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

[ruggers]

Just looking at high efficiency low modulation gas boilers vs modern ASHP for a self build with around 4kw heat loss with MVHR installed running weather comp.

 

Two in mind would be a Viessmann 200-W system boiler vs Vaillant Arotherm plus 7Kw.

Before ASHP's became popular in the UK, how did we manage/reduce short cycling of gas boilers in low heat loss homes?

 

A Viessmann will modulate down to 1900W.

This means that the gas boiler will be at its minimum modulation & starting to short cycle for anything over 9°C outdoors while maintaining a 21°C indoor temperature.

 

How big of an issue is short cycling really? Are ASHP's better at minimising short cycling issues or more suited than gas boilers?

 

Taking away green credentials, ASHP doesn't save anything. Todays prices the heat pump is £125 per year cheaper to run for me, but the annual service is up to 2 to 3x more than a gas boiler. The unit costs more possibly even after a grant.

[Chablais]

Hi I have a Viessman 200-W system boiler installed with weather comp, no internal stats, building heatloss is 3.7kw and have MVHR installed. You are correct, even though the boiler modulates to 1900w, it does in fact, in the words of the Germans, micro start. We have UFH at 150mm centres, in 100mm concrete screed across approx 128m2. Plus a basement with UFH same spec but 33m2. We maintain 21deg internal temp. When first installed the boiler was firing circa 100 times a day!! I now time the heating periods, 7.30am-10.30am and then 2.30pm-9.30pm. I have had to adjust the set back temp to 4deg to prevent the bolier firing all night. Our floor just about coasts through the night. So with a fair bit of faffing we are down to about 20 fires in a 24hr period on the above heating times. HW is timed once a day for 1/2hr, we have 300lt UVC. I am toying with the idea of a buffer to try and smooth out the feed to the UFH. Today with outside temp at 12deg the flow temp into the UFh is 26deg. The UFH is one zone for the whole house, plus one zone for the basement, they are both mixed circuits controlled by the boiler. So we have no actuators and all circuits are run fully open. It is a great system, but does micro fire in the shoulder seasons. I had words with Viessman in the UK and Germany as they said what I am experiencing is normal and not a problem! the boilers heating rating has been turned down to 25%. Hope this helps.

[JohnMo]

28 minutes ago, ruggers said:

how did we manage/reduce short cycling of gas boilers in low heat loss homes?

A, there weren't many low energy homes.

B, gas was cheap and no one knew any better.  

C, People that knew better installed a buffer, I assume as they are not new.

 

 

29 minutes ago, ruggers said:

modulate down to 1900W.

This means that the gas boiler will be at its minimum modulation & starting to short cycle for anything over 9°C outdoors while maintaining a 21°C indoor temperature.

May even start short cycling before that at very low flow temps.  You either need a big capacity system or a buffer.  Most heat pump manufacturers mandate you have a buffer, or a minimum volume, and have a min flow rate all the time.

 

33 minutes ago, ruggers said:

How big of an issue is short cycling really

I can still get it with a gas boiler and 160L buffer at very low flow temps.  Have just increased buffer temp to compensate.

 

36 minutes ago, ruggers said:

Are ASHP's better at minimising short cycling issues or more suited than gas boilers?

Think when all heat pumps were non modulating the issue was seen and installs had to have a buffer.  With inverter drives, there is more modulation but not as much as a gas boiler.  Both will short cycle at low flow temps and low heat loading, unless you design around it.

[ruggers]

@Chablais

 

All good feedback thanks. Mine will be 88m2 of UFH ground floor at 100 centres and 55mm screed and upstairs 125m2 8 radiators including the towel rail. Looking to fit 2 mixed units to control a curve for each floor.
 

Heat loss is 6Kw without MVHR, around 4kw with, but need to fine tune this figure & update a few things. 100 fires sounds a lot, how long does the building retain the heat before it needs to fire up once its reached 21C with such a low heat loss? 

 

Have you tried lowering the curve, I thought the idea of weather comp was to have it on all the time but with a night set set back 3 degrees less.
Do you think the heat loss is too low for the boiler then? I'm not sure if a heat pumps any better in these situations or equally as bad? My knowledge of ASHP's is very basic.

 

What your outdoor design temp in Cumbria?

[ruggers]

Oops, I've posted the same post twice.

 

@JohnMo

 

Whats your house heat loss again, I remember it being low? My understanding is, no short cycling is best, a little bit isn't too big of an issue, but lots of it repeating all day is going to be a problem.

 

I've heard heat pumps dont modulate as well as gas boilers but don't understand enough about them to know if they are affected as badly as gas or worse due to them needing to fire up and build pressure?

 

I'm going to need to try to calculate the flow rate and volume of the system for an idea. I can get all of the pipe lengths for 90% accuracy i think and diameters, can size the radiators but not sure how you know the volume they hold.

[JohnMo]

Heat loss on a -5 day is about 3kW.

 

The issue with short cycling, 

having to heat up the metal and any water that has cooled while the boiler is off.  That all takes energy.  In the heat pump most that metal is outside, on boiler I think a lot of heat gets wasted out the exhaust.  By getting on top of short cycling and boiler control I have halved my gas consumption.

 

Lots of cycles also mean additional wear and component life reduction.

 

[ReedRichards]

9 hours ago, ruggers said:

I've heard heat pumps dont modulate as well as gas boilers but don't understand enough about them to know if they are affected as badly as gas or worse due to them needing to fire up and build pressure?

 

According to my installer, heat pumps don't generally have the modulation range of a gas boiler (i.e their minimum output is a larger percentage of their maximum output}.  But it should be much easier to get a heat pump with a small maximum output matched to a well-insulated house than it is to get a gas boiler with a small maximum output.

 

It seems to me that short cycling might be worse for the life a boiler that works via combustion compared to one that does not (i.e. a heat pump).

 

I got my heat pump with a third party controller.  Originally there wasn't the option to tell it that it was controlling a heat pump so I was advised to choose "oil boiler" which limited the number of cycles to 3/4 per hour.  Now there is an option to select "heat pump" but it does the same thing.  

[ruggers]

On 10/11/2022 at 22:36, Chablais said:

 I am toying with the idea of a buffer to try and smooth out the feed to the UFH. Today with outside temp at 12deg the flow temp into the UFh is 26deg. The UFH is one zone for the whole house, plus one zone for the basement, they are both mixed circuits controlled by the boiler. 

Did you ever get a buffer fitted or fix your cycling issue? How do you get a 26° flow temperature when the lowest boiler fliw is 30°?

[JohnMo]

Prior to changing to heat pump, I would run the boiler and batch charge the floor instead of weather comp. So basically used a 0.1 hysterisis thermostat, set flow temp to 35, and let the boiler run, until the thermostat hit hit 20.6 (set to 20.5) boiler would shut off, would get about 0.5 Deg overshoot and house would settle to 21. From 9am set the thermostat to 20 until midnight then set to 20.5 to 9am. Boiler generally restarted at midnight, run for as long as need to get house back to 20.5. Generally 1 or 2 starts per day. This method definitely dropped gas usage quite a bit.

[ruggers]

Interesting John. From memory, you have a large house low heat loss but was it one floor and no radiators, all UFH?

 

I had to pause my build after starting and a year on looking to restart and finding it hard to get advice for gas because everyone takes offence if not installing a heat pump if a new build not knowing that theres almost zero installers anywhere near me. With having radiators upstairs they'll heat differently for me so the load phasing you mention could leave me cold upstairs although i did want two mixed circuit heating zones running two curves over just one open loop preferably.

[RobLe]

Short cycling of heat pumps was a real issue, back in the bad old pre-inverter days.  The rule of thumb was that greater than 6 start-stop cycles an hour was likely to lead to fatigue failure of copper joints near the compressor due to the start up impulse.  I would expect that particular issue isn't much of a concern with a modern ASHP inverter drive, which I would expect to soft start.  GSHP are often still on/off relay driven simpler units, I think as noise is less of a concern for them, so they are more likely to still suffer fatigue failure with excessive cycling.

[JohnMo]

1 hour ago, ruggers said:

Interesting John. From memory, you have a large house low heat loss but was it one floor and no radiators, all UFH?

 

I had to pause my build after starting and a year on looking to restart and finding it hard to get advice for gas because everyone takes offence if not installing a heat pump if a new build not knowing that theres almost zero installers anywhere near me. With having radiators upstairs they'll heat differently for me so the load phasing you mention could leave me cold upstairs although i did want two mixed circuit heating zones running two curves over just one open loop preferably.

House is as you describe about 3kW heat loss at -9. But we also have a garden room attached to the heating system with fan coil. We now run WC mode on an ASHP.

 

I am flowing to the fan coil at the same temperature as UFH as it works pretty well. I have the circulation pump on 24/7 and the fan in the fan coil at min speed all the time also. The house floor is basically used as a buffer for the heat pump. Fan coil gets a flow temp of between 22 and 24 most the time and when the heat pump runs up to about 30.

 

A lot on here haven't bothered with radiators in upstairs bedrooms. But I would treat the gas boiler the same as the a heat pump. As in -

Design the radiators for low flow temp same as the floor. Treat as one big single circuit and run weather compensation. The bigger the system volume, the longer the run times. Then you need no fancy controls, no mixers run the whole lot from the boiler pump. Do a system boiler not a heat only boiler, then with a decent boiler you will get a modulating circulation pump, ideally a 4 pipe one or one with external diverter on PDHW, so the boiler runs two flow temperatures. Do not do S or Y plan. Run away from an installer that suggests it.

 

Cycling and short cycling are different, a short cycle will use plenty of energy and little heat to the house, normally in the range of a couple of minutes or less. A cycle is the just the boiler doing capacity control, you have a run time generally a good 10 mins or more. Run time defined by system water capacity and min modulation kW.

 

I would look at Atag boilers also. Out the box WC enabled, even come with the sensor. 3 port diverter valve ready. Just need a temp sensor for the cylinder and a single thermostat timer (ideally from Atag)

 

They work differently from most other boilers, as they slowly ramp up to temp, and you can define the rate of rise in degs per minute. You can get full flow temp instantly if you want.

 

And while your at it, do a heat pump cylinder, 3m² coil, so two options keep flow temp down for full condensation the whole heat period or super quick reheat times.

[ruggers]

I've looked into this a lot over the last two years, it's just narrowing it down so it's suited to either system. Change of circumstances means my budget is tight and heat pump quotes were coming in very expensive even with the £7500 grant it's more expensive than gas. Theres just no local installer within a 1.5 hr drive for heat pumps or Viessmann. Everything will be designed for DT5 at max MWT 37.5c either way so it's heat pump ready. .

 

The only suitable boiler to stand a chance on a 5 to 5.65Kw heat loss is the Viessmann 200W 11 Kw system boiler which has the 1.9Kw min modulation and 30 degrees minimum flow temperature, 17.9Kw DHW, this is what I've based things on but don't want the short cycling issue, so might need to look at a buffer or 1st floor UFH. I just need it finalised so I can budget. My house will have a high level of airtightness and MVHR, but the insulation will be fixed at whats been planned and I don't think it will be exceptional and require no need to heat the upstairs like some builds. Some heat engineers like open loop, others have told me it's better to have two mixed circuits than open loop because often the radiators upstairs will require less flow temperature than the ground floor, Vaillant renewables suggested this too.

 

I wondered what the benefits of an ASHP would be over the V200 since it would cost me more to install, the heat geek calculations suggest the same running costs, and then the annual services cost 3x as much. It's just that gas will be easier to get services and repairs done.

 

My preference was to be able to control each floor independently but within 2 or 3 degrees of each other so that I can run the ground floor UFH later in the evening without heating upstairs when kids are in bed with a set back temperature. All rooms will be sized for 21C so that the upstairs bedrooms/WFH office are warm during the day

 

If I was to go ASHP, it will be a Viessmann 150A 6Kw, or a Vaillant Arotherm plus 5Kw.

Edited October 21 by ruggers

[ruggers]

37 minutes ago, JohnMo said:

And while your at it, do a heat pump cylinder, 3m² coil, so two options keep flow temp down for full condensation the whole heat period or super quick reheat times.

I don't quite understand this quoted suggestion? it will be PDHW so how would this help with a boiler?

[JohnMo]

1 hour ago, ruggers said:

I don't quite understand this quoted suggestion? it will be PDHW so how would this help with a boiler?

The bigger your heat transfer area in the cylinder (the coil) the easier the heat source (boiler, heat pump etc) can heat the cylinder. A normal gas boiler cylinder a has a very small coil and requires high flow temperature. 

 

But I would 

Buy a the smallest a Panasonic heat pump you can get away with. Don't bother with the grant. £2.5k, you need a cylinder anyway, heating system sorted.

[ruggers]

If I install a heat pump I'd have to instal a heat pump cylinder with a large coil. Some of the 250L ones I've seen are around 41Kw coil, but the heat pumps don't produce anywhere near that output even in summer which is confusing. I've noticed the 300L heat pump cylinders have only 230L of usable hot water. 250L is 200 or 210L usable.

 

I looked at heat pump cylinders even if I was to install a boiler, so that it's one less thing to think about if a heat pump is later installed but then thought, it's probably better to just size one to the boiler now, because in 15 years time you'd probably want a newer cylinder that matches the heat pump technology at that time.

 

If the boiler PDHW output is 17.9Kw having a larger coil doesn't bring many benefits on reheat times.

 

What output is your heat pump in PDHW and its min modulation? Did you install it yourself?

[JohnMo]

1 minute ago, ruggers said:

If I install a heat pump I'd have to instal a heat pump cylinder with a large coil. Some of the 250L ones I've seen are around 41Kw coil

Look at the data sheet and look at m² coil area. I just used an Ideal one 210L slim line, was about £800 last year. There are plenty of options out there.

 

Or take a look here

https://www.cylinders2go.co.uk/product-category/renewable-energy/heat-pumps-cylinders/?_gl=1*1ttl432*_up*MQ..&gclid=Cj0KCQjwmt24BhDPARIsAJFYKk3KvzDtUhGD5_9hmadZrcLf87Wv0TgcqsSxCr7QIu5Lmp0Sgy3PhBYaAniOEALw_wcB

 

4 minutes ago, ruggers said:

 

If the boiler PDHW output is 17.9Kw having a larger coil doesn't bring many benefits on reheat times

Will reduce reheat time, plus as more energy is transferred to water the return temp drops more compared to a small coil, so you get slightly better efficiency from the boiler.

 

 

[ruggers]

Thanks for sharing John 

[marshian]

On 21/10/2024 at 14:02, ruggers said:

I don't quite understand this quoted suggestion? it will be PDHW so how would this help with a boiler?

 

I have Viessmann 100W "Heat Only" 16 kW with HW demand box and WC

 

It will raise the tank temp from 25 deg C to 50 Deg C in 15-18 mins with a std vented Cylinder Coil

 

In autumn,winter and spring I will run the boiler with DHWP - I want it done in as short a time possible so boiler gets back to CH running weather compensation at low flow temps

 

Summer when I'm only running HW I will revert to heating HW low and slow as there is a small reduction in gas consumption compared to DHWP when the boiler hits the HW as hard as it can

 

Next year I will look to replace the current HW tank for one with a bigger coil (it should shorten the time for HW in any season but enable the boiler to condense better in summer with a lower flow temp)

 

I will experiment with range rating the boiler because I can't lower the flow temp of the boiler (80 deg C) when it's heating HW - it maybe I can't lower the output either but I'll find out soon enough

[ruggers]

@marshian I don't know how much the 100W differs from the 200 W system boiler on a 4 pipe set up, but if using viessmanns own cylinder with the temperature sensors within, the cylinder will modulate in DHW mode too based on the demand. This might be possible with 3rd party cylinders with the right components, but generally 3rd party cylinders can't speak to the boiler and have to rely on, on/off stat which will provide max output to reheat.

 

Which cylinder are you using?

How are you changing between PDHW and low and slow modes in summer? I though priority hot water just meant what it says in the title, so it takes priority if it's in space heating mode. But in summer it will always be DHW only.

[marshian]

1 hour ago, ruggers said:

@marshian I don't know how much the 100W differs from the 200 W system boiler on a 4 pipe set up, but if using viessmanns own cylinder with the temperature sensors within, the cylinder will modulate in DHW mode too based on the demand. This might be possible with 3rd party cylinders with the right components, but generally 3rd party cylinders can't speak to the boiler and have to rely on, on/off stat which will provide max output to reheat.

 

Which cylinder are you using?

How are you changing between PDHW and low and slow modes in summer? I though priority hot water just meant what it says in the title, so it takes priority if it's in space heating mode. But in summer it will always be DHW only.

100-W heat only is two pipe - it doesn’t go down as low as the 200 (min 3.2 kW) 

 

I’m using a std vented 115 litre copper cylinder (50 mm foam coated) with basic tank stat so doesn’t have the largest heating coil in it (maybe 3kW max)

The boiler does modulate down as the return temps get closer to flow but start of cycle it throws everything it has kW wise (rated at 16kW but my readings from the gas meter over a short period of time says everything is just shy of 21 kW 😉 

 

In summer in the installation settings I’ll just switch the boiler back to dumb (B5 then C7 and then select 13 rather than 4 and it’s back to a normal boiler with full control of the flow temps, no weather compensation and no HW demand function) so in a nutshell no wiring changes

 

[JohnMo]

23 minutes ago, marshian said:

summer in the installation settings I’ll just switch the boiler back to dumb (B5 then C7 and then select 13 rather than 4 and it’s back to a normal boiler with full control of the flow temps, no weather compensation and no HW demand function) so in a nutshell no wiring changes

Why do you need to flip setting between summer and winter?

 

You should be able to define an outside temperature (if doing WC) your boiler stops doing heating, then it just does DHW. 

 

When you are doing PDHW, you are really operating your boiler more akin to a combi boiler, your diverter valve is outside the casing instead of inside. The heat exchanger is a cylinder instead of plate pack exchanger inside the boiling casing.

 

So the heating is just treated differently to CH and different heating parameters. But you can do each independent of the other. So nothing to change between sumner and winter.

[marshian]

1 hour ago, JohnMo said:

Why do you need to flip setting between summer and winter?

 

You should be able to define an outside temperature (if doing WC) your boiler stops doing heating, then it just does DHW. 

 

When you are doing PDHW, you are really operating your boiler more akin to a combi boiler, your diverter valve is outside the casing instead of inside. The heat exchanger is a cylinder instead of plate pack exchanger inside the boiling casing.

 

So the heating is just treated differently to CH and different heating parameters. But you can do each independent of the other. So nothing to change between sumner and winter.

 

Because DHWP maximises efficiency on CH but sacrifices a small amount of efficiency when heating HW

 

So when you are using CH the gains on CH running weather comp flow temps out weigh the drop in efficiency when doing HW

 

In the summer you have no need to use CH so I can revert the boiler to non DHWP and run the flow temp at 55 deg C - the boiler runs for 1 hour to get the tank from 25 to 45 (maybe 48 max) it's in condensing mode all the time and the gas usage is lower compared to DHWP when the boiler throws everything at the HW circuit in order to minimise the time spent doing HW and get back to CH asap

 

I've had the opportunity to run the boiler in both modes and I'm pretty sure my data doesn't lie to me.

 

How do you run a heat pump to get the best SCOP or COP - low and slow

 

WC is all about minimising boiler flow temps - keeping the boiler in condensing mode maximises efficiency but to do that and not lose heat in the house you want min down time from heating hence DHWP

 

DMHP for water is actually the exact opposite - chucking everything the boiler has at the HW circuit is not low and slow and there is energy lost to the Flue - why would I want to continue that wasted energy thro the non heating months?

[JohnMo]

9 hours ago, marshian said:

summer you have no need to use CH so I can revert the boiler to non DHWP and run the flow temp at 55 deg C - the boiler runs for 1 hour to get the tank from 25 to 45 (maybe 48 max) it's in condensing mode all the time and the gas usage is lower compared to DHWP when the boiler throws everything at the HW circuit in order to minimise the time spent doing HW and get back to CH asap

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.

 

 

[marshian]

21 minutes ago, 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.

 

 

I asked Viessmann technical help if I could

 

1. set the max flow temp lower for HW (when demand box was being used) their response was no - it’s preset to max flow temp

 

2. Use the max output setting on the boiler (range rate effectively) to reduce the kW being used when doing HW (using the demand box) their response was no - this would only impact the CH side the demand box calls for max boiler output and only modulates down when the return temps mean it has too.

 

As a result I shall do what I said I would do earlier in the thread and in none heating months switch back to dumb boiler mode.

 

I will say it’s reheat time is bloody fast - this morning tank was 20 deg C (we’d used all the water yesterday - bath night) and in 18 mins it was 54 deg - I don’t need that speed in the summer 🙂