW set to 20 mins on a timed slot - hence my end of HW temps can vary a little (anywhere between 46 and 54 depending on tank starting temp) but I can live with that.

 

I always heat water before it's needed, I never have a HW demand at the same time as water heating

 

I think the Cyl Stat hysteresis is quite wide - It's just a std Dayton HTS3 probably around 8 deg it also doesn't like being set below 50 deg as it doesn't have a positive off so it's really easy to think you've cracked a position for 45 deg and then find you have no HW because the next day it's Off

 

I don't have an NTS sensor 

 

It's how I've always done HW always happy to hear alternatives to my current process but what I do works for us in the house.

 

Sorry probably a bit wordy in reply!!!